Clearing Registers in x86-64 Assembly#

Methods for setting registers to zero in NASM.

Fully working program that only purpose is to clear registers…

section .text
global _start

  _start:
      xor rax, rax    ; Clear RAX

      mov rax, 60     ; sys_exit
      xor rdi, rdi    ; exit code 0
      syscall

Overview#

Clearing registers (setting them to zero) is a fundamental operation

Methods#

XOR Method#

xor rax, rax    ; Clear RAX - preferred method

SUB Method#

sub rax, rax    ; Alternative method

MOV Method#

mov rax, 0      ; Less efficient - larger instruction

Performance Comparison#

Clearing Methods Comparison#

Method

Code

Size

XOR

xor reg, reg

3 bytes

SUB

sub reg, reg

3 bytes

MOV

mov reg, 0

7-10 bytes
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