Assembly

Quick references

These notes target Linux on x86-64 (AMD64), using Intel syntax with the GNU assembler (as). Keep in mind that assembler syntax and directives may differ when using another assembler, such as NASM, or when targeting a different CPU architecture.

Quick references for X86-64 AMD64 Linux Assembly: registers, instructions, Linux system calls, ...

Registers

Register Common role Important details
rax
  • Function return value
  • Syscall number before syscall
  • Syscall result after syscall
 Caller-saved
rbx General-purpose storage Callee-saved
rcx
  • Fourth normal function argument
  • Counts and temporary values
  • Caller saved
  • Destroyed by syscall
rdx
  • Third function argument
  • Third syscall argument
  • Second return register in some cases
 Caller-saved
rsi
  • Second function argument
  • Second syscall argument
  • Source pointer by convention in some instructions
 Caller-saved
rdi
  • First function argument
  • First syscall argument
  • Destination pointer by convention in some instructions
 Caller-saved
rsp Stack pointer Must be restored before returning from a function
rbp General-purpose or optional frame pointer Callee-saved
r8
  • Fifth normal function argument
  • Fifth syscall argument
 Caller-saved
r9
  • Sixth normal function argument
  • Sixth syscall argument
 Caller-saved
r10 Fourth syscall argument Caller-saved
r11 Temporary value
  • Caller-saved
  • Destroyed by syscall
r12-r15 Long-lived general-purpose values Callee-saved
rip Address of the next instruction Changed by jumps, calls, returns and exceptions
rflags Condition and control flags Read by conditional jumps and setcc

Partial register access

64-bit Low 32-bit Low 16-bit High 8-bit Low 8-bit
rax eax ax ah al
rbx ebx bx bh bl
rcx ecx cx ch cl
rdx edx dx dh dl
rsp esp sp spl
rbp ebp bp bpl
rsi esi si sil
rdi edi di dil
r8 r8d r8w r8b
r9 r9d r9w r9b
r10 r10d r10w r10b
r11 r11d r11w r11b
r12 r12d r12w r12b
r13 r13d r13w r13b
r14 r14d r14w r14b
r15 r15d r15w r15b

Function registers: System V AMD64

Value Register
Integer/pointer return value rax
Argument 1 rdi
Argument 2 rsi
Argument 3 rdx
Argument 4 rcx
Argument 5 r8
Argument 6 r9
Caller-saved rax, rcx, rdx, rsi, rdi, r8, r9, r10, r11
Callee-saved rbx, rbp, r12, r13, r14, r15

Linux x86-64 syscall registers

Value Register
  • syscall number
  • syscall return value
 rax
Argument 1 rdi
Argument 2 rsi
Argument 3 rdx
Argument 4 r10
Argument 5 r8
Argument 6 r9

Operand sizes

x86 name Bits Bytes Low part of rax Memory specifier
Byte 8 1 al BYTE PTR
Word 16 2 ax WORD PTR
Doubleword 32 4 eax DWORD PTR
Quadword 64 8 rax QWORD PTR

Unsigned and signed ranges

Width Unsigned range Two's complement signed range
8 bits 0 to 255 −128 to 127
16 bits 0 to 65,535 −32,768 to 32,767
32 bits 0 to 4,294,967,295 −2,147,483,648 to 2,147,483,647
64 bits 0 to 18,446,744,073,709,551,615 −9,223,372,036,854,775,808 to 9,223,372,036,854,775,807

Assembler directives

Directive Purpose Example
.section Select a section .section .rodata
.global Export a symbol .global solve
.type Attach a symbol type .type solve, @function
.byte Emit one or more bytes .byte 0x41, 0x42
.word Emit 16-bit values .word 0x1234
.long Emit 32-bit values .long 0x12345678
.quad Emit 64-bit values .quad 0x1122334455667788
.ascii Emit string bytes without terminating zero .ascii "Hello\n"
.asciz Emit string bytes followed by a zero byte .asciz
.equ /.set Define aan assembly-time constant .equ BUFFER_SIZE, 128

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