📂 Assembly File I/O Basics

Working with files in Assembly means using Linux syscalls directly. You manually open, read/write, and close files using registers and interrupts.

🔑 Key Syscalls

sys_open (eax=5) — open a file
sys_read (eax=3) — read data from a file descriptor
sys_write (eax=4) — write data to a file descriptor
sys_close (eax=6) — close a file descriptor

📚 Example Code: Open, Read, Write, Close

section .data
    filename db "test.txt",0
    buffer   times 128 db 0

section .text
    global _start

_start:
    ; Open file (read-only)
    mov eax, 5           ; sys_open
    mov ebx, filename    ; filename pointer
    mov ecx, 0           ; flags = O_RDONLY
    int 0x80
    mov ebp, eax         ; save file descriptor

    ; Read file
    mov eax, 3           ; sys_read
    mov ebx, ebp         ; fd
    mov ecx, buffer      ; buffer to read into
    mov edx, 128         ; bytes to read
    int 0x80

    ; Write to stdout
    mov eax, 4           ; sys_write
    mov ebx, 1           ; stdout fd
    mov ecx, buffer      ; buffer to write
    mov edx, 128         ; bytes to write
    int 0x80

    ; Close file
    mov eax, 6           ; sys_close
    mov ebx, ebp
    int 0x80

    ; Exit
    mov eax, 1           ; sys_exit
    mov ebx, 0
    int 0x80

Output will appear here...

💡 Pro Tip: You must pass pointers to buffers and filenames, and carefully manage registers during syscalls.