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"