I am attempting to write a simple OS from scrath, but I have stumbled into a problem.
I wrote a simple procedure that runs through a string and prints it on the screen.
print_string:
pusha
cmp byte[bx], 0
je exit
mov ah, 0x0e
mov al, [bx]
int 0x10
inc bx
jmp print_string
exit:
mov ah, 0x0e
mov al, 13
int 0x10
mov al, 10
int 0x10
popa
ret
And I am including it on the main file.
[org 0x7c00]
mov bx, hello
call print_string
mov bx, hi
call print_string
jmp $
%include "print_string.s"
hello:
db "Hello, World!",0
hi:
db "This is a test.",0
times 510-($-$$) db 0
dw 0xaa55
But for some reason, instead of printing Hello, World! This is a test. it just prints Hello World!
When I remove the pusha and popa from print_string.s and place it on the main file like this:
[org 0x7c00]
mov bx, hello
pusha
call print_string
popa
mov bx, hi
pusha
call print_string
popa
jmp $
%include "print_string.s"
hello:
db "Hello, World!",0
hi:
db "This is a test.",0
times 510-($-$$) db 0
dw 0xaa55
It works just fine. Why?
>Solution :
print_string is called in the loop, on each iteration is does pusha. But there is only one popa instruction for multiple pusha. A fix:
print_string:
pusha
next_char:
mov al, [bx]
or al, al
je exit
mov ah, 0x0e
int 0x10
inc bx
jmp next_char