Instruction FormatAlthough the basic word size of the EDSAC was 18 bits, only 17 bits could actually be used due to circuit set-up times. The opcode (i.e. one of the instruction codes explained below) was specified in 5 bits, and the address in 10 bits (giving an address range of 1024 short words). The length bit specified the operand length : most instructions could operate on either a 17-bit short word (length = 0), or a 35-bit full word (length = 1). ![]() Instruction CodesThe EDSAC Instruction Set 1949
Number FormatThe were four number formats which could be used in the EDSAC: short and long integers, and short and long fractions. Short numbers were 17-bits in length (for reasons explained above) and long ones 35-bits. Whereas the multiplier and multiplicand registers each had a capacity of 35 bits, the accumulator provided 71 bits. This means that long numbers can be multiplied without overflow. ![]() Character Codes |
Editor Window | Output Window | Binary | Decimal | ||
Letter shift |
Figure shift |
Letter shift |
Figure shift |
||
P | 0 | P | 0 | 00000 | 0 |
Q | 1 | Q | 1 | 00001 | 1 |
W | 2 | W | 2 | 00010 | 2 |
E | 3 | E | 3 | 00011 | 3 |
R | 4 | R | 4 | 00100 | 4 |
T | 5 | T | 5 | 00101 | 5 |
Y | 6 | Y | 6 | 00110 | 6 |
U | 7 | U | 7 | 00111 | 7 |
I | 8 | I | 8 | 01000 | 8 |
O | 9 | O | 9 | 01001 | 9 |
J | J | 01010 | 10 | ||
Pi (type in as "#") | Figure shift | 01011 | 11 | ||
S | S | " | 01100 | 12 | |
Z | Z | + | 01101 | 13 | |
K | K | ( | 01110 | 14 | |
Erase (type in as "*") | Letter shift | 01111 | 15 | ||
Blank Tape (type in as "." ) | (no effect) | 10000 | 16 | ||
F | F | $ | 10001 | 17 | |
Theta (type in as "@") | Carriage return | 10010 | 18 | ||
D | D | ; | 10011 | 19 | |
Phi (type in as "!") | Space | 10100 | 20 | ||
H | + | H | £ | 10101 | 21 |
N | - | N | , | 10110 | 22 |
M | M | . | 10111 | 23 | |
Delta (type in as "&") | Line feed | 11000 | 24 | ||
L | L | ) | 11001 | 25 | |
X | X | / | 11010 | 26 | |
G | G | # | 11011 | 27 | |
A | A | - | 11100 | 28 | |
B | B | ? | 11101 | 29 | |
C | C | : | 11110 | 30 | |
V | V | = | 11111 | 31 |