imul

IMUL performs signed integer multiplication. For the single-operand form, the other operand and destination are implicit, in the following way:

• For IMUL r/m8, AL is multiplied by the given operand; the product is stored in AX.

• For IMUL r/m16, AX is multiplied by the given operand; the product is stored in DX:AX.

• For IMUL r/m32, EAX is multiplied by the given operand; the product is stored in EDX:EAX.

The two-operand form multiplies its two operands and stores the result in the destination (first) operand. The three-operand form multiplies its last two operands and stores the result in the first operand.

The two-operand form with an immediate second operand is in fact a shorthand for the three-operand form, as can be seen by examining the opcode descriptions: in the two-operand form, the code /r takes both its register and r/m parts from the same operand (the first one).

In the forms with an 8-bit immediate operand and another longer source operand, the immediate operand is considered to be signed, and is sign-extended to the length of the other source operand. In these cases, the BYTE qualifier is necessary to force NASM to generate this form of the instruction.

Unsigned integer multiplication is performed by the MUL instruction: see MUL.