rredpoppy/compiling_asm.md

## compiling_asm.md

      
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              compiling_asm.md
            
          
    32-bit ELF binary

how to assemble and link:
nasm -f elf32 -o <filename>.o <filename>.asm
ld -m elf_i386 -o <filename> <filename>.o
template code (hello world):
section .text
global _start

%define system_call int 0x80

_start:
    mov ebx, 0x1
    mov ecx, hello
    mov edx, helloLen
    mov eax, 0x4
    system_call

    xor ebx, ebx
    mov eax, 0x1
    system_call

section .data
    hello db "Hello World", 0xa
    helloLen equ $-hello
64-bit ELF binary

how to assemble and link:
nasm -f elf64 -o <filename>.o <filename>.asm
ld -o <filename> <filename>.o
template code (hello world):
section .text
global _start

_start:
    mov rdi, 0x1
    mov rsi, hello
    mov rdx, helloLen
    mov rax, 0x1
    syscall

    xor rdi, rdi
    mov rax, 0x3c
    syscall

section .data
    hello db "Hello World", 0xa
    helloLen equ $-hello
32-bit ELF binary with libc

how to assemble and link:
nasm -f elf32 -o <filename>.o <filename>.asm
ld -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o <filename> -lc <filename>.o
template code (hello world):
extern puts
extern exit

section .text
global _start

%macro call_func 1
    call %1
    add esp, 0x4
%endmacro

_start:
    push hello
    call_func puts

    mov eax, 0xa
    call exit

section .data
    hello db "Hello World"
64-bit ELF binary with libc

how to assemble and link:
nasm -f elf64 -o <filename>.o <filename>.asm
ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o <filename> -lc <filename>.o
template code (hello world):
extern puts
extern exit

section .text
global _start

%macro call_func 1
    call %1
%endmacro

_start:
    mov rdi, hello
    call_func puts

    mov rax, 0xa 
    call exit

section .data
    hello db "Hello World"

NOTE :

Assembler Directives: instructions for the assembler (e.g. NASM) and not a part of the ISA

extern: declare a symbol which is defined in another module that will later be linked with current one
section: identifies the section the code you write will go into. Examples of common ones are .text, .data, and .bss
global: defines symbols such that other modules that EXTERN those symbols can correctly reference them after linking
macro: a name that represents a snippet of code
define: a single-line macro