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- ;
- ; This program runs in 32-bit protected mode.
- ; build: nasm -f elf -F stabs name.asm
- ; link: ld -o name name.o
- ;
- ; In 64-bit protected mode you can use 64-bit registers (e.g. rax instead of eax, rbx instead of ebx, etc..)
- ; Also change "-f elf " for "-f elf64" in build command.
- ;
- section .data ; section for initialized data
- str: db 'Hello world!', 0Ah ; message string with new-line char at the end (10 decimal)
- str_len: equ $ - str ; calcs length of string (bytes) by subtracting this' address ($ symbol) from the str's start address
- section .text ; this is the code section
- global _start ; _start is the entry point and needs global scope to be 'seen' by the linker -equivalent to main() in C/C++
- _start: ; procedure start
- mov eax, 4 ; specify the sys_write function code (from OS vector table)
- mov ebx, 1 ; specify file descriptor stdout -in linux, everything's treated as a file, even hardware devices
- mov ecx, str ; move start _address_ of string message to ecx register
- mov edx, str_len ; move length of message (in bytes)
- int 80h ; tell kernel to perform the system call we just set up - in linux services are requested through the kernel
- mov eax, 1 ; specify sys_exit function code (from OS vector table)
- mov ebx, 0 ; specify return code for OS (0 = everything's fine)
- int 80h ; tell kernel to perform system call
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