last update: December 1997
A microprocessor generally means a CPU on a single silicon chip, but exceptions have been made (and are documented) when the CPU includes particularly interesting design ideas, and is generally the result of the microprocessor design philosophy. However, towards the more modern designs, design from other fields overlap, and this criterion becomes rather fuzzy. In addition, parts that used to be separate (FPU, MMU) are now usually considered part of the CPU design.
Another note on terminology - because of the muddling of the term "RISC" by marketroids, I've avoided using those terms here to refer to architectures. And anyway, there are in fact four architecture families, not two. So I use "memory-data" and "load-store" to refer to CISC and RISC architectures. This file is not intended as a reference work, though all attempts (well, many attempts) have been made to ensure its accuracy. It includes material from text books, magazine articles and papers, authoritative descriptions and half remembered folklore from obscure sources (and net.people who I'd like to thank for their many helpful comments). As such, it has no bibliography or list of references.
In other words, "For entertainment use only".
Enjoy, criticize, distribute and quote from this list freely.
By: John Bayko (Tau).
An explanation of the version numbers:
##.##.## | | | | | +-- small, usually 2 sentences or less. | +--- changes a paragraph or more, or several descriptions +---- CPU added or deleted.
Processors:Architectures: Virtual Machines: Definitions And Explanations
The 4004 had 46 instructions, using only 2,300 transistors in a 16-pin DIP. It ran at a clock rate of 740kHz (eight clock cycles per CPU cycle of 10.8 microseconds) - the original goal was 1MHz, to allow it to compute BCD arithmetic as fast (per digit) as a 1960's era IBM 1620.
The 4040 was an enhanced version of the 4004, adding 14 instructions, larger (8 level) stack, 8K program space, and interrupt abilities (including shadows of the first 8 registers).
[for additional information, see Appendix D]
The 8080 was used in the Altair 8800, the first widely-known personal computer (though the definition of 'first PC' is fuzzy. Some claim that the 12-bit LINC (Laboratory INstruments Computer) was the first 'personal computer'. Developed at MIT (Lincoln Labs) in 1963 using DEC components, it inspired DEC to design its own PDP-8 in 1965, also considered an early 'personal computer'). 'Home computer' would probably be a better term here, though).
Intel updated the design with the 8085 (1976), which added two instructions to enable/disable three added interrupt pins (and the serial I/O pins), and simplified hardware by only using +5V power, and adding clock generator and bus controller circuits on-chip.
Clock speeds ranged from the original Z-80 2.5MHz to the Z80-H (later called Z80-C) at 8MHz.
Like many processors (including the 8085), the Z-80 featured many undocumented instructions. In some cases, they were a by-product of early designs (which did not trap invalid op codes, but tried to interpret them as best they could), and in other cases chip area near the edge was used for added instructions, but fabrication made the failure rate high. Instructions that often failed were just not documented, increasing chip yield. Later fabrication made these more reliable.
But the thing that really made the Z-80 popular in designs was the memory interface - the CPU generated its own RAM refresh signals, which meant easier design and lower system cost, the deciding factor in its selection for the TRS-80 Model 1. That and its 8080 compatibility, and CP/M, the first standard microprocessor operating system, made it the first choice of many systems.
Embedded varients of the Z-80 were also produced. Hitachi produced the 64180 (1984) with added components (two 16 bit timers, two DMA controllers, three serial ports, and a segmented MMU mapping a 20 bit (1M) address space to any three variable sized segments in the 16 bit (64K) Z-80 memory map), a design Zilog and Hitachi later refined to produce the Z-180 and HD64180Z (1987?) which were compatible with Z-80 peripheral chips, plus variants (Z-181, Z-182). The Z-280 was a 16 bit version introduced about July, 1987 (loosely based on the ill-fated Z-800), with a paged (like Z-180) 24 bit (16M) MMU (8 or 16 bit bus resizing), user/supervisor modes and features for multitasking, a 256 byte (4-way) cache, 4 channel DMA, and a huge number of new op codes tacked on (total of almost 3,500, including previously undocumented Z-80 instructions), though the size made some very slow. Internal clock could be run at twice the external clock (ex. 16MHz CPU with a 8MHz bus), and additional on-chip components were available. A 16/32 bit Z-380 version also exists (1994) with added 32-bit linear addressing mode (not Z-80 compatible).
The Z-8 (1979) was an embedded processor with on-chip RAM (actually a set of 124 general and 20 special purpose registers) and ROM (often a BASIC interpreter), and is available in a variety of custom configurations up to 20MHz. Not actually related to the Z-80.
Unlike the 8080 and its kind, the 6502 (and 6800 had very few registers. It was an 8 bit processor, with 16 bit address bus. Inside was one 8 bit data register, two 8 bit index registers, and an 8 bit stack pointer (stack was preset from address 256 ($100 hex) to 511 ($1FF)). It used these index and stack registers effectively, with more addressing modes, including a fast zero-page mode that accessed memory addresses from address 0 to 255 ($FF) with an 8-bit address that speeded operations (it didn't have to fetch a second byte for the address).
Back when the 6502 was introduced, RAM was actually faster than microprocessors, so it made sense to optimize for RAM access rather than increase the number of registers on a chip. It also had a lower gate count (and cost) than its competitors.
The 650x also had undocumented instructions.
The CMOS 65C02/65C02S fixed some original 6502 design flaws, and the 65816 (officially W65C816S, both designed by Bill Mensch of Western Design Center Inc.) extended the 650x to 16 bits internally, including index and stack registers, with a 16-bit direct page register (similar to the 6809), and 24-bit address bus (16 bit registers plus 8 bit data/program bank registers). It included an 8-bit emulation mode. Microcontroller versions of both exist, and a 32-bit version (the 65832) is planned. Various licensed versions are supplied by GTE (16 bit G65SC802 (pin compatible with 6502), and G65SC816 (support for VM, I/D cache, and multiprocessing)) and Rockwell (R65C40), and Mitsubishi has a redesigned compatible version. The 6502 remains surprisingly popular largely because of the variety of sources and support for it.
The 6502-based Apple II line (not backwards compatible with the Apple I) was among the first microcomputers introduced and became the longest running PC line, eventually including the 65816-based Apple IIgs The 6502 was also used in the Nintendo entertainment system (NES), and the 65816 is in the 16-bit successor, the Super NES.
Other features were one of the first multiplication instructions of the time, 16 bit arithmetic, and a special fast interrupt. But it was also highly optimized, gaining up to five times the speed of the 6800 series CPU. Like the 6800, it included the undocumented HCF (Halt Catch Fire) instruction to incrementally strobe the address lines for bus testing ("jump to accumulator (A or B)" in the 6800, implemented and documented as $00 in the 68HC11 which is described below).
The 6800 and 6809, like the 6502 series, used a single clock cycle (the base cycle, plus a cycle rotated 90 degrees out of phase) to generate the timing for four internal execution stages, so that there were instructions which executed in one external 'cycle' (this is different from clock-doubling, which uses a phase-locked-loop to generate a faster internal clock which is synchronised with an external clock). Most CPUs, such as the 8080, used the external clock directly, so an equivalent instruction would take four cycles, meaning a 2MHz 6809 would be roughly equivalent to a 8MHz 8080. The 680x and 650x only accessed memory every other cycle, allowing a peripheral (such as video, or even a second cpu) to access the same memory without conflict. Motorola later produced CPUs in this line with a standard four-cycle clock.
The 6800 lived on as well, becoming the 6801/3, which included ROM, some RAM, a serial I/O port, and other goodies on the chip (as an embedded controller, minimizing part counts - but expensive at 35,000 transistors. The 6805 was a cheaper 6801/3, dropping seldom used instructions and features). Later the 68HC11 version (two 8 bit/one 16 bit data register, two 16 bit index, and one 16 bit stack register, and an expanded instruction set with 16 bit multiply operations) was extended to 16 bits as the 68HC16 (and a lower cost 16 bit 68HC12 (May 1996)). It remains a popular embedded processor (with over 2 billion 6800 varients sold), and radiation hardened versions of the 68HC11 have been used in communications satellites. But the 6809 was a faster and more flexible chip, particularly with the addition of the OS-9 operating system.
Of course, I'm a 6809 fan myself...
The Am2901, from Advanced Micro Devices, was a popular 4-bit-slice processor. It featured sixteen 4-bit registers and a 4-bit ALU, and operation signals to allow carry/borrow or shift operations and such to operate across any number of other 2901s. An address sequencer (such as the 2910) could provide control signals with the use of custom microcode in ROM.
The Am2903 featured hardware multiply.
Legend holds that some Soviet clones of the PDP-11 were assembled from Soviet clones of the Am2901.
AMD also produced what is probably the first floating point "coprocessor" for microprocessors, the AMD 9511 "arithmetic circuit" (1979), which performed 32 bit (23 + 7 bit floating point) RPN-style operations (4 element stack) under CPU control - the 64-bit 9512 (1980) lacked the transcendental functions. It was based on a 16-bit ALU, performed add, subtract, multiply, and divide (plus sine and cosine), and while faster than software on microprocessors of the time (about 4X speedup over a 4MHz Z-80), it was much slower (at 200+ cycles for 32*32->32 bit multiply) than more modern math coprocessors are.
It was used in some CP/M (Z-80)
systems, and on a S-100 bus math
card for NorthStar systems. Calculator circuits (such as the National
Semiconductor MM57109 (1980), actually a 4-bit NS COP400 processor with
floating point routines in ROM) were also sometimes used, with emulated
keypresses sent to it and results read back, to simplify programming
rather than for speed.
While the 8048 used 1-byte instructions, the 8051 has a more flexible 2-byte instruction set. It has eight 8-bit registers, plus an accumulator A. Data space is 128 bytes accessed directly or indirectly by a register, plus another 128 above that in the 8052 which can only be accessed indirectly (usually for a stack). External memory occupies the same address space, and can be accessed directly (in a 256 byte page via I/O ports) or through the 16 bit DPTR address register much like in the RCA 1802. Direct data above location 32 is bit-addressable. Although complicated, these memory models allow flexibility in embeded designs, making the 8051 very popular (over 1 billion sold since 1988).
The Siemens 80C517 adds a math coprocessor to the CPU which provides 16 and 32 bit integer support plus basic floating point assistance (32 bit normalise and shift), reminiscent of the old AMD 9511. The Texas Instruments TMS370 is similar to the 8051, Adding a B accumulator and some 16 bit support.
The PIC has a large register set (from 25 to 192 8-bit registers, compared to the Z-8's 144). There are up to 31 direct registers, plus an accumulator W, though R1 to R8 also have special functions - R2 is the PC (with implicit stack (2 to 16 level)), and R5 to R8 control I/O ports. R0 is mapped to the register R4 (FSR) points to (similar to the ISAR in the F8, it's the only way to access R32 or above).
The 16x is very simple and RISC-like (but less so than the RCA 1802 or the more recent 8-bit Atmel AVR microcontroller which is a canonical simple load-store design - 16-bit instructions, 2-stage pipeline, thirty-two 8-bit data registers (six usable as three 16-bit X, Y, and Z address registers), load/store architecture (plus data/subroutine stack)). It has only 33 fixed length 12-bit instructions, including several with a skip-on-condition flag to skip the next instruction (for loops and conditional branches), producing tight code important in embedded applications. It's marginally pipelined (2 stages - fetch and execute) - combined with single cycle execution (except for branches - 2 cycles), performance is very good for its processor catagory.
The 17x has more addressing modes (direct, indirect, and relative - indirect mode instructions take 2 execution cycles), more instructions (58 16-bit), more registers (232 to 454), plus up to 64K-word program space (2K to 8K on chip). The high end versions also have single cycle 8-bit unsigned multiply instructions.
The PIC 16x is an interesting look at an 8 bit design made with slightly newer design techniques than other 8 bit CPUs in this list - around 1978 by General Instruments (the 1650, a successor to the more general 1600). It lost out to more popular CPUs and was later sold to Microchip Technology, which still sells it for small embedded applications. An example of this microprocessor is a small PC board called the BASIC Stamp, consisting of 2 ICs - an 18-pin PIC 16C56 CPU (with a BASIC interpreter in 512 word ROM (yes, 512)) and 8-pin 256 byte serial EEPROM (also made by Microchip) on an I/O port where user programs (about 80 tokenized lines of BASIC) are stored.
It had sixteen 16-bit registers, which could be accessed as thirty-two 8 bit registers, and an accumulator D used for arithmetic and memory access - memory to D, then D to registers, and vice versa, using one 16-bit register as an address. This led to one person describing the 1802 as having 32 bytes of RAM and 65535 I/O ports. A 4-bit control register P selected any one general register as the program counter, while control registers X and N selected registers for I/O Index, and the operand for current instruction. All instructions were 8 bits - a 4-bit op code (total of 16 operations) and 4-bit operand register stored in N.
There was no real conditional branching (there were conditional skips which could implement it, though), no subroutine support, and no actual stack, but clever use of the register set allowed these to be implemented - for example, changing P to another register allowed jump to a subroutine. Similarly, on an interrupt P and X were saved, then R1 and R2 were selected for P and X until an RTI restored them.
A later version, the 1805, was enhanced, adding several Forth language primitives (Forth is commonly used in control applications).
The use of the ISAR register allowed a subroutine to be entered without saving a bunch of registers, speeding execution - the ISAR would just be changed. Special purpose registers were stored in the second cell (regs 9-15), and the first eight registers were accessed directly (globally).
The windowing concept was useful, but only the register pointed to by the ISAR could be accessed - to access other registers the ISAR was incremented or decremented through the window.
Fairchild ended up as part of National Semiconductor, before being spun off again in 1997.
