asmtutorials/x86/functions.asm

69 lines
2.1 KiB
NASM

;; This is an includes file. It doesn't have its own compilation
;; capabilities. There is no namespacing in assembly language;
;; try not to use these names in your own code.
;; sys/unistd_32.h
%define SYS_write 4
%define SYS_exit 1
;; unistd.h
%define STDOUT 1
;; strlen() function. Takes eax as an argument - the pointer to
;; the initial string. Returns eax as the result - the length of
;; the string.
strlen:
push ebx ; We'll be borrowing this register, so we put its
; current value on the stack.
mov ebx, eax
;; Note that even though these have underscores to indicate that
;; outside users should not use them, they're still globally
;; accesible in this program's namespace. A user could
;; theoretically call _strlen_done from anywhere. Assembly gives
;; you ALL the opportunities to shoot yourself in the foot!
_strlen_next:
cmp byte [eax], 0
jz _strlen_done
inc eax
jmp _strlen_next
_strlen_done:
sub eax, ebx
pop ebx ; Restore the register
ret
;; Puts() function - puts a string to the console. Takes EAX as
;; its only argument - the pointer to the beginning of the string.
puts:
push edx
push ecx
push ebx
push eax
call strlen ; Uses EAX as the pointer to the beginning of the string.
; Returns EAX as the length of the string
mov edx, eax ; Move the length of the string into EDX, where WRITE expects
pop eax ; Restore EAX from the stack
push eax ; Put the value BACK on the stack.
mov ecx, eax ; Put the pointer to the message into ECX, where WRITE expects
mov ebx, STDOUT
mov eax, SYS_write
int 80h
pop eax ; Restore registers in reverse order.
pop ebx
pop ecx
pop edx
ret
;; exit(). Straight from the original.
exit_program:
mov ebx, 0
mov eax, SYS_exit
int 80h