IDIV-DIV

Table of Contents

1. IDIV-DIV (idiv / div)
   1. Basic purpose
   2. DIV instruction format (unsigned)
   3. Behavioral characteristics
   4. Examples

IDIV-DIV (idiv / div)

Basic purpose

IDIV → signed division DIV → unsigned division
Result layout:

• Quotient → eax
• Remainder → edx

(or rax / rdx)

IDIV (signed) format

1
idiv r/m32
2
; edx:eax ÷ r/m32
3
; 有符号除法

Before execution:

• Fill EDX with the sign extension of EAX (cdq instruction)

DIV instruction format (unsigned)

1
div r/m8
2
; ax  ÷ r/m8
3
; al = 商
4
; ah = 余数
5

6
div r/m32
7
; edx:eax ÷ r/m32
8
; eax = 商
9
; edx = 余数

Note: before execution, edx must be cleared to zero (if the dividend is an unsigned dword).

Behavioral characteristics

• Divisor is 0 → divide-by-zero exception
• Quotient or remainder out of range → overflow exception
• Implicitly uses registers (AL/AX/EAX/RAX and AH/EDX/RDX)

Examples

1
mov eax, 100
2
mov ecx, 7
3
xor edx, edx
4
div ecx
5
; eax = 14, edx = 2

Signed:

1
mov eax, -30
2
mov ecx, 4
3
cdq
4
idiv ecx
5
; eax = -7, edx = -2