The unique feature was the ability to completely share a system bus with other processors. Most processors of the time assumed they were the only ones accessing memory or I/O devices. Multiple SC/MPs (as well as other intelligent devices, such as DMA controllers) could be hooked up to the bus. A control line (ENOUT (Enable Out) to ENIN) could be chained along the processors to allow cooperative processing. This was very advanced for the time, compared to other CPUs, but the bit-serial CPU was slow (even simple instruction took 5-7 cycles, while memory access was 2 cycles, which allowed them to share a memory bus without saturating it, as opposed to a 6502 which could share memory with at most one other CPU, and only then because of the way the CPU clock was used). However this feature was almost never used for multiprocessing.
In addition to I/O ports like the 8080, the SC/MP also had instructions and one pin for serial input and one for output.
National Semiconductor eventually replaced the SCMP with the COP4 (4 bit) and COP8 (8 bit) embedded controllers, with only two index registers, but adding stack support.
The unique feature of the F100-L was that it had a complete control bus available for a coprocessor that could be added on. Any instruction the F100-L couldn't decode was sent directly to the coprocessor for processing. Applications for coprocessors at the time were limited, but the design is still used in some modern processors, such as the National Semiconductor 320xx series (the predecessor of the Swordfish processor, described later), which included FPU, MMU, and other coprocessors that could just be added to the CPU's coprocessor bus in a chain. Other units not foreseen could be added later.
Ferranti no longer makes microprocessors.
The ALU chip contained twenty six 8 bit registers and an 8 bit ALU, while the control unit supervised the moving of data, memory access, and other control functions. The ROM allowed the chip to function as either an 8 bit chip or 16 bit, with clever use of the 8 bit ALU. Even more, microcode allowed the addition of floating point routines (40 + 8 bit format), simplifying programming (and possibly producing a Floating Point Coprocessor).
Two standard microcode ROMS were available. This flexibility was one reason it was also used to implement the DEC LSI-11 processor as well as the WD Pascal Microengine.
The 6100 was a 12 bit processor, which had exactly three registers - the PC, AC (an accumulator), and MQ. All 2 operand instructions read AC and MQ, and wrote back to AC. It had a 12 bit address bus, limiting RAM to only 4K. Memory references were 7 bit (128 word) offset either from address 0, or the PC.
It had no stack. Subroutines stored the PC in the first word of the subroutine code itself, so recursion wasn't possible without fancy programming.
4K RAM was pretty much hopeless for general purpose use. The 6102 support chip (included on chip in the 6120) added 3 address lines, expanding memory to 32K the same way that the PDP-8/E expanded the PDP-8. Two registers, IFR and DFR, held the page for instructions and data respectively (IFR always used until a data address was detected). At the top of the 4K page, the PC wrapped back to 0, so the last instruction on a page had to load a new value into the IFR if execution was to continue.
The PDP-8 itself was succeeded by the PDP-11 (though a PDP-12 version of the PDP-8 was produced. The IMS 6120 was used in the DECmate (1980), DEC's original competition for the IBM PC, but lacked the processor and RAM capacity (a Z-80 or 8086 card could be added (reducing the 6120 to an I/O coprocessor) but lacked IBM PC compatability). DEC also tried competing with the 8086 based Rainbow, and the PDP-11 based PRO-325 personal computers, but none caught on.
Intersil was eventually bought by Harris Semiconductors, which produces versions of the 8088 and 8086, 1802, and 68HC05.
The NOVA had four 16-bit accumulators, AC0 to AC3. There were also 15-bit system registers - Program Counter, Stack pointer, and Stack Frame pointer (the last two were only on the MicroNova and Nova 3, not the original Nova or Fairchild CPU). AC2 and AC3 could be used for indexed addresses. Apart from the small register set, the NOVA was an ordinary CPU design.
Another CPU, the PACE, was based on the NOVA design, but featured 16 bit addressing, more addressing modes, and a 10 level stack (like the 8008).
The 32 bit ECLIPSE (pre 1983) was Data General's successor to the 16 bit Nova. Like the Nova, the ECLIPSE had four 32 bit integer accumulators, added four stack registers, and four 64 bit floating point registers (in the MV series). There are twelve special purpose registers. The ECLIPSE was eventually implemented in a microprocessor form as well.
This has nothing to actually do with the Nova CPU, but is a little
bit interesting anyway.
It also had a subroutine stack of eight 15 bit elements, with no provision for spilling over into memory.
Signetics was bought by Valvo, which was later bought by Phillips.
It had only 16 pins, less than a typical RAM chip, and ran at 1 MHz.
The PDP-11 had eight general purpose 16-bit registers (R0 to R7 - R6 was also the SP and R7 was the PC). It featured powerful register oriented (little-endian, byte addressable) addressing modes. Since the PC was treated as a general purpose register, constants were loaded using an indirect mode on R7 which had the effect of loading the 16 bit word following the current instruction, then incrementing the PC to the next instruction before fetching. The SP could be accessed the same way (and any register could be used for a user stack (useful for FORTH)). A CC (or PSW) register held results from every instruction that executed.
Adjascent registers could be implicitly grouped into a 32 bit register for multiply and divide results (Multiply result stored in two registers if destination is an even register, not if it's odd. Divide source must be grouped - quotient is stored in high order (low number) register, remainder in low order).
A floating point unit could be added which contains six 64 bit accumulators (AC0 to AC5, can also be used as six 32-bit registers - values can only be loaded or stored using the first four registers).
PDP-11 addresses were 16 bits, limiting program space to 64K, though an MMU could be used to expand total address space (18-bits and 22-bits in different PDP-11 versions).
The LSI-11 (1975-ish) was a popular microprocessor implementation of the PDP-11 using the Western Digital MCP1600 microprogrammable CPU, and the architecture influenced the Motorola 68000, NS 320xx, and Zilog Z-8000 microprocessors in particular. There was also a 32-bit PDP-11 plan as far back as its 1969 introduction. The PDP-11 was finally replaced by the VAX architecture, (early versions included a PDP-11 emulation mode, and were called VAX-11).
It had a 15 bit address space and two internal 16 bit registers. One unique feature, though, was that all user registers were actually kept in memory - this included stack pointers and the program counter. A single workspace register pointed to the 16 register set in RAM, so when a subroutine was entered or an interrupt was processed, only the single workspace register had to be changed - unlike some CPUs which required a dozen or more register saves before acknowledging a context switch.
This was feasible at the time because RAM was often faster than the CPUs. A few modern designs, such as the INMOS Transputers, use this same design using caches or rotating buffers, for the same reason of improved context switches. Other chips of the time, such as the 650x series had a similar philosophy, using index registers, but the TMS 9900 went the farthest in this direction. Later versions added a write-through register buffer/cache.
That wasn't the only positive feature of the chip. It had good interrupt handling features and very good instruction set. Serial I/O was available through address lines. In typical comparisons with the Intel 8086, the TMS9900 had smaller and faster programs. The only disadvantage was the small address space and need for fast RAM.
Despite the very poor support from Texas Instruments, the TMS 9900 had the potential at one point to surpass the 8086 in popularity. TI also produced an embedded version, the TMS 9940.
Internally, the Z-8000 had sixteen 16 bit registers, but register size and use were exceedingly flexible - the first eight Z-8000 registers could be used as sixteen 8 bit subregisters (identified RH0, RL0, RH1 ...), or all sixteen could be grouped into eight 32 bit registers (RR0, RR2, RR4 ...), or four 64 bit registers. They were all general purpose registers - the stack pointer was typically register 15, with register 14 holding the stack segment (both accessed as one 32 bit register (RR14) for painless address calculations). The instruction set included 32-bit multiply (into 64 bits) and divide.
The Z-8000 was one of the first to feature two modes, one for the operating system and one for user programs. The user mode prevented the user from messing about with interrupt handling and other potentially dangerous stuff (each mode had its own stack register).
Finally, like the Z-80, the Z-8000 featured automatic RAM refresh circuitry. Unfortunately the processor was somewhat slow, but the features generally made up for that.
A later version, the Z-80000, was introduced about at the beginning of 1986, at about the same time as the 32 bit MC68020 and Intel 80386 CPUs, though the Z-80000 was quite a bit more advanced. It was fully expanded to 32 bits internally, giving it sixteen 32 bit physical registers (the 16 bit registers became subregisters), doubling the number of 32 bit and 64 bit registers (sixteen 8-bit and 16-bit subregisters, 32-bit physical registers, eight 64-bit double registers). The system stack remained in RR14.
In addition to the addressing modes of the Z-8000, larger 24 bit (16Mb) segment addressing was added, as well as an integrated MMU (absent in the 68020 but added later in the 68030) which included an on chip 16 line 256-byte fully associated write-through cache (which could be set to cache only data, instructions, or both, and could also be frozen by software once 'primed' - also found on later versions of the AMD 29K). It also featured multiprocessor support by defining some memory pages to be exclusive and others to be shared (and non-cacheable), with separate memory signals for each (including GREQ (Global memory REQuest) and GACK lines). There was also support for coprocessors, which would monitor the data bus and identify instructions meant for them (the CPU had two coprocessor control lines (one in, one out), and would produce any needed bus transactions).
Finally, the Z-80000 was fully pipelined (six stages), while the fully pipelined 80486 and 68040 weren't introduced until 1991.
But despite being technically advanced, the Z-8000 and Z-80000 series never met mainstream acceptance, due to initial bugs in the Z-8000 (the complex design did not use microcode - it used only 17,500 transistors) and to delays in the Z-80000. There was a radiation resistant military version, and a CMOS version of the Z-80000 (the Z-320). Zilog eventually gave up and became a second source for the AT&T WE32000 32-bit (1986) CPU instead (a VAX-like microprocessor derived from the Bellmac 32A minicomputer, which also became obsolete).
The Z-8001 was used for Commodore's CBM 900 prototype, but the Unix based machine was never released - instead, Commodore bought Amiga, and released the 68000 based machine it was designing. A few companies did produce Z-8000 based computers, with Olivetti being the most famous, and the Plexus P40 being the last - the 68000 quickly became the processor of choice.
Looking back it was a logical design decision, since most 8 bit processors featured direct 16 bit addressing without segments.
The 68000 had sixteen 32-bit registers, split into eight data and address registers. One address register was reserved for the Stack Pointer. Data registers could be used for any operation, including offset from an address register, but not as the source of an address itself. Operations on address registers were limited to move, add/subtract, or load effective address.
Like the Z-8000, the 68000 featured a supervisor and user mode (each with its own Stack Pointer). The Z-8000 and 68000 were similar in capabilities, but the 68000 was 32 bit units internally (16 bit ALUs, two in parallel for 32-bit data, one for addresses), making it faster and eliminating forced segments. It was designed for expansion, including specifications for floating point and string operations (floating point was added in the 68040 (1991), with eight 80 bit floating point registers compatible with the 68881/2 coprocessors). Like many other CPUs of the time, the 68000 could fetch the next instruction during execution (a 2 stage pipeline).
The 68010 added virtual memory support (the 68000 couldn't restart interrupted instructions) and a special loop mode - small decrement-and-branch loops could be executed from the instruction fetch buffer. The 68020 (1984) expanded external data and address bus to 32 bits,simple 3-stage pipeline, and added a 256 byte cache, while the 68030 brought the MMU onto the chip (it supported two level pages (logical, physical) rather than the segment/page mapping of the Intel 80386 and IBM S/360 mainframe). The 68040 (1989/90) added fully cached Harvard busses (4K(?) each for data and instructions), 6 stage pipeline, and on chip FPU.
The 68060 (late 1994) expanded the design to a superscalar version, like the Intel Pentium and NS320xx (Swordfish) series before it. Like the Nx586, AMD K5, and Intel's "Pentium Pro", the the third stage of the 10-stage 68060 pipeline translates the 680x0 instructions to a decoded RISC-like form (stored in a 16 entry buffer in stage four), and uses resource renaming (with fourty rename registers) to reorder instructions. There is also a branch cache, and branches are folded into the decoded instruction stream like the AT&T Hobbit and other more recent processors, then dispatched to two pipelines (three stages: Decode, addr gen, operand fetch) and finally to two of three execution units - 2 integer, 1 floating point) before reaching two 'writeback' stages. Cache sizes are doubled over the 68040.
The 68060 also also includes many innovative power-saving features (3.3V operation, execution unit pipelines could actually be shut down, reducing power consumption at the expense of slower execution, and the clock could be reduced to zero) so power use is lower than the 68040 (4-6 watts vs. 3.9-4.9). Another innovation is that simple register-register instructions which don't generate addresses may use the the address stage ALU to execute 2 cycles early.
The embedded market became the main market for the 680x0 series after workstation venders (and the Apple Macintosh) turned to faster load-store processors, so a variety of embedded versions were introduced. Later, Motorola designed a successor called Coldfire (early 1995), in which complex instructions and addressing modes (added to the 68020) were removed and the instruction set was recoded, simplifying it at the expense of compatibility (source only, not binary) with the 680x0 line.
The Coldfire 52xx architecture resmbles a stripped (single pipeline) 68060, The 5 stage pipeline is literally folded over itself - after two fetch stages and a 12-byte buffer, instructions pass through the decode and address generate stages, then loop back so the decode becomes the operand fetch stage, and the address generate becomes the execute stage (so only one ALU is required for address and execution calculations). Simple (non-memory) instructions don't need to loop back. There is no translator stage as in the 68060 because Coldfire instructions are already in RISC-like form. The earlier 51xx version has a straight 68040-based 6 stage pipeline and includes 680x0 instructions (in user mode).
At a quarter the physical size and a fraction of the power consumption, Coldfire is about as fast as a 68040 at the same clock rate, but the smaller design allows a faster clock rate to be acheived.
The Macintosh was to include the best features of the Lisa, but at an affordable price - in fact the original Macintosh came with only 128K of RAM and no expansion slots. Cost was such a factor that the 8 bit Motorola 6809 was the original design choice, and some prototypes were built, but they quickly realised that it didn't have the power for a GUI based OS, and they used the Lisa's 68000, borrowing some of the Lisa low level functions (such as graphics toolkit routines) for the Macintosh.
