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imul

Signed multiply

Multiplies signed integers, producing a full implicit product with one operand or a destination-width result with two or three.

8086 and later

Intel's reference: IMUL

Used in
CFPFAFZFSFDFOF ●????–●
  • ● set from the result
  • ? undefined afterwards
  • – not changed

Forms

Operands

r/m
A general-purpose register or memory location matching the operation’s size, for example rax or qword [rbp-8]. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, including when the memory address uses r8–r15.
reg
A general-purpose register matching the operation’s size, for example al, ax, eax or rax. The byte registers ah, bh, ch and dh cannot be used with a REX prefix, which is required by spl, bpl, sil, dil and r8–r15.
imm
An immediate value encoded in the instruction, written as a number, character or label. Its encoded width and any sign extension depend on the instruction; arithmetic immediates may be sign-extended, while shift counts are used without sign extension.

With one explicit source, its width selects the implicit factor and full product: 8 bits multiplies al into ax; 16 bits multiplies ax into dx:ax; 32 bits multiplies eax into edx:eax; 64 bits multiplies rax into rdx:rax.

With two operands, the destination register is multiplied by the source and keeps only the low destination-width bits. With three, the last two operands are multiplied into the first. imul rax, 5 is shorthand for imul rax, rax, 5.

With one operand, CF and OF are set when the high half of the product is more than a sign extension of the low half, so the product does not fit the operand width. With two or three, they are set when the product had to be truncated to fit the destination. SF, ZF, AF and PF are undefined.

An 8-bit immediate used with a wider operand is sign-extended from −128 through 127. For a 64-bit operation, the longer immediate field is only 32 bits and is also sign-extended; there is no full 64-bit immediate form. The encoded bits determine the value used by the operation. For example, a 32-bit field of 0xffffffff becomes 0xffffffffffffffff (−1), not positive 4294967295. An out-of-range constant can produce a warning and truncated bits; use a register for a value that cannot be represented by these sign extensions.

NASM normally chooses the short immediate encoding when it preserves the value; a byte qualifier is not required. strict with an immediate size can prevent that shortening.

global _start

section .text
_start:
        mov rax, 12
        mov rbx, 5
        mov rcx, 3
        mov rsi, 9

        imul rax

        mov rax, 60         ; syscall 60: exit
        xor rdi, rdi
        syscall