I am working with a simple assembly snippet to print a character to screen using the BIOS as a part of a bootloader. Here is the bootloader code.
[org 0x7c00]
[bits 16]
%include "a20_check.asm"
mov ah, 0x0e
mov al, 'H'
int 0x10
times 510 - ($-$$) db 0
dw 0xaa55
This is the code for the a20_check.asm function, taken from osdev.org.
[bits 16]
; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
; 1 in ax if the a20 line is enabled (memory does not wrap around)
check_a20:
pushf
push ds
push es
push di
push si
cli
xor ax, ax ; ax = 0
mov es, ax
not ax ; ax = 0xFFFF
mov ds, ax
mov di, 0x0500
mov si, 0x0510
mov al, byte [es:di]
push ax
mov al, byte [ds:si]
push ax
mov byte [es:di], 0x00
mov byte [ds:si], 0xFF
cmp byte [es:di], 0xFF
pop ax
mov byte [ds:si], al
pop ax
mov byte [es:di], al
mov ax, 0
je check_a20__exit
mov ax, 1
check_a20__exit:
pop si
pop di
pop es
pop ds
popf
ret
The problem appears to be in the last label, check_a20__exit. If I comment out ret, then the character prints to the screen. Otherwise, it does not print.
Can someone explain this problem to me?