x86asm notes

Sections

• .data: Stores initialized data (constants, static variables)
• .bss: Stores uninitialized data or variables that should be zeroed out at runtime
• .text: Contains the executable code of the program
• .rodata: Stores read-only data such as string literals

In 64-bit architecture .data and .bss serve the same purposes

Defining data

• db: Defines a byte (8bits) of data
• dw: Defines a word (2 bytes) of data
• dd: Defines a double word (4 bytes) of data
• dq: Defines a quad word (8 bytes) of data
• dt: Defines a ten-byte value typically used for extended precision floating-point numbers

Reserving space

• resb: Reserves a cenrtain number of bytes in memory (typicaly used with buffers)
• resw: Reserves space for a number of words (2 bytes each)
• resd: Reserves space for a number of double words (4 bytes each)
• resq: Reserves space for a number of quad words (8 bytes each)

Constants

equ is used to define constants

CONSTANT_NAME equ value_or_expression

Registers

• eax, ebx, ecx, edx (general purpose registers)
• esp (stack pointer)
• ebp (base pointer)
• esi, edi (index registers)
• eip (instruction pointer)

Instructions

• mov (move data)
• add, sub (atirthmetic operations)
• push, pop (stack operations)
• cmp, jmp, je, jne (comparison and jumps)

Including

Data

; data.asm
section .data
my_data db "Hello World", 0

; main.asm
section .data
    include "data.asm"

Procedures

; utils.asm
section .text
global my_proc ; make the proc exportable to be available to other files

; main.asm
extern my_proc ; now i can use this procedure

Syscalls

eax: Contains the syscall number

ebx, ecx, edx, esi, edi: These registers hold arguments ti the syscall

eax: On return this register holds the result of the syscall (0 success / <0 error)

Numbers

• sys_exit (exit program): Number: 1 Arguments: Exit code (in EBX).

• sys_fork (create a child process): Number: 2 Arguments: None.

• sys_read (read from a file descriptor): Number: 3 Arguments: File descriptor (in EBX) Buffer (in ECX) Number of bytes to read (in EDX).

• sys_write (write to a file descriptor): Number: 4 Arguments: File descriptor (in EBX) Buffer (in ECX) Number of bytes to write (in EDX).

• sys_open (open a file): Number: 5 Arguments: File path (in EBX) Flags (in ECX) Mode (in EDX).

• sys_close (close a file descriptor): Number: 6 Arguments: File descriptor (in EBX).

Macros

Syntax:

%macros MACRO_NAME <n> ; number of arguments
    ; code
    ; %1, %2, ... , %n arguments
%endmacro

Invokation:

MACRO_NAME arg1, arg2, ... , argn