Competing personal computers such as the Amiga and Atari ST, and
early workstations by Sun, Apollo, NeXT and most others also used 680x0
CPUs (including one of the earliest workstations, the Tandy TRS-80
Model 16, which used a 68000 CPU and Z-80
for I/O and VM support).
It was similar to the 68000 in basic features, such as byte addressing, 24-bit address bus in the first version, memory to memory instructions, and so on (The 320xx also includes a string and array instruction). Unlike the 68000, the 320xx had eight instead of sixteen 32-bit registers, and they were all general purpose, not split into data and address registers. There was also a useful scaled-index addressing mode, and unlike other CPUs of the time, only a few operations affected the condition codes (as in more modern CPUs).
Also different, the PC and stack registers were separate from the general register set - they were special purpose registers, along with the interrupt stack, and several "base registers" to provide multitasking support - the base data register pointed to the working memory of the current module (or process), the interrupt base register pointed to a table of interrupt handling procedures anywhere in memory (rather than a fixed location), and the module register pointed to a table of active modules.
The 320xx also had a coprocessor bus, similar to the 8-bit Ferranti F100-L CPU, and coprocessor instructions. Coprocessors included an MMU, and a Floating Point unit which included eight 32-bit registers, which could be used as four 64-bit registers.
The series found use mainly in embedded applications, and was expanded to that end, with timers, graphics enhancements, and even a Digital Signal Processor unit in the Swordfish version (1991), among the first superscalar processors, with two 4-stage integer units, one floating point add and one multiplier/DSP unit. The Swordfish also has dynamic bus resizing (8, 16, 32, or 64 bits, allowing 2 instructions to be fetched at once) and clock doubling, 2 DMA channels, and in circuit emulation (ICE) support for debugging.
Like other 16 bit designs of the time, 1750 was inspired by the PDP-11, but differs significantly. Sixteen 16-bit registers were specified, and any adjascent pairs (such as R0+R1 or R1+R2) could be used as 32-bit registers (the Z-8000 and PDP-11 could only use even pairs, and the PDP-11 only for specific uses) for integer or floating point (FP) values (no separate FP registers), or triples for 48-bit extended precision FP (with the mantissa concatenated after the exponent - eg. 32-bit FP was [1s][23mantissa][8exp], 48-bit was [1s][23mantissa][8exp][16ext], meaning any 48-bit FP was also a valid 32-bit FP, only losing the extra precision). Also, only the upper four registers (R12 to R15) could be used as an address base (2 instruction bits instead of 4), and R0 can't be used as an index (using R0 implies no indexing, similar to the PowerPC. R15 is used as an implicit stack pointer, the program counter is not user accessible.
Address space is 16 bit word addressed (not bytes), but the design allows for an MMU to extend this to 20 bits. In addition, program and data memory can be separated using the MMU. A 4-bit Address State field in the processor status word (PSW) selects one of sixteen page groups, each containing sixteen registers for data memory and another sixteen for program memory (16x16x2 = 512 total). The top 4 bits of an address selects a register from the current AS group, which provides the upper 8 bits of a 20 bit address.
Each page register also has a 4-bit access key. While other CPUs at the time provided user and supervisor modes, the 1750 provided for sixteen modes, from supervisor (mode 0, could access all pages), fourteen user modes (1 to 14 can only access page with same key, or key 15), and an unprivledged mode (mode 15 can only access page with key 15). Program memory can occupy the same logical address space as data, but will select from the program page registers. Pages can also be write or execute protected.
Several I/O instructions are also included, and are used to access processor state registers.
The 1750 is a very practical 16 bit design, and is still being produced, mainly in expensive radiation resistant forms. It did not achieve widespread acceptance, likely because of the rapid advance of technology and the rise of the RISC paradigm.
It featured four 16 bit general registers, which could also be accessed as eight 8 bit registers, and four 16 bit index registers (including the stack pointer). The data registers were often used implicitly by instructions, complicating register allocation for temporary values. It featured 64K 8-bit I/O (or 32K 16-bit) ports and fixed vectored interrupts. There were also four segment registers that could be set from index registers.
The segment registers allowed the CPU to access 1 meg of memory through an odd process. Rather than just supplying missing bytes, as most segmented processors, the 8086 actually added the segment registers ( X 16, or shifted left 4 bits) to the address. As a strange result of this unsuccessful attempt at extending the address space without adding address bits, it was possible to have two pointers with the same value point to two different memory locations, or two pointers with different values pointing to the same location, and limited typical data structures to less than 64K. Most people consider this a brain damaged design (a better method might have been that developed for the MIL-STD-1750 MMU).
Although this was largely acceptable for assembly language, where control of the segments was complete (it could even be useful then), in higher level languages it caused constant confusion (ex. near/far pointers). Even worse, this made expanding the address space to more than 1 meg difficult. The 80286 (1982?) expanded the design to 32 bits only by adding a new mode (switching from 'Real' to 'Protected' mode was supported, but switching back required using a bug in the original 80286, which then had to be preserved) which greatly increased the number of segments by using a 16 bit selector for a 'segment descriptor', which contained the location within a 24 bit address space, size (still less than 64K), and attributes (for Virtual Memory support) of a segment.
But all memory access was still restricted to 64K segments until the 80386 (1985), which included much improved addressing: base reg + index reg * scale (1, 2, 4 or 8 bits) + displacement (8 or 32 bit constant = 32 bit address (in the form of paged segments (using six 16-bit segment registers), like the IBM S/360 series, and unlike the Motorola 68030). It also had several processor modes (including separate paged and segmented modes) for compatibility with the previous awkward design. In fact, with the right assembler, code written for the 8008 can still be run on the most recent Pentium Pro. The 80386 also added an MMU, security modes (called "rings" of privledge - kernal, system services, application services, applications) and new op codes in a fashion similar to the Z-80 (and Z-280).
The 80486 (1989) added full pipelines, single on chip 8K cache, integrated FPU (based on the eight element 80-bit stack-oriented FPU in the 80387 FPU), and clock doubling versions (like the Z-280). The Pentium (late 1993) was superscalar (up to two instructions at once in dual integer units and single FPU) with separate 8K I/D caches.
The Pentium was the name Intel gave the 80586 version because it could not legally protect the name "586" to prevent other companies from using it - and in fact, the Pentium compatible CPU from NexGen is called the Nx586 (early 1995). Due to its popularity, the 80x86 line has been the most widely cloned processors, from the NEC V20/V30 (slightly faster clones of the 8088/8086 (could also run 8085 code)), AMD and Cyrix clones of the 80386 and 80486, to versions of the Pentium within less than two years of its introduction.
MMX (initially reported as MultiMedia eXtension, but later said by Intel to mean Matrix Math eXtension) is very similar to the earlier SPARC VIS or HP-PA MAX instructions) - they perform integer operations on vectors of 8, 16, or 32 bit words, using the 80 bit FPU stack elements as eight 64 bit registers (switching between FPU and MMX modes as needed - it's very difficult to use them as a stack and as MMX registers at the same time). The P55C Pentium version (January 1997) is the first Intel CPU to include MMX instructions, followed by the AMD K6, and Pentium II. Cyrix also added these instructions in its M2 CPU (6x86MX, June 1997), as well as IDT with its C6.
Interestingly, the old architecture is such a barrier to improvements that most of the Pentium compatible CPUs (NexGen Nx586/Nx686, AMD K5, IDT-C6), and even the "Pentium Pro" (Pentium's successor, late 1995) don't clone the Pentium, but emulate it with specialized hardware decoders which convert Pentium instructions to RISC-like instructions which are executed on specially designed superscalar RISC-style cores faster than the Pentium itself. Intel also used BiCMOS in the Pentium and Pentium Pro to achieve clock rates competitive with CMOS load-store processors (the Pentium P55C (early 1997) version is a pure CMOS design).
Persistant rumours that IBM was developing hardware to translate Pentium instructions for the PowerPC as part of the PowerPC 615 CPU eventually died.
The Cyrix 6x86 (early 1996), initially manufactured by IBM before Cyrix merged with National Semiconductor, still directly executes 80x86 instructions in two pipelines, but partly out of order, making it faster than a Pentium. Cyrix also sells an integrated version with graphics and audio on-chip called the MediaGX. MMX instructions were added to the 6x86MX, and 3D graphics instructions to the 6x86MXi.
The Pentium Pro (code named "P6") is a 1 or 2-chip (CPU plus 256K or 512K L2 cache - I/D L1 cache (8K each) is on the CPU), 14-stage superpipelined processor. It uses extensive multiple branch prediction and speculative execution via register renaming. Three decoders (one for complex instructions, two for simpler ones (four or fewer micro-ops)) each decode one 80x86 instruction into micro-ops (one per simple decoder + up to four from the complex decoder = three to six per cycle). Up to five (usually three) micro-ops can be issued in parallel and out of order (six units - FPU, 2 integer, 2 address, 1 load/store), but are held and retired (results written to registers or memory) as a group to prevent an inconsistant state (equivalent to half an instruction being executed when an interrupt occurs, for example). 80x86 instructions may produce several micro-ops in CPUs like this (and the Nx586 and AMD K5), so the actual instruction rate is lower. In fact, due to problems handling instruction alignment in the Pentium Pro, emulated 16-bit instructions execute slower than on a Pentium. The Pentium II (April 1997) version of the Pentium Pro added MMX instructions, doubled cache to 32K, and was packaged in a processor card instead of an IC package.
The AMD K5 translates 80x86 code to ROPs (RISC OPerations), which execute on a RISC-style core based on the superscalar AMD 29K. Up to four ROPs can be dispatched to six units (two integer, one FPU, two load/store, one branch unit), and five can be retired at a time. The complexity led to low clock speeds for the K5, prompting AMD to buy NexGen and integrate its designs for the next generation K6.
The NexGen/AMD Nx586 (early 1995) is unique by being able to execute its micro-ops (called RISC86 code) directly, allowing optimised RISC86 programs to be written which are faster than an equivalent x86 program would be, but this feature is seldom used. It also features two 16K I/D L1 caches, a dedicated L2 cache bus (like that in the Pentium Pro 2-chip module) and an off-chip FPU (either separate chip, or later as in 2-chip module).
The Nx586 sucessor, the K6 (April 1997) actually has three caches - 32K each for data and instructions, and a half-size 16K cache containing instruction decode information. It also brings the FPU on-chip and eliminates the dedicated cache bus of the Nx586, allowing it to be pin-compatible with the P54C model Pentium. Another decoder is added (two complex decoders, compared to the Pentium Pro's one complex and two simple decoders) producing up to four micro-ops and issuing up to six (to seven units - load, store, complex/simple integer, FPU, branch, multimedia) and retiring four per cycle. It includes MMX instructions, licensed from Intel, and 3D graphics extensions are planned.
Centaur, a subsidiary of Integrated Device Technology, will produce the IDT-C6 (May 1997), which uses a much simpler (6-stage, 2 way integer/simple-FPU execution) desgn than Intel and AMD translation-based designs by using micro-ops more closely resembling 80x86 than RISC code, which allows for a higher clock rate and larger L1 (32K each I/D) and TLB caches in a lower cost, lower power consumption design. Simplifications include replacing branch prediction (less important with a short pipeline) with an eight entry call/return stack, depending more on caches. The FPU unit includes MMX support (the C6+ version added a second FPU/MMX unit and 3D graphics enhancements).
Intel, with partner Hewlett-Packard, has begun development of a next generation 64-bit processor (code named Merced or IA-64). It is expected to be a variable length instruction group (VLIG or what HP/Intel call EPIC (Explicit Parallel Instruction Computing)) with instruction dependencies grouped from 1 to 9+, in 128 bit bundles of three instructions plus three "template bits" which indicate dependancies. Most instructions are predicated, a design very similar to the TI 320C6x DSP, but with 128 general 64-bit and 128 floating point registers, and 64 predicate bits (a type of condition code). To reduce page faults, speculative load instructions execute a load, but does not trap if there is an exception until a second instruction completes it - if the second instruction is predicated and never executes, then a page fault is avoided, and loads can be rescheduled more flexibly. It's expectged to be compatible in some way with both the PA-RISC and 80x86 - it will include 80x86 data and segment registers, with additional instructions to switch between instruction/register sets and transfer data between 80x86 and IA-64 registers. It is expected to translate 80x86 instructions into VLIW instructions (or directly to decoded instructions) the same way that Pentium Pro and AMD K5/K6 CPUs do, but with a larger number of instructions issued using the VLIW design, it should be faster. However, native IA-64 code should be even faster, and this may finally produce the incentive to let the 80x86 architecture finally fade away.
Other factors were the 8-bit 8088 (1979) version, which could use existing low cost 8085-type components, and allowed the computer to be based on a modified 8085 design. 68000 components were not widely available, though it could use 6800 components to an extent. After the failure and expense of the IBM 5100 (1974/5/6? - their first attempt at a peronal computer - discrete random logic CPU with no bus, built in BASIC and APL as the OS), cost was a large factor in the design of the PC.
The availability of CP/M-86 is also likely a factor, since CP/M was the operating system standard for the computer industry at the time. However Digital Research founder Gary Kildall was unhappy with the legal demands of IBM, so Microsoft, a programming language company, was hired instead to provide the operating system (initially known at varying times as QDOS, SCP-DOS, and finally 86-DOS, it was purchased by Microsoft from Seattle Computer Products and renamed MS-DOS).
Digital Research did eventually produce CP/M 68K for the 68000 series, making the operating system choice less relevant than other factors.
Intel bubble memory
was on the market for a while, but faded away
as better and cheaper memory technologies arrived.
The basic design is scalable, from 32 to 48 and 64 bit designs, with 16 general purpose registers. It is a memory-data instruction set, but an elegant one. One early design was the Mitsubishi M32 (mid 1987), which optimised the simple and often used TRON instructions, much like the 80486 and 68040 did. It featured a 5 stage pipeline, dynamic branch prediction with a target branch buffer similar to that in the AMD 29K. It also featured an instruction prefetch queue, but being a prototype, had no MMU support or FPU.
