SUB

Table of Contents

1. SUB (sub)
   1. Basic function
   2. Instruction format
   3. Instruction execution process
   4. Example
   5. Equivalent expansion
   6. Common uses

SUB (sub)

Basic function

The SUB instruction performs subtraction, writing the result of x1 - x2 back to x1.

Instruction format

1
sub x1, x2
2
x1 = x1 - x2

x1 and x2 can be:

• Register
• Memory
• Immediate value

Restrictions:

• They cannot both be memory operands at the same time

Instruction execution process

1
x1 ← x1 - x2
2
EFLAGS ← 根据结果更新

Affected flags:

• OF (signed overflow)
• SF (sign)
• ZF (whether the result is 0)
• CF (used to determine whether a borrow occurred)
• AF, PF

Example

1
sub eax, ebx
2
sub rax, 0x100
3
sub [rbp-0x4], 1

Equivalent expansion

1
sub rax, rbx
2
; 等价于
3
tmp = rax - rbx
4
rax = tmp
5
更新 EFLAGS

Common uses

• Decrementing, loop counting
• Moving the stack pointer upward (such as the reverse of a sub operation)
• Numeric computation
• Address offsetting (such as moving backward within a structure)