Registers

Overview

An x86-64 CPU contains a set of 16 general-purpose registers storing 64-bit values. They are used to store integers and pointers.

1 Byte	2 Bytes	4 Bytes	8 Bytes	Purpose
%al	%ax	%eax	%rax	Return value
%bl	%bx	%ebx	%rbx	Callee saved
%cl	%cx	%ecx	%rcx	4th argument
%dl	%dx	%edx	%rdx	3rd argument
%sil	%si	%esi	%rsi	2nd argument
%dil	%di	%edi	%rdi	1st argument
%bpl	%bp	%ebp	%rbp	Callee saved
%spl	%sp	%esp	%rsp	Stack pointer
%r8b	%r8w	%r8d	%r8	5th argument
%r9b	%r9w	%r9d	%r9	6th argument
%r10b	%r10w	%r10d	%r10	Caller saved
%r11b	%r11w	%r11d	%r11	Caller saved
%r12b	%r12w	%r12d	%r12	Callee saved
%r13b	%r13w	%r13d	%r13	Callee saved
%r14b	%r14w	%r14d	%r14	Callee saved
%r15b	%r15w	%r15d	%r15	Callee saved

Condition Codes

The CPU also maintains a set of single-bit condition code registers describing attributes of the most recent arithmetic or logical operation.

Code	Name	Description
CF	Carry flag	The most recent operation generated a carry out of the most significant bit.
ZF	Zero flag	The most recent operation yielded zero.
SF	Sign flag	The most recent operation yielded a negative value.
OF	Overflow flag	The most recent operation caused a two’s-complement overflow.

Bibliography

• Bryant, Randal E., and David O’Hallaron. Computer Systems: A Programmer’s Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.