Commercial versions such as the Gmicro/200 (and other Gmicro/) from Fujitsu and Toshiba Tx1 were also introduced, but didn't catch on in the non-Japanese market. In addition, newer techniques (such as load-store designs) overshadowed the TRON standard. Companies such as Hitachi switched to load-store designs, and many American companies (Sun, MIPS) licensed their (faster) designs openly to Japanese companies. TRON's promise of a unified architecture (when complete) was less important to companies than raw performance and immediate compatibility (Unix, MS-DOS, Macintosh), and has not become significant in the industry, though TRON operating system development continues as an embedded distributed operating system (such as Intelligent House projects) implemented on non-TRON CPUs.
NEC produced a similar memory-data design around the same time, the V60/V70 series, using thirty two registers, a seven stage pipeline, and preprocessed branches. NEC later became a source of MIPS load-store processors.
SPARC was not the first RISC processor. The AMD 29000 (see below) came before it, as did the MIPS R2000 (based on Stanford's experimental design) and Hewlett-Packard PA-RISC CPU, among others. The SPARC design was radical at the time, even omitting multiple cycle multiply and divide instructions (added in later versions), using single-cycle "step" instructions instead, while most RISC CPUs were more conventional.
SPARC usually contains about 128 or 144 registers, (memory-data designs typically had 16 or less). At each time 32 registers are available - 8 are global, the rest are allocated in a 'window' from a stack of registers. The window is moved 16 registers down the stack during a function call, so that the upper and lower 8 registers are shared between functions, to pass and return values, and 8 are local. The window is moved up on return, so registers are loaded or saved only at the top or bottom of the register stack. This allows functions to be called in as little as 1 cycle. Like most RISC processors, global register zero is wired to zero to simplify instructions, and SPARC is pipelined for performance (a new instruction can start execution before a previous one has finished), but not as deeply as others - like the MIPS CPUs, it has branch delay slots. Also like previous processors, a dedicated CCR holds comparison results.
SPARC is 'scalable' mainly because the register stack can be expanded (up to 512, or 32 windows), to reduce loads and saves between functions, or scaled down to reduce interrupt or context switch time, when the entire register set has to be saved. Function calls are usually much more frequent than interrupts, so the large register set is usually a plus, but compilers now can usually produce code which uses a fixed register set as efficiently as a windowed register set across function calls.
SPARC is not a chip, but a specification, and so there are various designs of it. It has undergone revisions, and now has multiply and divide instructions. Original versions were 32 bits, but 64 bit and superscalar versions were designed and implemented (beginning with the Texas Instruments SuperSparc in late 1992), but performance lagged behind other load-store and even Intel 80x86 processors until the UltraSPARC (late 1995) from Texas Instruments and Sun, and superscalar HAL/Fuji SPARC64 multichip CPU.
The UltraSPARC is a 64-bit superscalar processor series which can issue up to four instructions at once (but not out of order) to any of nine units: two integer units, two of the five floating point/graphics units, the branch and load/store unit. The UltraSparc also added a block move instruction which bypasses the caches (2-way 16K instr, 16K direct mapped data), to avoid disrupting it, and specialized pixel operations (VIS - the Visual Instruction Set) which can operate in parallel on 8, 16, or 32-bit integer values packed in a 64-bit floating point register (for example, four 8 X 16 -> 16 bit multiplications in a 64 bit word, a sort of simple SIMD/vector operation. More extensive than the Intel MMX instructions, or earlier HP PA-RISC MAX and Motorola 88110 graphics extensions, VIS also includes some 3D to 2D conversion, edge processing and pixes distance (for MPEG, pattern-matching support).
The HAL/Fuji SPARC64 can issue up to four in order instructions simultaneously to four buffers, then to four integer, two floating point, two load/store, and the branch unit, and may complete out of order (an instruction completes when it finishes without error, is committed when all instructions ahead of it have completed, and is retired when its resources are freed - these are 'invisible' stages in the SPARC64 pipeline). A combination of register renaming, a branch history table, and processor state storage (like in the Motorola 88K) allow for speculative execution while maintaining precise exceptions/interrupts (renamed integer, floating, and CC registers - trap levels are also renamed and can be entered speculatively).
The 29000 has 64 global registers, in comparison to the SPARC's eight. In addition, the 29000 allows variable sized windows allocated from the 128 register stack cache. The current window or stack frame is indicated by a stack pointer (a modern version of the ISAR register in the Fairchild F8 CPU), a pointer to the caller's frame is stored in the current frame, like in an ordinary stack (directly supporting stack languages like C, a CISC-like philosophy). Spills and fills occur only at the ends of the cache, and registers are saved/loaded from the memory stack. This allows variable window sizes, from 1 to 128 registers. This flexibility, plus the large set of global registers, makes register allocation easier than in SPARC (optimised stack operations also make it ideal for a stack-oriented interpreted languages such as PostScript, making it popular as a laser printer controller).
There is no special condition code register - any general register is used instead, allowing several condition codes to be retained, though this sometimes makes code more complex. An instruction prefetch buffer (using burst mode) ensures a steady instruction stream. Branches to another stream can cause a delay, so the first four new instructions are cached - next time a cached branch (up to sixteen) is taken, the cache supplies instructions during the initial memory access delay.
Registers aren't saved during interrupts, allowing the interrupt routine to determine whether the overhead is worthwhile. In addition, a form of register access control is provided. All registers can be protected, in blocks of 4, from access. These features make the 29000 useful for embedded applications, which is where most of these processors are used, allowing it at one point to claim the title of 'the most popular RISC processor'. The 29000 also includes an MMU and support for the 29027 FPU. The superscalar 29050 version in 1990 integrated a redesigned FPU (4 instructions could be dispatched to execute out of order and speculatively).
In late 1995 Advanced Micro Devices dropped development of the 29K in favour of its more profitable clones of Intel 80x86 processors, although much of the development of the superscalar core for a new AMD 29000 (including FPU designs from the 29050) was shared with the 'K5' (1995) Pentium compatible processor (the 'K5' translates 80x86 instructions to RISC-like instructions, and dispatches up to five at once to two integer units, one FPU, a branch and a load/store unit).
The 80C166 has sixteen 16 bit registers, with the lower eight usable as sixteen 8 bit registers, which are stored in overlapping windows (like in the SPARC) in the on-chip RAM (or register bank), pointed to by the Context Pointer (CP) (similar to the SP in the AMD 29K). Unlike the SPARC, register windows can overlap by a variable amount (controlled by the CP), and the there are no spills or fills because the registers are considered part of the RAM address space (like in the TMS 9900), and could even extend to off chip RAM. This eliminates wasted registers of SPARC style windows.
Address space (18 to 24 bits) is segmented (64K code segments with a separate code segment register, 16K data segments with upper two bits of 16 bit address selecting one of four data segment registers).
The 80C166 has 32 bit instructions, while it's a 16 bit processor (compared to the Hitachi SH, which is a 32 bit CPU with 16 bit instructions). It uses a four stage pipeline, with a limited (one instruction) branch cache.
Like the AMD 29000 and DEC Alpha, the R2000 has no condition code register considering it a potential bottleneck. The PC is user readable. The CPU includes an MMU unit that can also control a cache, and the CPU was one of the first which could operate as a big or little endian processor. An FPU, the R2010, is also specified for the processor.
Newer versions included the R3000 (1988), with improved cache control, and the R4000 (1991) (expanded to 64 bits and is superpipelined (twice as many pipeline stages do less work at each stage, allowing a higher clock rate and twice as many instructions in the pipeline at once, at the expense of increased latency when the pipeline can't be filled, such as during a branch, (and requiring interlocks added between stages for compatibility, making the original "I" in the "MIPS" acronym meaningless))). The R4400 and above integrated the FPU with on-chip caches. The R4600 and later versions abandoned superpipelines.
The superscalar R8000 (1994) was optimised for floating point operation, issuing two integer or load/store operations (from four integer and two load/store units) and two floating point operations simultaneously (FP instructions sent to the independent R8010 floating point coprocessor (with its own set of thirty-two 64-bit registers and load/store queues)).
The R10000 and R12000 versions (early 1996 and May 1997) added multiple FPU units, as well as almost every advanced modern CPU feature, including separate 2-way I/D caches (32K each) plus on-chip secondary controller (and high speed 8-way split transaction bus (up to 8 transactions can be issued before the first completes)), superscalar execution (load four, dispatch five instructions (may be out of order) to any of two integer, two floating point, and one load/store units), dynamic register renaming (integer and floating point rename registers (thirty two in the R10K, fourty eight in the R12K)), and an instruction cache where instructions are partially decoded when loaded into the cache, simplifying the processor decode (and register rename/issue) stage. This technique was first implemented in the AT&T CRISP/Hobbit CPU, described later. Branch prediction and target caches are also included.
The 2-way (int/float) superscalar R5000 (January, 1996) was added to fill the gap between R4600 and R10000, without any fancy features (out of order or branch prediction buffers). For embedded applications, MIPS and LSI Logic added a compact 16 bit instruction set which can be mixed with the 32 bit set (same as the ARM Thumb 16 bit extension), implemented in a CPU called TinyRISC (October 1996), as well as MIPS V and MDMX (MIPS Digital Multimedia Extensions, announced October 1996)). MIPS V adds parallel floating point (two 32 bit fields in 64 bit registers) operations (compared to similar HP MAX integer extensions), MDMX adds integer 8 or 16 bit subwords in 64 bit FPU registers and a 24 and 48 bit subwords in a 192 bit accumulator for multimedia instructions (a MAC instruction on an 8-bit value can produce a 24-bit result, hence the large accumulator). Vector-scalar operations (ex: multiply all subwords in a register by subword 3 from another register) are also supported. These extensive instructions are partly derived from Cray vector instructions (Cray is owned by SGI, the parent company of MIPS). Future versions are expected to add Java virtual machine support.
Much of the RISC philosophy was independently invented at HP from lessons learned from FOCUS (pre 1984), HP's (and the world's) first fully 32 bit microprocessor. It was a huge (at the time) 450,000 transistor chip with a stack based instruction set, described as "essentially a gigantic microcode ROM with a simple 32 bit data path bolted to its side". Performance wasn't spectacular, but it was used in a pre-Unix workstation from HP.
It's almost the cannonical load-store design, similar except in details to most other mainstream load-store processors like the Fairchild/Intergraph Clipper (1986), and the Motorola 88K in particular. It has a 5 stage pipeline, which (unlike early MIPS (R2000) processors) had hardware interlocks from the beginning for instructions which take more than one cycle, as well as result forwarding (a result can be used by a previous instruction without waiting for it to be stored in a register first).
It is a load/story architecture, originally with a single instruction/data bus, later expanded to a Harvard architecture (separate instruction and data buses). It has thirty-two 32-bit integer registers (GR0 wired to constant 0, GR31 used as a link register for procedure calls), with seven 'shadow registers' which preserve the contents of a subset of the GR set during fast interrupts (also like ARM), and thirty-two 64-bit floating point registers (also as sixty-four 32-bit and sixteen 128-bit), in an FPU (which could execute a floating point instruction simultaneously, from the Apollo-designed Prism architecture (1988?) after Hewlett-Packard acquired the company). Later versions (the PA-RISC 7200 in 1994) added a second integer unit (still dispatching only two instructions at a time to any of the three units). Addressing originally was 48 bits, and expanded to 64 bits, using a segmented addressing scheme.
The PA-RISC 7200 also included a tightly integrated cache and MMU, a high speed 64-bit 'Runway' bus, and a fast but complex fully associative 2KB on-chip assist cache, between the simpler direct-mapped data cache and main memory, which reduces thrashing (repeatedly loading the same cache line) when two memory addresses are aliased (mapped to the same cache line). Instructions are predecoded into a separate instruction cache (like the AT&T CRISP/Hobbit).
The PA-RISC 8000 (April 1996), intended to compete with the R10000, UltraSparc, and others) expands the registers and architecture to 64 bits (eliminating the need for segments), and adds aggressive superscalar design - up to 5 instructions out of order, using fifty six rename registers, to ten units (five pairs of: ALU, shift/merge, FPU mult/add, divide/sqrt, load/store). The CPU is split in two, with load/store (high latency) instructions dispatched from a separate queue from operations (except for branch or read/modify/write instructions, which are copied to both queues). It also has a deep pipeline and speculative execution of branches (many of the same features as the R10000, in a very elegant implementation).
The PA-RISC 8500 breaks with HP tradition (in a big way) and adds on-chip cache - 1.5Mb L1 cache.
Although typically sporting fewer of the advanced (and promised) features of competing CPUs designs, a simple elegant design and effective instruction set has kept PA-RISC performance among the best in its class (of those actually available at the same time) since its introduction.
HP pioneered the addition of multimedia instructions with the MAX-1 (Multimedia Acceleration eXtension) extensions in the PA-7100LC (pre-1994) and 64-bit (version 2.0) MAX-2 extensions in the PA-8000, which allowed vector operations on two or four 16-bit subwords in 32-bit or 64-bit integer registers (this only required circuitry to slice the integer ALU (similar to bit-slice processors, such as the AMD 2901), adding only 0.1 percent to the PA-8000 CPU area - using the FPU registers like Sun's VIS and Intels MMX do would have required duplicating ALU functions. 8 and 32-bit support, multiplication, and complex instructions were also left out in favour of powerful 'mix' and 'permute' packing/unpacking operations).
In the future Hewlett-Packard plans to pursue a "post-VLIW" (Very Long Instruction Word) design in conjunction with Intel code named "Merced" or IA-64, possibly expanding on the idea of MAX or MMX operations. Some of the newer CPUs which execute Intel 80x86 instructions (The AMD 'K5' and NexGen Nx586, for example) treat 80x86 instructions as VLIW instructions, decoding them into RISC-like instructions and executing several concurrently. However, it would more likely follow the VLIW design of the TI 320C6x DSP.
