Editing EDSAC (section)

===Memory and instructions===
[[File:EDSAC (9).jpg|thumb|upright|[[Maurice Wilkes]] inspecting the mercury [[delay-line memory|delay line]] of the EDSAC in construction]]
[[File:EDSAC (12).jpg|thumb|Maurice Wilkes and Bill Renwick in front of the complete EDSAC]]
The EDSAC's main memory consisted of 1024 locations, though only 512 locations were initially installed. Each contained 18&nbsp;bits, but the topmost bit was always unavailable due to timing problems, so only 17&nbsp;bits were used. An instruction consisted of a five-bit instruction code, one spare bit, a 10-bit operand (usually a memory address), and one length bit to control whether the instruction used a 17-bit or a 35-bit operand (two consecutive words, [[Endianness|little-endian]]). All instruction codes were by design represented by one mnemonic letter, so that the ''Add'' instruction, for example, used the EDSAC character code for the letter&nbsp;A.

Internally, the EDSAC used [[two's complement]] [[Binary number|binary]] numbers. Numbers were either 17&nbsp;bits (one word) or 35&nbsp;bits (two words) long. Unusually, the [[Binary multiplier|multiplier]] was designed to treat numbers as [[fixed-point arithmetic|fixed-point]] fractions in the range −1 ≤ ''x'' &lt; 1, i.e. the binary point was immediately to the right of the sign. The [[Accumulator (computing)|accumulator]] could hold 71&nbsp;bits, including the sign, allowing two long (35-bit) numbers to be multiplied without losing any precision.

The instructions available were:
* Add
* Subtract
* Multiply-and-add
* AND-and-add (called "Collate")
* Shift left
* Arithmetic shift right
* Load multiplier register
* Store (and optionally clear) accumulator
* Conditional goto
* Read input tape
* Print character
* Round accumulator
* No-op
* Stop
There was no division instruction (but various division subroutines were supplied) and no way to directly load a number into the accumulator (a "Store and zero accumulator" instruction followed by an "Add" instruction were necessary for this). There was no unconditional jump instruction, nor was there a procedure call instruction&nbsp;– it had not yet been invented.

[[Maurice Wilkes]] discussed relative addressing modes for the EDSAC in a paper published in 1953. He was making the proposals to facilitate the use of [[subroutine]]s.<ref>Proceedings of the Cambridge Philosophical Society, Vol.&nbsp;49, Pt.&nbsp;1, p.&nbsp;84–89.</ref>