Instruction Format

Although EDSAC has an 18 bit word length, the first bit is unusable (due to circuit setup times), so the instructions are represented with 17 bits divided as follows:

Opcode: The character code value for the letter representing the instuction.

Address: Location of one of the 1024 words of memory.

Length: Whether the instruction operates on a short (F) or a long (D).

Typical instructions are A 43 F (Add the short at location 43 into the accumulator) and S 192 D (Subtract the long at 192 from the accumulator). Note that for address zero the number is omitted, e.g. T F means store the short in the accumulator at location 0.