The 88000 has thirty-two 32 bit user registers, with up to 8 distinct internal function units - an ALU and a floating point unit (sharing the single register set) in the 88100 version, multiple ALU and FPU units (with thirty-two 80-bit FPU registers) and two-issue instuctions were added to the 88110 to produce one of the first superscalar designs (following the 320xx Swordfish). Other units could be designed and added to produce custom designs for customers, and the 88110 added a graphics/bit unit which pack or unpack 4, 8 or 16-bit integers (pixels) within 32-bit words, and multiply packed bytes by an 8-bit value. But it was introduced late and never became as popular in major systems as the MIPS or HP processors. Development (and performance) has lagged as Motorola favoured the PowerPC CPU, coproduced with IBM.
Like the most modern processors, the 88000 is pipelined (with interlocks), and has result forwarding (in the 88110 one ALU can feed a result directly into another for the next cycle). Loads and saves in the 88110 are buffered so the processor doesn't have to wait, except when loading from a memory location still waiting for a save to complete. The 88110 also has a history buffer for speculatively executing branches and to make interrupts 'precise' (they're imprecise in the 88100). The history buffer is used to 'undo' the results of speculative execution or to restore the processor to 'state' when the interrupt occurred - a 1 cycle penalty, as opposed to 'register renaming' which buffers results in another register and either discards or saves it as needed, without penalty.
The C100 was a three-chip set like the Motorola 88000 (but predating it by two years), with a Harvard architecture CPU and separate MMU/cache chips for instruction and data. It differed from the 88K and HP PA-RISC in having sixteen 32-bit user registers and eight 64-bit FPU registers, rather than the more common thirty-two, and 16 and 32 bit instruction lengths.
The only other distinguishing features of the Clipper are a bank of sixteen supervisor registers which completely replace the user registers, (the ARM replaces half the user registers on an FIRQ interrupt) and the addition of some microcode instructions like in the Intel i960.
The original ARM (ARM1, 2 and 3) was a 32 bit CPU, but used 26 bit addressing. The newer ARM6xx spec is completely 32 bits. It has user, supervisor, and various interrupt modes (including 26 bit modes for ARM2 compatibility). The ARM architecture has sixteen registers (including user visible PC as R15) with a multiple load/save instruction, though many registers are shadowed in interrupt modes (2 in supervisor and IRQ, 7 in FIRQ) so need not be saved, for fast response. The instruction set is reminiscent of the 6502, used in Acorns earlier computers.
A feature introduced by the ARM is that every instruction is predicated, using a 4 bit condition code (including 'never execute', not officially recommended), an idea later used in some HP PA-RISC instructions and the TI 320C6x DSP. Another bit indicates whether the instruction should set condition codes, so intervening instructions don't change them. This easily eliminates many branches and can speed execution. Another unique and useful feature is a barrel shifter which operates on the second operand of most ALU operations, allowing shifts to be combined with most operations (and index registers for addressing), effectively combining two or more instructions into one.
These features make ARM code both dense (unlike most load-store processors) and efficient, despite the relatively low clock rate and short pipeline - it is roughly equivalent to a much more complex 80486 in speed. And like the Motorola Coldfire, ARM has developed a low cost 16-bit version called Thumb, which recodes a subset of ARM CPU instructions into 16 bits (decoded to native 32-bit ARM instructions without penalty - similar to the CISC decoders in the newest 80x86 compatible and 68060 processors, except they decode native instructions into a newer one, while Thumb does the reverse). Thumb programs can be 30-40% smaller than already dense ARM programs. Native ARM code can be mixed with Thumb code when the full instruction set is needed.
The ARM series consists of the ARM6 CPU core (35,000 transistors, which can be used as the basis for a custom CPU) the ARM60 base CPU, and the ARM600 which also includes 4K 64-way set-associative cache, MMU, write buffer, and coprocessor interface (for FPU). A newer version, the ARM7 series (Dec 1994), increases performance by optimising the multiplier, and adding DSP extensions including 32 bit and 64 bit multiply and multiply/accumulate instructions (operand data paths lead from registers through the multiplier, then the shifter (one operand), and then to the integer ALU for up to three independent operations). It also doubles cache size to 8K, includes embedded In Circuit Emulator (ICE) support, and raises the clock rate significantly.
A full DSP coprocessor (codenamed Piccolo, expected second half 1997) adds an independent set of sixteen 32-bit registers (also accessable as thirty two 16 bit registers), four which can be used as 48 bit registers, and a complete DSP instruction set (including four level zero-overhead loop operations), using a load-store model similar to the ARM itself. The coprocessor has its own program counter, interacting with the CPU which performs data load/store through input/output buffers connected to the coprocessor bus (similar but more intelligent than the address unit in a typical DSP (such as the Motorola 56K) supporting the data unit). The coprocessor shares the main ARM bus, but uses a separate instruction buffer to reduce conflict. Two 16 bit values packed in 32 bit registers can be computed in parallel, similar to the HP PA-RISC MAX-1 multimedia instructions.
The ARM CPU was chosen for the Apple Newton handheld system because of its speed, combined with the low power consumption, low cost and customizable design (the ARM610 version used by Apple includes a custom MMU supporting object oriented protection and access to memory for the Newton's NewtOS). DEC has also licensed the architecture, and has developed the SA-110 (StrongARM) (February 1996), running a 5-stage pipeline at 100 to 233MHz (using only 1 watt of power), with 5-port register file, faster multiplier, single cycle shift-add, and Harvard architecture (16K each 32-way I/D caches). To fill the gap between ARM7 and DEC StrongARM, ARM also developed the ARM8/800 which includes many StrongARM features, and the ARM9 with Harvard busses, write buffers, and flexible memory protection mapping.
An experimental asynchronous version of the ARM6 (operates without an external or internal clock signal) called AMULET has been produced by Steve Furber's research group at Manchester university. The first version (AMULET1, early 1993) is about 70% the speed of a 20MHz ARM6 on average (using the same fabrication process), but simple operations (multiplication is a big win at up to 3 times the speed) are faster (since they don't need to wait for a clock signal to complete). AMULET2e (October 1996, 93K transistor AMULET2 core plus four 1K fully associative cache blocks) is 30% faster (40 MIPS, 1/2 the performance of a 75MHz ARM810 using same fabrication), uses less power, and includes features such as branch prediction. AMULET3 is expected to be a commercial product in 1999.
The Hitachi SH series was meant to replace the 8-bit and 16-bit H8 microcontrollers, a series of PDP-11-like (or National Semiconductor 32032/32016-like) memory-data CPUs with sixteen 16-bit registers (eight in the H8/300), usable as sixteen 8-bit or combined as eight 32-bit registers (for addressing, except H8/300), with many memory-oriented addressing modes. The SH is also designed for the embedded marked, and is similar to the ARM architecture in many ways. It's a 32 bit processor, but with a 16 bit instruction format (different than Thumb, which is a 16 bit encoding of a subset of ARM 32 bit instructions, or the NEC V800 series, which mixes 16 and 32 bit instruction formats), and has sixteen general purpose registers and a load/store architecture (again, like ARM). This results in a very high code density, similar to the 680x0 and 80x86 CPUs, and about half that of the PowerPC. Because of the small instruction size, there are no immediate load instruction, but a PC-relative addressing mode is supported to load 32 bit values (unlike ARM or PDP-11, the PC is not otherwise visible). The SH also has a Multiply ACcumulate (MAC) instruction, and MACH/L (high/low word) result registers - 42 bit results (32 low, 10 high) in the SH1, 64 bit results (both 32 bit) in the SH2 and later. The SH3 includes an MMU and 2K to 8K of unified cache. A 2-way 64-bit superscalar SH4 is being developed (target of 360 MIPS).
The SH is used in many of Hitachi's own products, as well as being a pioneer of wide popularity for a Japanese CPU outside of Japan. It's most prominently featured in the Sega Saturn video game system (which uses two SH2 CPUs) and many Windows CE palmtop computers (SH3 chip set).
Although the first implementation was not superscalar, the 960 was designed to allow dispatching of instructions to multiple (undefined, but generally including at least one integer) execution units, which could include internal registers (such as the four 80 bit registers in the floating point unit (32, 64, and 80 bit IEEE operations)) - the 960 CA version (1989) was superscalar. There are sixteen 32 bit global registers which can be shared by all excution units and sixteen register "caches" - similar to the SPARC register windows, but not overlapping (originally four banks). It's a load/store Harvard architecture (32-bit flat addressing), but has some complex microcoded instructions (such as CALL/RET). There are also thirty-two 32 bit special function registers.
It's a very clean embedded architecture, not designed for high level applications, but very effective and scalable - something that can't be said for all Intel's processor designs.
The 860 has several modes, from regular scaler mode to a superscalar mode that executes two instructions per cycle and a user visible pipeline mode (instructions using the result register of a multi-cycle op would take the current value instead of stalling and waiting for the result). It can use the 8K data cache in a limited way as a small vector register (like those in supercomputers). The unusual cache uses virtual addresses, instead of physical, so the cache has to be flushed any time the page tables changes, even if the data is unchanged. Instruction and data busses are separate, with 4 G of memory, using segments. It also includes a Memory Management Unit for virtual storage.
The 860 has thirty two 32 bit registers and thirty two 32 bit (or sixteen 64 bit) floating point registers. It was one of the first microprocessors to contains not only an FPU as well as an integer ALU, and also included a 3-D graphics unit (attached to the FPU) that supports lines drawing, Gouraud shading, Z-buffering for hidden line removal, and operations in conjunction with the FPU. It was also the first able to do an integer operation, and a (unique at the time) multiply and add floating point instruction, for the equivalent of three instructions, at the same time.
However actually getting the chip at top speed usually requires using assembly language - using standard compilers gives it a speed closer to other processors. Because of this, it was used as a coprocessor, either for graphics, or floating point acceleration, like add in parallel units for workstations. Another problem with using the Intel 860 as a general purpose CPU is the difficulty handling interrupts. It is extensively pipelined, having as many as four pipes operating at once, and when an interrupt occurs, the pipes can spill and lose data unless complex code is used to clean up. Delays range from 62 cycles (best case) to 50 microseconds (almost 2000 cycles).
The branch unit is the heart of the CPU, and enables multiple instructions (up to four in the original POWER1, more commonly two or three) to be executed at once. It contains the condition code register, a loop register (can decrement and branch on zero with no penalty - a feature usually only found on DSPs like the TMS320C30), and performs branches. The condition code register has eight fields - in POWER1 two were reserved for the fixed and floating point units, the other six could be set separately (or combined from several instructions), and can be checked several instructions later. It also dispatches multiple instructions (out of order if possible) to available execution units (each unit has a buffer allowing instructions to be dispatched to a unit still executing a complex instruction).
The branch unit can speculatively take branches (using a prediction bit in the POWER1 and PowerPC 601 (1993), and using dynamic prediction and a Branch History Table in the PowerPC 604 (mid 1995) and newer versions), dispatching instructions and then canceling them if the branch is not taken (3 cycle maximum penalty) while buffering the other instruction path to reduce latency. The branch unit also manages procedure calls and returns on a program counter stack, allowing effective zero-cycle calls when overlapped with other instructions. Finally, it handles interrupts (except floating point exceptions) without software intervention.
The integer unit(s) perform integer operations, as well as some complex string instructions in the POWER1 and 2 and 64-bit PowerPC-AS (AS/400, mid 1996), and loads and stores in the POWER1 and PowerPC 601 (newer versions added a separate concurrent load/store unit) - including 'update' forms of load/stores (similar to post-increment/decrement memory-data addressing modes). Most versions contain thirty two 32 bit registers (the POWER1/2 design included a special MQ register for extended precision integer multiply/divides (like the MIPS HI/LO registers), but it was removed in PowerPC CPUs after the 601 as a potential bottleneck), while the PowerPC 620 (delivered late, April 1996, then withdrawn for redesign) and AS registers are 64 bits (with appropriate new instructions). R0 is sometimes treated as a constant 0, like other load-store processors, but can also contain a value for other operations. The high end PowerPC-AS, intended for the AS/400 minicomputer series, also has decimal arithmetic and string instructions (but only for 64-bit instructions), and an interface for a matrix coprocessor (for possible future RS/6000 workstations). All integer units can forward results needed by subsequent instructions before the write stage occurs, and some versions (PowerPC 603 and later) include extra registers which are renamed for speculative or out of order instruction execution to prevent write conflicts, and make it easier to discard the results of a canceled instruction. A reorder buffer in the branch/dispatch unit tracks renamed integer and floating point registers.
The floating point unit contains thirty two 64 bit registers and performs all typical floating point operations (single or double precision in PowerPC, double only in POWER1 and POWER2), including DSP-like multiply/accumulate instructions and array multiply and add. The registers are loaded and stored by the fixed point unit in POWER1 and PowerPC 601, by the Load/Store unit in others (the multichip POWER2 has two dedicated floating point load/store units). The FPU also includes rename registers. Like some other CPUs, floating point traps are imprecise because of pipelining - normally, a trap bit is set on a floating point exception, and software can test for the condition to generate a trap - or ignore it if its a safe operation. For debugging, a slower precise trap mode is included.
Data buses range from 32 bits for early and low end versions to 256 bits (plus ECC bits) for the high bandwidth POWER2 multichip CPU (late 1993 - 8 chips, 23 million transistors including 256K cache, later combined into one chip, and then into 8 CPUs+cache on one chip) which issues up to six instructions and four simultaneous loads or stores (the POWER2 was later superceded by the lower cost PowerPC based POWER3). The PowerPC 601 used the Motorola 88000 microprocessor bus, more recent versions use PowerPC specific buses, some with a 128 bit 'backside' bus (620 and later versions) used to access a L2 cache.
