IMUL-MUL

Table of Contents

1. IMUL-MUL (imul / mul)
   1. Basic purpose
   2. IMUL instruction formats
   3. MUL instruction format (unsigned)
   4. Behavioral characteristics
   5. Example
   6. Common uses

IMUL-MUL (imul / mul)

Basic purpose

IMUL → signed multiplication
MUL → unsigned multiplication Both may involve a double-register result (EDX:EAX or RDX:RAX).

IMUL instruction formats

There are three forms:

1. Implicit form (result stored in edx:eax)

1
imul r/m32
2
; EDX:EAX = EAX * r/m32（有符号）

2. Explicit two-operand form

1
imul reg, r/m32
2
reg = reg * r/m32

3. Three-operand form

1
imul reg, r/m32, imm
2
reg = r/m32 * imm

MUL instruction format (unsigned)

1
mul r/m32      ; EDX:EAX = EAX * r/m32

MUL does not have an explicit reg = reg × x form.

Behavioral characteristics

• The high part of the result (EDX or RDX) is used to determine overflow
• If the high part is not 0, OF = CF = 1
• The implicit form must use the accumulator register (EAX / RAX) as an operand

Example

1
mov eax, 5
2
imul eax, 3      ; eax = 15
3

4
mov eax, -10
5
imul eax, -4     ; eax = 40（有符号乘法）

Implicit form:

1
mov eax, 0x10000
2
imul dword ptr [rbp-4]  ; rdx:rax = rax * [rbp-4]

Common uses

• Mathematical calculations
• Array index calculation (index × element_size)
• Struct offset calculation