Assembly

Introduction

  • Tight coupling between code and machine instructions
  • Not portable, code is written for a specific ISA
  • Can be extremely efficient if optimized properly

Registers

  • Internal CPU data storage
  • Very limited in number
  • Hold operands for asm instructions

Instructions

  • Generally operate on registers
  • May perform other functions call, ret, syscall

ISA

  • Instruction set architecture
  • Set of instructions provided by a CPU
  • Examples include x86, x86_64, ARM, and ARM64

System Call

  • Used to run kernel code from userspace
  • syscall instruction triggers kernel
  • Values are picked up from appropriate registers

System Call Registers

Register Purpose
%rax System call number
%rdi 1st parameter
%rsi 2nd parameter
%rdx 3rd parameter
%r10 4th parameter
%r8 5th parameter
%r9 6th parameter 
.globl _start
_start:
 mov $1, %rdi
 leaq message(%rip), %rsi
 mov $14, %rdx
 mov $1, %rax
 syscall
 xor %rdi, %rdi
 mov $60, %rax
 syscall

message:
 .string "Hello, world!\n"

Functions

  • call and ret perform jumps and adjust state
  • push and pop access stack
  • rax register is used for return value in x64 calling convention

Common Function Registers

Register Purpose
%rax 1st return register
%rdi Used to pass 1st argument to functions
%rsi Used to pass 2nd argument to functions
%rdx Used to pass 3rd argument to functions
%rcx Used to pass 4th argument to functions
%rsp Stack pointer 
.globl _start
_start:
 call get_message
 mov $1, %rdi
 mov %rax, %rsi
 mov $14, %rdx
 mov $1, %rax
 syscall
 xor %rdi, %rdi
 mov $60, %rax
 syscall

get_message:
 leaq message(%rip), %rax
 ret

message:
 .string "Hello, world!\n"