Very high clock rate (500MHz) BiCMOS 704 (based on a simplified 604 with only one integer, one FPU, one load/store unit) was being developed (expected early 1997) by Exponential Technology, expanding on the type of technology Intel found necessary to keep its Pentium and Pentium Pro CPUs competitive, but advances in CMOS and a slower initial product (410MHz) sharply reduced the clock speed advantages, cancelling the project (faster, lower power, fully CMOS Pentium and Pentium Pro CPUs have replaced earlier BiCMOS versions). Embedded versions have also been introduced by IBM (40x series) and Motorola (8xx and 50x series - ironically, Motorola now has a MPC860 CPU (on-chip communications support) with a viable future, in contrast to Intel's i860).
In direct response to Intel's MMX instructions, VMX (Video and Multimedia eXtension) are planned to be added to the fourth (G4) and possibly some third (G3) generation PowerPC CPUs. This consists of extending FPU registers to 128 bits, and adding SPARC-type VIS vector/multimedia operations (8, 16, or 32 bit subwords).
Overall the IBM POWER/PowerPC CPU is very powerful, reminiscent of mainframe designs, which almost qualifies it as "Weird and Innovative", and violates the RISC philosophy of simplicity and fewer instructions (at over a hundred (including identical pairs where one implicitly sets the CC registers and the other doesn't), versus only about 34 for the ARM and 52 for the Motorola 88000 (including FPU instructions)). The high complexity is very effective, but also limits the clock rate of the designs - an interesting tradeoff considering that a highly parallel 71.5 MHz POWER2 is faster than a 200MHz DEC Alpha 21064.
Alpha is a 64 bit architecture (32 bit instructions) that doesn't support 8- or 16-bit operations, but allows conversions, so no functionality is lost (Most processors of this generation are similar, but have instructions with implicit conversions). Alpha 32-bit operations differ from 64 bit only in overflow detection. Alpha does not provide a divide instruction due to difficulty in pipelining it. It's very much like the MIPS R2000, including use of general registers to hold condition codes. However, Alpha has an interlocked pipeline, so no special multiply/divide registers are needed, and Alpha is meant to avoid the significant growth in complexity which the R2000 family experienced as it evolved into the R8000 and R10000.
One of Alpha's roles is to replace DEC's two prior architectures - the MIPS-based workstations and VAX minicomputers (Alpha evolved from a VAX replacment project codenamed PRISM, not to be confused with the Apollo Prism acquired by Hewlett Packard). To do this, the chip provides both IEEE and VAX 32 and 64 bit floating point operations, and features Privileged Architecture Library (PAL) calls, a set of programmable (non-interruptable) macros written in the Alpha instruction set, similar to the programmable microcode of the Western Digital MCP-1600 or the AMD Am2910 CPUs, to simplify conversion from other instruction sets using a binary translator, as well as providing flexible support for a variety of operating systems.
Alpha was also designed for the future for a 1000-fold eventual increase in performance (10 X by clock rate, 10 X by superscalar execution, and 10 X by multiprocessing) Because of this, superscalar instructions may be reordered, and trap conditions are imprecise (like in the 88100). Special instructions (memory and trap barriers) are available to syncronise both occurrences when needed (different from the POWER use of a trap condition bit which is explicitly by software, but similar in effect. SPARC also has a specification for similar barrier instructions). And there are no branch delay slots like in the R2000, since they produce scheduling problems in superscalar execution, and compatibility problems with extended pipelines. Instead speculative execution (branch instructions include hint bits) and a branch cache are used.
The 21064 was introduced with one integer, one floating point, and one load/store unit. The 21164 (Early 1995) added one integer/load/store unit with byte vector (multimedia-type) instructions (replacing the load/store unit) and one floating point unit, and increased clock speed from 200 MHz to 300 MHz (still roughly twice that of competing CPUs), and introduced the idea of a level 2 cache on chip (8K each inst/data level 1, 96K combined level 2). The 21264 (early 1997) expanded this to four integer units (two add/logic/shift/branch (one also with multiply, one with multimedia) and two add/logic/load/store), two different floating point units (one for add/div/square root and for multiply), with the ability to load four, dispatch six, and retire eight instructions per cycle (and for the first time including 40 integer and 40 floating point rename registers and out of order execution), at up to 500MHz.
Multimedia extensions introduced with the 21264 are simple, but include VIS-type motion estimation (MPEG).
DEC's Alpha is in many ways the antithesis of IBM's POWER design, which gains performance from complexity, and the expense of a large transistor count, while the Alpha concentrates on the original RISC idea of simplicity and a higher clock rate - though that also has its drawback, in terms of very high power consumption.
The GDP was exclusively object oriented - normal linear memory access wasn't allowed, and there was hardware support for data hiding, methods, inheritance, late binding, and access protection, and it was promoted as being ideal for the Ada programming language. To enforce this, permission and type checks for every memory access (via a 2 stage segmentation) slowed execution (despite cached segment tables). It supported up to 2^24 segments, each limited to 64K in size (within a 2^32 address space), but the object oriented nature of the design meant that was not a real limitation. The stack oriented design meant the GDP had no user data registers. Instructions were bit encoded (and bit-aligned in memory), ranging from 6 bits to 321 bits long (the T-9000 has variable length byte encoded/aligned instructions) and could be very complex.
The BIU defined the bus, designed for multiprocessor support allowing up to 63 modules (BIU or MCU) on a bus and up to 8 independent buses (allowing memory interleaving to speed access). The MCU did automatic parity checking and ECC error correcting. The total system was designed to be fault tolerant to a large degree, and each of these parts contributes to that reliability.
Despite these advanced features, the 432 didn't catch on. The main reason was that it was slow, sometimes up to five or ten times slower than a 68000 or Intel's own 80286. Part of this was the lack of local (user) data registers, or a data cache. Part of this was the fault-tolerant BIU, which defined an (asynchronous protocol) clocked bus that resulted in 25% to 40% of the access time being used by wait states. The instructions weren't aligned on bytes or words, and took longer to decode. In addition, the protections imposed on the objects slowed data access. Finally, the implementation of the GDP on two chips instead of one produced a slower product. However, the fact that this complex design was produced and bug free is impressive.
Its high level architecture was similar to the Transputer systems, but it was implemented in a way that was much slower than other processors, while the T-414 wasn't just innovative, but much faster than other processors of the time.
This is not the only processor designed specifically for a language that is slow on other CPUs. Several specialized LISP processors, such as the Scheme-79 lisp processor, were created, but this chip is unique in its object oriented features. It also manages to support objects without the slowness of the Intel 432.
The Rekursiv processor features a writable instruction set, and is highly parallel. It uses 40 bits for objects, and 24 bit addressing, kind of. Memory can't be addressed directly, only through the object identifiers (segments), which are 40 bit tags. The hardware handles all objects in memory and on disk, and swapping them to disk. It has no real program - all data and code/methods are embedded in the objects, and loaded when a message is sent to them. There is a page table which stores the object tags and maps them into memory.
There is a 64k area, arranged as 16k X 128 bit words, for microcode, allowing an instruction set to be constructed on the fly. It can change for different objects.
The CPU hardware creates, loads, saves, destroys, and manipulates objects. The manipulation is accomplished with a standard AMD 29203 CPU, but the other parts are specially designed. It executes LINGO entirely fast enough, and is a perfect match between language and CPU, but it can execute more conventional languages, such as Smalltalk or C if needed - possible simultaneously, as separate complete objects.
Unfortunately, Linn did not have the resources to pursue this very promising (the prototype was "surprisingly easy" to implement) architecture.
The 320C30 is a 32 bit floating point DSP, based on the earlier 320C20/10 16 bit fixed point DSPs (1982). It has eight 40 bit extended precision registers R0 to R7 (32 bits plus 8 guard bits for floating, 32 bits for fixed), eight 32 bit auxiliary registers AR0 to AR7 (used for pointers) with two separate arithmetic units for address calculation, and twelve 32 bit control registers (including status, an index register, stack, interrupt mask, and repeat block loop registers).
It includes on chip memory in the form of one 4K ROM block, and two 1K RAM blocks - each bus has its own bus, for a total of three (compared to one instruction and one data bus in a Harvard architecture), which essentially function as programer controlled caches. Two arguments to the ALU can be from memory or registers, and the result is written to a register, through a 4 stage pipeline.
The ALU, address controller and control logic are separate - much clearer in the AT&T DSP32, ADSP 2100 and Motorola 56000 designs, and is even reflected in the MIPS R8000 processor FPU and IBM POWER architecture with its Branch Unit loop counter. The idea is to allow the separate parts to operate as independently as possible (for example, a memory access, pointer increment, and ALU operation), for the highest throughput, so instructions accessing loop and condition registers don't take the same path as data processing instructions.
The TMS320Cx0 series also includes the 320C8x (1994?), which can have two or four DSP cores on a single chip as well as a load-store CPU (thirty two 32-bit regs, load/store, plus FPU) for control.
The 320C6x (February 1997) expands this idea by integrating functional units more tightly with the 320C8x load-store core in variable length instruction groups (VLIG) - up to eight instructions can be dispatched (scheduled at compile time) to eight functional units (two multipliers and six DSP-type ALUs). Replacing hardware with compiler scheduling (some simple hardware support, in that all instructions are predicated like the ARM) requires only 550,000 transistors for the 320C6201, while improving performance by about 10 times, based on a FFT benchmark.
In fact, it's almost a completely different processor.
Like the TMS320C30, the 96002 has a separate program memory (RAM in this case, with a bootstrap ROM used to load the initial external program) and two blocks of data RAM, each with a separate data and address busses. The data blocks can also be switched to ROM blocks (such as sine and cosine tables). There's also a data bus for access to external memory. Separate units work independently, with their own registers (generally organised as three 32 bit parts of a single 96 bit register in the 96002 (where the '96' comes from).
The program control unit has a register containing 32 bit PC, status, and operating mode registers, plus 32 bit loop address and 32 bit loop counter registers (branches are 2 cycles, conditional branches are 3 cycles - with conditional execution support), and a fifteen element 64 bit stack (with separate 6 bit stack pointer).
The address generation unit has seven 96 bit registers, divided into three 32 bit (24 in the 56000/1) registers - R0-R7 address, N0-N7 offset, and M0-M7 modify (containing increment values) registers.
The Data Unit includes ten 96-bit floating point/integer registers, grouped as two 96 bit accumulators (A and B = three 32 bit registers each: A2, A1, A0 and B2, B1, B0) and two 64 bit input registers (X and Y = two 32 bit registers each: X1, X0 and Y1, Y0). Input registers are general purpose, but allow new operands to be loaded for the next instruction while the current contents are being used (accumulators are 8+24+24 = 56 bit in the 56000/1, where the '56' comes from). The DSP96000 was one of the first to perform fully IEEE floating point compliant operations.
The processor is not pipelined, but designed for single cycle independent execution within each unit (actually this could be considered a three stage pipeline). With multiple units and the large number of registers, it can perform a floating point multiply, add and subtract while loading two registers, performing a DMA transfer, and four address calculations within a two clock tick processor cycle, at peak speeds.
It's very similar to the Analog Devices ADSP2100 series - the latter has two address units, but replaces the separate data unit with three execution units (ALU, a multiplier, and a barrel shifter).
The DSP56K and 680xx CPUs have been combined in one package (similar idea as the TMS320C8x) in the Motorola 68456.
The DSP56K was part of the ill-fated NeXT system, as well as the lesser known Atari Falcon (still made in low volumes for music buffs).
The M17 CPU wasn't the first Forth microprocessor (the Novix NC4000/4016 (1985?) came before), but the M17 is a good example of low cost Forth CPUs. It featured two 16 bit stack pointers (Data and Return (subroutine) stacks), plus three 16-bit top of stack data registers (X, Y, Z, plus an extra LastX which could hold values popped from X). An I/O register buffered data during I/O while the ALU operated concurrently. Finally, there was an Index register which normally held the top element of the Return stack, but could also be used as a loop counter, and a 6 instruction buffer (for short loops, like the Motorola 68010).
Address space was 64K, but external memory could be either a single bank or up to five banks, signaled by status pins, depending on the context - data stack, return stack, program code, A or B buffers. Some other Forth processors include on chip stack memory, and while most (including the M17) were 16 bit, some 32 bit Forth processors have also been developed.
The simplicity of design allows the M17 (and most other Forth CPUs, such as the more recent 7,000 transistor MuP21, which includes a composite video generator on chip) to execute instructions in only two cycles (load, execute), or one cycle each from the instruction cache, making them faster than more complex CPUs (though instructions do less, the higher clock speed usually compensates). Stack advocates often cite this as the strongest advantage for stack based designs, though critics contend that the state nature of stacks compared to registers make conventional speedup tricks such as pipelining and superscalar execution far more complex than using a register array. As it is, register-based load-store processors dominate when it comes to speed.
Instead of registers, a thirty-two entry 32 bit two ported stack cache is provided. This is similar to the stack cache of the AMD 29000 (in Hobbit it's much smaller (64 32-bit words) but is easily expandable), and Hobbit has no global registers. Addresses can be memory direct or indirect (for pointers) relative to the stack pointer without extra instructions or operand bits. The cache is not optimised for multiprocessors.
Hobbit has an instruction prefetch buffer (3K in 92010, 6K in the 92020), like the 8086, but decodes the variable length (1, 3 or 5 halfword (16 bit)) instructions into a thirty-two entry instruction cache. Branches are not delayed, and a prediction bit directs speculative branch execution. The decode unit folds branches into the decoded instructions (which include next and alternate next PC), so a predicted branch does not take any clock cycles. The three stage execution unit takes instructions from the decode cache. Results can be forwarded when available to any prior stage as needed.
Though CISC in philosophy, the Hobbit is greatly simplified compared to traditional memory-data designs, and features some very elegant design features. AT&T prefers to call it a RISC processor, and performance is comparable to similar load-store designs such as the ARM. Its most prominent use was in the EO Personal Communicator, a competitor to Apple's Newton which uses the ARM processor.
