Annotated Hello, World Program in Assembly

hello.S

; To build and execute this program on Linux, you can use:
; nasm -f elf32 hello.S -o hello.o
; ld -m elf_i386 hello.o -o hello

section .data
  hello db 'Hello, World.', 0
section .text
  global _main

_main:
  ; In C, we could write a Hello, World program as
  ; write(1, hello, 13);
  ; write is a system call (sys_write). You can read more
  ; about how it works by reading its "man page":
  ; https://linux.die.net/man/3/write
  ; Because we are _not_ in C, we have to do the compiler's
  ; work ourselves. 
  ;
  ; The value of the first parameter goes in ebx:
  mov ebx, 1 ; the always-available file descriptor for stdout (the console)
  ; The value of the second parameter goes in ecx:
  mov ecx, hello ; set the address of the string "Hello, World.\n" into ecx
  ; The value of the third parameter goes in edx:
  mov edx, 13 ; set the length of the string "Hello, World.\0" into edx
  ; How did we know which registers to fill?
  ; https://github.com/torvalds/linux/blob/v3.13/arch/x86/ia32/ia32entry.S#L378-L397
  ;
  ; Ultimately there is only a generic mechanism to tell the Kernel
  ; that we want to execute a system call. We will need to set some
  ; specific information to tell the Kernel _which_ system call
  ; we want to execute. In order to do that, we set the system call
  ; number in eax:
  mov eax, 4 ; https://github.com/torvalds/linux/blob/d8ec26d7f8287f5788a494f56e8814210f0e64be/arch/x86/syscalls/syscall_32.tbl#L13
  ; Now that everything is set up, we will signal to the Kernel that
  ; we want to execute a system call. When the Kernel answers that signal,
  ; it knows to look in the eax register to see which system call we want to execute.
  int 0x80 ; 
  ; When we are here, the system call has been performed and our string is on
  ; the screen. All that's left to do is stop the program. Good news for us:
  ; stopping the program is just another system call! Take a stab at deciphering
  ; how the following lines of assembly code make the program exit:
  mov eax, 1
  xor ebx, ebx ; Note: This is just a _fancy_ way of setting the ebx register to 0
  int 0x80