Instruction Format

Although 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 Codes

The EDSAC Instruction Set 1949

A n	Add the number in storage location n into the accumulator
S n	Subtract the number in storage location n from the accumulator
H n	Copy the number in storage location n into the multiplier register
V n	Multiply the number in storage location n by the number in the multiplier register and add the product into the accumulator
N n	Multiply the number in storage location n by the number in the multiplier register and subtract the product from the accumulator
T n	Transfer the contents of the accumulator to storage location n and clear the accumulator
U n	Transfer the contents of the accumulator to storage location n and do not clear the accumulator
C n	Collate [logical and] the number in storage location n with the number in the multiplier register and add the result into the accumulator
R 2^(n-2)	Shift the number in the accumulator n places to the right
L 2^(n-2)	Shift the number in the accumulator n places to the left
E n	If the sign of the accumulator is positive, jump to location n; otherwise proceed serially
G n	If the sign of the accumulator is negative, jump to location n; otherwise proceed serially
I n	Read the next character from paper tape, and store it as the least significant 5 bits of location n
O n	Print the character represented by the most significant 5 bits of storage location n
F n	Read the last character output for verification
X	No operation
Y	Round the number in the accumulator to 34 bits
Z	Stop the machine and ring the warning bell

Number Format

The 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