While the transputers were originally faster than their contemporaries, recent load-store designs have surpassed them. The T-9000 was an attempt to regain the lead. It starts with the architecture of the T-800 which contains only three 32 bit integer and three 64 bit floating point registers which are used as an evaluation stack - they are not general purpose. Instead, like the TMS 9900, it uses memory, addressed relative to the workspace register (the 9900 workspace contained only sixteen registers, the Transputer workspace can be any length, though access slows down with every 4 bits used for offset from the workspace register - sixteen bytes can be accessed with just one instruction, 256 needs two, and so on). This allows very fast context switching, less than a microsecond, speeding and simplifying process scheduling enough that it is automated in hardware (supporting two priority levels and event handling (link messages and interrupts)). The Intel 432 also attempted some hardware process scheduling, but was unsuccessful.
Unlike the TMS 9900, the T-9000 is far faster than memory, so the CPU has several levels of high speed caches and memory types. The main cache is 16K, and is designed for 3 reads and 1 write simultaneously. The workspace cache is based on 32 word rotating buffers, allows 2 reads and 1 write simultaneously.
Instructions are in bytes, consisting of 4 bit op code and 4 bit data (usually a 16 byte offset into the workspace), but prefix instructions can load extra data for an instruction which follows, 4 bits at a time. Less frequent instructions can be encoded with 2 (such as process start, message I/O) or more bytes (CRC calculations, floating point operations, 2D block copies and scheduler queue management). The stack architecture makes instructions very compact, but executing one instruction byte per clock can be slow for multibyte instructions, so the T-9000 has a grouper which gathers instruction bytes (up to eight) into a single CISC-type instruction then sent into the 5 stage pipeline (fetching four per cycle, grouping up to 8 if slow earlier instructions allow it to catch up). For example, two concurrent memory loads (simple or indexed), a stack/ALU operation and a store (a[i] = b + c) can be grouped.
The T-9000 contains 4 main internal units, the CPU, the VCP (handling the individual links of the previous chips, which needed software for communication), the PMI, which manages memory, and the Scheduler.
This processor is ideal for a model of parallel processing known as systolic arrays (a pipeline is a simple example). Even larger networks can be created with the C104 crossbar switch, which can connect 32 transputers or other C104 switches into a network hundreds of thousands of processors large. The C104 acts like a instant switch, not a network node, so the message is passed through, not stored. Communication can be at close to the speed of direct memory access.
Like the many CPUs, the Transputers can adapt to a 64, 32, 16, or 8 bit bus. They can also feed off a 5 MHz clock, generating their own internal clock (up to 50MHz for the T-9000) from this signal, and contain internal RAM, making them good for high performance embedded applications.
Unfortunately excessive delays in the T-9000 design (partly because of the stack based design) left it uncompetitive with other CPUs (roughly 36 MIPS at 50 MHz). The T-4xx and T-8xx architecture still exist in the SGS-Thomson ST20 microcore family.
At 100MHz, the microprocessing unit (MPU) executes about one instruction per cycle, without normal instruction/data caches. Byte instructions are loaded in groups of four (32 bits), and executed sequentially. The problem of loading constants is handled in a unique way. The 68000 and PDP-11 could load a constant stored in program memory following the current instruction, and the Hitachi SH uses a similar PC-relative mode to load constants. Processors like the Mips R3000 load half a constant at a time using two instructions. Transputers always contain 4 bits of data and 4 bits of op code in each byte instruction.
The ShBoom loads single bytes of data from the rightmost bytes of the current instruction group, and words from program memory following the current group. For example, a load byte instruction could be in position one, two or three from the left, the data would always be in the fourth (rightmost) byte. Four consecutive load word instructions would be grouped together, and the constants taken fromthe four 32 bit words following the group. This ensures data alignment without extra circuitry (but may get in the way in the future, such as for 64 bit versions).
There are sixteen 32 bit global registers (g0 to g15), a sixteen register local stack (r0 to r14 can be used as a stack frame (R15 is not user visible), or as a Forth return stack), and an eighteen element operand stack (s0 to s17, accessed only by data stack operations) - the stacks automatically spill and refill to and from memory, s0 and r0 can also be used as index registers, g0 is used for multiply and divide instructions. There's also an extra index register x, a loop counter ct, and a mode register (like a CC or PSW register).
The CPU also contains an I/O coprocessor on chip for simultanious I/O (much more advanced than the I/O buffer register of the M17, but the same idea), which communicates with the MPU via the global data registers. It's a simple, independent unit which executes small data transfer programs until I/O is complete. There are also a programmable memory interface, 8 channel DMA controller, and interrupt controller.
The ShBoom architecture is a very innovative and elegant attempt at combining stack and register oriented architectures, with emphasis on the stack operation simplicity. It would give Java a good home.
The picoJava I (early 1997) is a stack oriented CPU core like the JVM, with a 64 entry stack cache (similar to the Patriot Scientific ShBoom PSC1000), but there are interesting differences between it and FORTH-style stack CPUs. Java only uses a single stack (like many languages such as C, which the AT&T Hobbit and AMD 29K were designed to support) and the picoJava CPU enhances performance with a 'dribbler' unit which constantly updates a complete copy of the stack cache in memory, without affecting other CPU operations (similar to a write-back cache), so stack frames can be added without waiting for a stack frame to be stored. Some Java instructions are complex, so the CPU has microcoded instructions, and a 4 stage pipeline (fetch, decode, execute/cache, stack writeback). Finally, picoJava groups (or 'folds') load and stack operations together, executing both at once (treating the top of stack as an accumulator) (this is a much simpler version of instruction grouping tried in the Transputer T-9000), This usually eliminates 60% of stack operation inefficiency. Seldom used instructions aren't implemented, but are emulated using trap handlers.
The picoJava II (October 1997) core is used in the first actual CPU from Sun, the microJava701. It extends the pipeline to 6 stages, and can four up to four instructions into one operation. It also adds a FPU and separate 16Kb I/D caches.
Accumulator | Accumulator +Index / \ Stack Memory- (CISC) Data | Load- (RISC) StoreAt that time, RISC and "load-store" were often synonymous, but RISC usually referred to a list of features:
By contrast, the CISC philosophy has been that if added hardware can
result in an overall increase in speed, it's good - the ultimate goal
of mapping every high level language statement on to a single CPU
instruction. The disadvantage is that it's harder to increase the
clock speed of a complex chip. The is a
good example of this idea applied to a load-store architecture.
IBM System 360/370/390: The Mainframe(1964)
The IBM System/360 is a sort of geologic feature in the computer
world, and isn't at all a microprocessor, but was certainly
influential (and enough people asked for it to be included in this
was designed to be an "all around" (as in, 360 degrees) system usable
for any computing task, and as a result created many of the standards
for the computing industry, such as 8-bit bytes and byte addressable
memory, 32-bit words, segmented
and paged memory (see the DEC VAX, except that in the S/360
the PSW includes the program counter (24 bits in the S/360, 31 bits in
the S/370 XA (eXtended Architecture, pre 1983) and later versions). The
S/370 (pre 1977) also includes sixteen control registers used by the
A two stage pipeline was first introduced in the IBM 3033 (1977). Instructions are fetched from the cache into three 32 bit buffers. The Instruction Pre-Processing Function (IPPF) then decodes them, generates operand addresses and stores them in operand address registers, and places source operands in operand buffers. Decoded instructions were placed into a 4 entry queue until the execution unit was ready.
In some models (such as 360/91) when a conditional branch occurs, the most likely next instruction is loaded into the IPPF buffer, but the previous next instruction is not discarded, so either can be executed without penalty. Two speculative branches can be buffered this way. Some had a "loop mode" like the Motorola 68010.
Addressing was originally 24 bit, but was extended to 31 bits (the high bit indicated whether to use 24 or 32 bits) with the XA architecture (This caused problems with software which stored type information in the unused 8 bits of a 32 bit word. The same thing happened when the Motorola 68000 was expanded from 24 to 32 bit addressing). The S/360 used completely position independent (register+offset and register+index) addressing modes. Virtual memory was added in the S/370, and used a segment and paging method - the first 8 bits of an address indicated an entry in a segment table which is added to the next 4 or 8 bits to get the page table index which contains the upper (12 or 20) bits of the physical memory address, and the rest of the address provides the lower 12 bits (the Intel 80386 uses a similar method, while the Motorola 68030 uses fixed length logical/physical pages instead of variable length segments).
Like the DEC VAX, the S/370 has been implemented as a microprocessor. The Micro/370 discarded all but 102 instructions (some supervisor instructions differed), with a coprocessor providing support for 60 others, while the rest are emulated (as in the MicroVAX). The Micro/370 had a 68000 compatible bus, but was otherwise completely unique (some legends claim it was a 68000 with modified microcode plus a modified 8087 as the coprocessor, others say IBM started with the 68000 design and completely replaced most of the core, keeping the bus interface, ALU, and other reusable parts, which is more likely).
More recently, with increased microprocessor complexity, a complete S/390 superscalar microprocessor with 64K L1 cache (at up to 350MHz, a higher clock rate than the 200MHz Intel's Pentium Pro available at the time) has been designed.
The VAX was a 32 bit architecture, with a 32 bit address range (split into 1G sections for process space, process specific system space, system space, and unused/reserved for future use). Each process has its own 1G process and 1G process system address space, with memory allocated in pages.
It features sixteen user visible 32 bit registers. Registers 12 to 15 are special - AP (Argument Pointer), FP (Frame Pointer), SP and PC (user, supervisor, executive, and kernal modes have separate SPs in R14, like the 68000 user and supervisor modes). All these registers can be used for data, addressing and indexing. A 64 bit PSL (Program Status Longword) keeps track of interrupt levels, program status, condition codes, and access mode (kernal (hardware management), executive (files/records), supervisor (interpreters), user (programs/data)).
The VAX 11 featured an 8 byte instruction prefetch buffer, like the 8086, while the VAX 8600 has a full 6 stage pipeline. Instructions mimic high level language constructs, and provide dense code. For example, the CALL instruction, which not only handles the argument list itself, but enforces a standard procedure call for all compilers. However, the complex instructions aren't always the fastest way of doing things. For example, the INDEX instruction was 45% to 60% faster when by replaced by simpler VAX instructions. This was one inspiration for the RISC philosophy.
Further inspiration came from the MicroVAX (VAX 78032) implementation, since in order to reduce the architecture to a single (integer) chip, only 175 of the 304 instructions (and 6 of 14 native data types) were implemented (through microcode), while the rest were emulated - this subset included 98% of instructions in a typical program. The optional FPU implemented 70 instructions and 3 VAX data types, which was another 1.7% of VAX instructions. All remaining VAX instructions were only used 0.2% of the time, and this allowed MicroVAX designs to eventually exceed the speed of full VAX implementations, before being replaced by the Alpha architecture.
The CDC 6600 was a 60-bit machine ('bytes' were 6 bits each), with an 18-bit address range. It had eight 18 bit A and 18 bit B (address) and eight 60 bit X (data) registers, with useful side effects - loading an address into A1, A2, A3, A4 or A5 caused a load from memory at that address into registers X1, X2, X3, X4 or X5. Similarly, A6 and A7 registers had a store effect on X6 and X7 registers - loading an address into A0 had no side effects. As an example, to add two arrays into a third, the starting addresses of the source could be loaded into A2 and A3 causing data to load into X2 and X3, the values could be added to X6, and the destination address loaded into A6, causing the result to be stored in memory. Incrementing A2, A3, and A6 (after adding) would step through the array. Side effects such as this are decidedly anti-RISC, but very nifty. This vector-oriented philosophy is more directly expressed in later Cray computers.
Only one instruction could be issued per cycle, but multiple independent functional units in the CDC 6600 meant instruction execution in different units could overlap (a scoreboard register prevented instructions from issuing to a unit if the operands weren't available). The units weren't pipelined until the CDC 7600 (1969), at which point instructions could be issued without waiting for operands (they would wait for them in the functional unit if necessary). Compared to the variable instruction lengths of other machines, instructions were only 15 or 30 bits, packed within 30 bit half-words (a 30 bit instruction could not occupy the upper 15 bit "parcel" of one half-word and the lower 15 bits of the next, so the compiler would insert NOPs to align instructions) to simplify decoding (a RISC-like feature). Branches were 60-bit-word aligned. Like the DEC Alpha, there were no byte or character operations, until later versions added a CMU (Compare and Move Unit) for character, string and block operations.
The 801 had thirty two 32 bit registers, but no floating point unit/registers, and no separate user/supervisor mode, since it was an experimental system - security was enforced by the compiler. It implemented Harvard architecture with separate data and instruction caches, and had flexible addressing modes.
IBM tried to commercialise the 801 design starting in 1977 (before RISC workstations first became popular) with the ROMP CPU (Research OPD (Office Products Division) Mini Processor), 1986, first chips early as 1981) used in the PC/RT workstation, but it wasn't successful. Originally designed for wordprocessor systems, changes to reduce cost included eliminating the caches and Harvard architecture (but adding 40 bit virtual memory), reducing registers to sixteen, variable length (16/32 bit) instructions (to increase instruction density), and floating point support via an adaptor to an NS32081 FPU (later, a 68881 or 68882 were available). This allowed a small CPU, only 45,000 transistors, but an average instruction took around 3 cycles.
The 801 itself morphed into an I/O processor for the IBM 3090 mainframes
This wasn't the only innovative design developed by IBM which never saw daylight. Slightly earlier (around 1971?) the Advanced Computer System pioneered superscalar (seven issue) design, speculative execution, delayed condition codes, multithreading, imprecise traps and instruction streamed interrupts, and load/store buffers, plus compiler optimisation to support these features. It was expensive and incompatible with the System/360, so was not pursued, but many ideas did find its way into the expensive high end mainframes.
