A platform chip is the modern equivalent of a microcontroller: it is a flexible chip that be programmed up to serve in a number of embedded applications. The set of components remains the same as for the microcontroller, but each has far more complexity: e.g. 32 bit prfocessor instead of 8. In addition, rather than putting a microcontroller on a PCB as the heart of a system, the whole system is placed on the same piece of silicon as the platform components. This gives us a system on a chip (SoC).
The example illustrated has two ARM processors and two DSP processors. Each ARM has a local cache and both store their programs and data in the same off-chip DRAM.
The left-hand-side ARM is used as an I/O processor and so is connected to a variety of standard peripherals. In any typical application, many of the peripherals will be unused and so held in a power down mode.
The right-hand-side ARM is used as the system controller. It can access all of the chip's resources over various bus bridges. It can access off-chip devices, such as an LCD display or keyboard via a general purpose A/D local bus.
The bus bridges map part of one processor's memory map into that of another so that cycles can be executed in the other's space, albeit with some delay and loss of performance. A FIFO bus bridge contains its own transaction queue of read or write operations awaiting completion.
The twin DSP devices run completely out of on-chip SRAM. Such SRAM may dominate the die area of the chip. If both are fetching instructions from the same port of the same RAM, then they had better be executing the same program in lock-step or else have some own local cache to avoid huge loss of performance in bus contention.
The rest of the system is normally swept up onto the same piece of silicon and this is denoted with the `special function peripheral.' This would be the one part of the design that varies from product to product. The same core set of components would be used for all sorts of different products, from iPODs, digital cameras or ADSL modems.