Address-format code for Y=(A+B)*(C+D)

Assumptions: the operands are memory locations, R1, R2, ... are CPU registers, T1, T2, ... are temporary memory locations, and Y receives the result. Multiplication and division are done before addition and subtraction, and equal-priority operators go left to right.

Fully bracketed: Y = (A+B)*(C+D). Postfix (reverse Polish) form: A B + C D + *.

Three-address instructions (OP dest, src1, src2)

ADD R1, A, B   ; R1 = A + B
ADD R2, C, D   ; R2 = C + D
MUL Y, R1, R2  ; Y = R1 * R2

Number of instructions: 3. Shortest program, but each instruction is long (three addresses).

Two-address instructions (OP dest, src; dest = dest OP src)

MOV R1, A   ; R1 = A
ADD R1, B   ; R1 = R1 + B
MOV R2, C   ; R2 = C
ADD R2, D   ; R2 = R2 + D
MUL R1, R2  ; R1 = R1 * R2
MOV Y, R1   ; Y = R1

Number of instructions: 6. A MOV is needed first because the destination operand is overwritten.

One-address instructions (accumulator AC is the implied operand)

LOAD A    ; AC = A
ADD B     ; AC = AC + B
STORE T1  ; T1 = AC
LOAD C    ; AC = C
ADD D     ; AC = AC + D
MUL T1    ; AC = AC * T1
STORE Y   ; Y = AC

Number of instructions: 7. Intermediate results that cannot stay in AC are saved in memory temporaries.

Zero-address instructions (stack machine)

PUSH A  ; push A
PUSH B  ; push B
ADD     ; pop two, compute, push result
PUSH C  ; push C
PUSH D  ; push D
ADD     ; pop two, compute, push result
MUL     ; pop two, compute, push result
POP Y   ; Y = top of stack

Number of instructions: 8. Only PUSH and POP carry an address; ADD, SUB, MUL, DIV work on the top two stack items, so the program follows the postfix form directly.

Summary: 3-address = 3, 2-address = 6, 1-address = 7, 0-address = 8 instructions. Fewer addresses per instruction means shorter instructions but more of them.