The Berkeley project also produced an instruction cache with some innovative features, such as instruction line prefetch that identified jump instructions, frequently used instructions compacted in memory and expanded upon cache load, multiple cache chips support, and bits to map out defective cache lines.
The Stanford MIPS project was the basis for the MIPS R2000, and like the case with Berkeley project, there are close similarities. MIPS stood for Microprocessor without Interlocked Pipeline Stages, using the compiler to eliminate register conflicts. Like the R2000, the MIPS had no condition code register, and a special HI/LO multiply and divide register pair.
Unlike the R2000, the MIPS had only 16 registers, and two delay slots for LOAD/STORE and branch instructions. The PC and last three PC values were tracked for exception handling. In addition, instructions were 'packed' (like the Berkeley RISC), in that many instructions specified two operations that were dispatched in consecutive cycles (not decoded by the cache). In this way, it was a 2 operation VLIW, but executed sequentially. User assembly language was translated to 'packed' format by the assembler.
Being experimental, there was no support for floating point operations.
SOAR (Smalltalk On A RISC) modified the RISC II design to support Smalltalk.
Complex/ Simple/ CISC____________________________________________________________RISC | 14500B* 4-bit | *Am2901 | *4004 | *4040 8-bit | 6800,650x *1802 | 8051* * *8008 * SC/MP | Z8 * * *F8 | F100-L* 8080/5 2650 | * *NOVA * *PIC16x | MCP1600* *Z-80 *6809 IMS6100 16-bit| *Z-280 *PDP11 80C166* *M17 | *8086 *TMS9900 | *Z8000 *65816 | *56002 | 32016* *68000 ACE HOBBIT Clipper R3000 32-bit|432  96002 *68020 * * * * *29000 * *ARM | * *VAX * 80486 68040 *PSC i960 *SPARC *SH | Z80000* * * TRON48 PA-RISC | PPro Pent* -- - About here, from left to right, the Swordfish and 68060.
-*------- * *88100 | * * --<860>-*--*----- * *88110 64-bit|Rekurs POWER PowerPC * CDC6600 *R4000 | 620* U-SPARC * *R8000 *Alpha | R10000
Okay, an explanation. Since this is only a 2-dimensional graph, and I want to get a lot more across than that allows, design features 'pull' a CPU along the RISC/CISC axis, and the complexity of the design (given the number of bits and other considerations) also tug it - thus the much of the POWER's RISC-ness is offset by its inherently complex (though effective) design. And it also depends on my mood that day - hey, it's ultimately subjective anyway.
Because virtual machines have to be mapped on to the widest range of hardware possible, they have to make as few assumptions as they can (such as number of CPU registers in particular). This is the main reason why most virtual machines are stack based designs - almost all processors can implement one or two stacks fairly easilly.
The inverse isn't true. Some programming languages are based entirely on stack operations (Forth), but most are based on stack frames (C, Pascal, and their common ancestor ALGOL), or patternless memory access (FORTRAN, Smalltalk). Forth processors are effective because of the simplicity which comes from eliminating non-Forth features, but implementing a stack frame can be a real headache.
The Forth virtual machine contains two stacks. The first is the data stack, which consists of 16 bit entries (double entries can hold 32 bit values). The second is the return stack, used to hold PC values during subroutines.
The Forth equivalent to an instruction is a 'word', and can either be a predefined operation, or a programmer defined word made up of a sequence of executable words (the Forth version of subroutines, similar to Smalltalk). Forth also allows a word to be deleted with the "forget" word, normally only used for interactive Forth development (the language INTERCAL also includes a FORGET statement, but it is used for more evil purposes). Operations typically pull operands from the stack and push the results back onto it, which reduces instruction size since operands don't need to be specified. A subroutine is called by pushing the operands and executing the subroutine word, which leaves the results in the stack.
Operations can be either 16 bit or 32 bit, but there are two cases where types can be mixed - mixed multiplication will multiply two 16 bit numbers and leave a 32 bit result on the stack, while mixed division will divide a 16 bit (top of stack) number into a 32 bit number, producing a 16 bit quotient and 16 bit remainder (note that these two operations are directly supported by the PDP-11 architecture). There are I/O instructions as well.
The Forth two-stack machine has been implemented in the M17 CPU, among many others. The Transputer is stack oriented to a lesser extent (single evaluation stack only), and provides direct memory access abilities (for stack frames and other structures) without penalty.
As for Forth, although it has dedicated advocates, it's explicit stack orientation and its lack of modularity limit the scale of Forth programs. One of the largest Forth efforts was an integrated operating system called Valdocs (Valuable Document System) on the Epson QX-10. The software remained buggy couldn't be updated quickly enough for the machine to remain competitive - although you could just as easilly blame the computer's Z-80 processor (since at the time the 8088 based IBM PC and 68000 based Apple Macintosh were being introduced) and difficulty in finding experienced Forth programmers. Whatever the cause, this soured the acceptance of Forth for large scale projects.
Pascal, like Algol and C, is a stack frame oriented language, and so the p-Machine is a stack oriented machine. Memory is arranged from the top down as follows: p-System operating system code, system stack (growing down), relocatable p-Code pool, system heap (growing up), a series of process stacks as needed (growing down), a series of global data segments, and the p-Machine interpreter. The code pool contains compiled procedure segments in a linked list. Segments can be swapped into and out of memory, and relocated - if the stack needs more space to grow, the highest code segment can be relocated below the code pool. Similarly if the heap needs more space, code segments can be relocated upwards, and if both stack and heap need memory, code segments can be swapped out of memory altogether.
The UCSD p_System used a 64K memory map (standard for microcomputers of the time), but could also keep code in a separate 64K bank, freeing up data memory. The p-System also defined terminal I/O, a simple file system, serial and printer I/O, and allowed other device drivers to be added like any other operating system. It included an interactive program development system (all written in Pascal).
Western Digital implemented the p-Machine in the WD9000 Pascal Microengine (1980), based on the WD MCP-1600 programmable processor.
The JVM contains a stack stack used for parameters and instruction operands as in Forth, and a 'vars' register which points to the memory segment containing any number of local variables (like the workspace register in the Transputers).
Data typing is strongly enforced - while in Forth pushing two integers on the stack and treating them as a double is allowed, the JVM prohibits this. Object oriented support is also defined in the JVM, but not the architectual mechanisms, so implementation can vary. Objects are dynamically linked and can be swapped in or out (similar to the UCSD p-Machine, but the p-Machine segments are not grouped like objects and methods, and must be part of the program being executed, while JVM objects can be linked from external sources at run time). The other main difference between the JVM and the p-Machine is that the JVM memory segments (heap (data) and method area (code)) are not tied to a memory map, but may be allocated any way the operating or run-time system supports. Apart from that, the concept and implementation are quite similar (including multitasking support).
The Java language relies heavily on garbage collection, which is accomplished using a background thread and is not part of the JVM itself.
One other thing about the Java Virtual Machine is that some versions need to run code of unknown reliability which has been transferred over networks, and so includes security features to prevent a program from unauthorised access to the computer that it's running on.
Sun intends to produced Java processors (starting with the picoJava CPU) to execute Java bytecode directly, faster than a virtual machine or recompiled code.
RISC processors use a load/store architecture instead - to add
memory to a register, it must be loaded into an intermediate register
Smaller caches are faster, so often a small level 1 cache is used, with a larger but slower level 2 cache supporting it. Level 3 caches can even be used in some cases.
Some cache controllers monitor the memory bus to detect when a
cached memory value has been modified by another CPU, or a peripheral.
Implementations generally use either 'horizontal' or 'vertical'
microcode, which differ mainly in number of bits. Microinstructions
include a condition code and jump address (jump if condition is true,
next instruction if false), and the operation to be performed.
In horizontal microcode, each operation bit triggers an individual
control line (simple CPU controller but large microcode storage), in
vertical microcode, the operation field is decoded to produce the
control signals (smaller microcode but more complex controller). Some
CPUs used a combination.
1: add r1,r2->r8 2: sub r8,r3->r3 3: add r4,r5->r8 4: sub r8,r6->r6
Instructions 1 and 3 can be executed in parallel if r8 is
renamed, and instructions 2 and 4 can then be
executed in parallel. Instruction 3 is executed before 2, out of the
order which they appear in the program.
The circutry required to keep track of renamed registers can be
NEW PRODUCTS FEATURE PRODUCT COMPUTER ON A CHIP Intel has introduced an integrated CPU complete with a 4-bit parallel adder, sixteen 4-bit registers, an accumula- tor and a push-down stack on one chip. It's one of a family of four new ICs which comprise the MCS-4 micro computer system--the first system to bring the power and flexibility of a dedicated general-purpose computer at low cost in as few as two dual in-line packages. MSC-4 systems provide complete computing and con- trol functions for test systems, data terminals, billing machines, measuring systems, numeric control systems and process control systems. The heart of any MSC-4 system is a Type 4004 CPU, which includes a set of 45 instructions. Adding one or more Type 4001 ROMs for program storage and data tables gives a fully functioning micro-programmed com- puter. Add Type 4002 RAMs for read-write memory and Type 4003 registers to expand the output ports. Using no circuitry other than ICs from this family of four, a system with 4096 8-bit bytes of ROM storage and 5120 bits of RAM storage can be created. For rapid turn-around or only a few systems, Intel's erasable and re-programmable ROM, Type 1701, may be substituted for the Type 4001 mask-programmed ROM. MCS-4 systems interface easily with switches, key- boards, displays, teletypewriters, printers, readers, A-D converters and other popular peripherals. For further information, circle the reader service card 87 or call Intel at (408) 246-7501. Circle 87 on Reader Service Card COMPUTER/JANUARY/FEBRUARY 1972/71There was also an ad for the 4004 in Electronic News, Nov. 1971.
The age of the affordable computer. MITS announces the dawning of the Altair 8800 Computer. A lot of brain power at a price that's bound to create love and understanding. To say nothing of excitement. The Altair 8800 uses a parallel, 8-bit processor (the Intel 8080) with a 16-bit address. It has 78 basic machine instructions with variances over 200 instructions. It can directly address up to 65K bytes of memory and it is fast. Very fast. The Altair 8800's basic instruction cycle time is 2 microseconds. Combine this speed and power with Altair's flexibility (it can directly address 256 input and 256 output devices) and you have a computer that's competitive with most mini's on the market today. The basic Altair 8800 Computer includes the CPU, front panel control board, front panel lights and switches, power supply (enough to power any additional cards), and expander board (with room for 3 extra cards) all enclosed in a handsome, alum- inum case. Up to 16 cards can be added inside the main case. Options now available include 4K dynamic mem- ory cards, 1K static memory cards, parallel I/O cards, three serial I/O cards (TTL, R232, and TTY), octal to binary computer terminal, 32 character alpha-numeric display terminal, ASCII keyboard, audio tape interface, 4 channel storage scope (for testing), and expander cards. Options under development include a floppy disc system, CRT terminal, line printer, floating point processor, vectored interrupt (8 levels), PROM programmer, direct memory access controller and much more. PRICE Altair 8800 Computer: $439.00* kit $621.00* assembled prices and specifications subject to change without notice For more information or our free Altair Systems Catalogue phone or write: MITS, 6328 Linn N.E., Albuquerque, N.M. 87108, 505/265-7553. *In quantities of 1 (one). Substantial OEM discounts available.[Picture of computer, with switches and lights]
A bubble can be formed by reversing the field in a small spot, and can be destroyed by increasing the field.
The bubbles are anchored to tiny magnetic posts arranged in lines. Usually a 'V V V' shape or a 'T T T' shape. Another magnetic field is applied across the chip, which is picked up by the posts and holds the bubble. The field is rotated 90 degrees, and the bubble is attracted to another part of the post. After four rotations, a bubble gets moved to the next post:
o o o \/ \/ \/ \/ \/ \/ \/ \/ o o_|_ _|_ _|_ _|_ _|_o _|_ _|_ o _|_ _|_ o_|_ | o | | | |I hope that diagram makes sense.
These bubbles move in long thin loops arranged in rows. At the end of the row, the bits to be read are copied to another loop that shift to read and write units that create or destroy bubbles. Access time for a particular bit depends on where it is, so it's not consistent.
One of the limitations with bubble memories, why they were superceded, was the slow access. A large bubble memory would require large loops, so accessing a bit could require cycling through a huge number of other bits first. The speed of propagation is limited by how fast magnetic fields could be switched back and forth, a limit of about 1 MHz. On the plus side, they are non-volatile, but eeproms, flash memories, and ferroelectric technologies are also non-volatile and and are faster.
Core memories consist of ferromagnetic rings strung together on tiny wires. The wires will induce magnetic fields in the rings, which can later be read back. Usually reading this memory will erase it, so once a bit is read, it is written back. This type of memory is expensive because it has to be constructed physically, but is very fast and non-volatile. Unfortunately it's also large and heavy, compared to other technologies.
Ferroelectric materials retain an electric field rather than a magnetic field. like core memories, they are fast and non-volatile, but bits have to be rewritten when read. Unlike core memories, ferroelectric memories can be fabricated on silicon chips.
Legend reports that a Swedish jet prototype (the Viggen I believe) once crashed, but the magnetic tape flight recorders weren't fast enough to record the cause of the crash. The flight computers used core memory, though, so they were hooked up and read out, and the still contained the data microseconds before the crash occurred, allowing the cause to be determined. A similar trick was used when investigating the crash of the Space Shuttle Challenger.
On a similar note, the IBM 7740 communication controller was shipped with diagnostics code in its core memory, so it could be checked out on arrival without a host machine being operational.
Interestingly enough, newer flight recorders have replaced magnetic tape with flash memories, which is a newer and more reliable form of EEPROM (Electronically Erasable Programmable ROM). This actually has nothing to do with either ferromagnetic or ferroelectric memories, though. Oh well, this is an appendix. Who reads appendices anyway?