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. Left-to-right evaluation gives (A + (B×C)) + D.

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

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

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

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

Number of instructions: 5. No temporary location T is needed here, because every intermediate result can stay in AC (addition is commutative, so B×C is computed first and A is added to it).

Zero-address instructions (stack machine)

PUSH A  ; push A
PUSH B  ; push B
PUSH C  ; push C
MUL     ; pop two, compute, push result
ADD     ; pop two, compute, push result
PUSH D  ; push D
ADD     ; 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 = 5, 0-address = 8 instructions. Fewer addresses per instruction usually means shorter instructions but more of them.