I'm trying to write a real mode bootloader and I'm currently having problems trying to enable the A20 line. Here's my code so far, I'm assembling with NASM:
[bits 16]
[global _start]
jmp _start
bios_print:
lodsb
test al, al
jz bios_print_done
mov ah, 0x0E
mov bh, 0
int 0x10
jmp bios_print
bios_print_done:
ret
a20_is_enabled:
push ds
push si
push es
push di
xor ax, ax
mov ds, ax
mov si, BOOT_ID_OFFS
mov ax, BOOT_ID_OFFS_PLUS_1MB_SEGM
mov es, ax
mov di, BOOT_ID_OFFS_PLUS_1MB_OFFS
cmp word [es:di], BOOT_ID
mov ax, 1
jne a20_is_enabled_done
mov ax, word [ds:si]
xor ax, ax
mov [ds:si], ax
cmp word [es:di], BOOT_ID
push ax
xor ax, ax
mov [ds:si], ax
pop ax
mov ax, 1
jne a20_is_enabled_done
mov ax, 0
a20_is_enabled_done:
pop di
pos es
pop si
pop ds
ret
a20_enable_bios:
mov ax, 0x2403
int 0x15
jc a20_enable_bios_failure
test ah, ah
jnz a20_enable_bios_failure
mov ax, 0x2401
int 0x15
jc a20_enable_bios_failure
test ah, ah
jnz a20_enable_bios_failure
mov ax, 1
jmp a20_enable_bios_done
a20_enable_bios_failure:
mov ax, 0
a20_enable_bios_done:
ret
a20_enable:
push si
mov si, word MSG_A20_TRY_BIOS
call bios_print
pop si
call a20_enable_bios
test ax, ax
jz a20_enable_failure
call a20_is_enabled
test ax, ax
jnz a20_enable_success
a20_enable_failure:
push si
mov si, word MSG_A20_FAILURE
call bios_print
pop si
mov ax, 0
jmp a20_enable_done
a20_enable_success:
push si
mov si, word MSG_A20_SUCCESS
call bios_print
pop si
mov ax, 1
a20_enable_done:
ret
_start:
xor ax, ax
mov ds, ax
cld
cli
push si
mov si, word MSG_GREETING
call bios_print
pop si
call a20_enable
test ax, ax
jz boot_error
; TODO
boot_error:
jmp boot_error
BOOT_ID equ 0xAA55
BOOT_ID_OFFS equ 0x7DFE
BOOT_ID_OFFS_PLUS_1MB_SEGM equ 0xFFFF
BOOT_ID_OFFS_PLUS_1MB_OFFS equ BOOT_ID_OFFS + (0x1 << 20) - (BOOT_ID_OFFS_PLUS_1MB_SEGM << 4)
MSG_GREETING db 'Hello from the bootloader', 0xA, 0xD, 0
MSG_A20_TRY_BIOS db 'Trying to enable A20 line via BIOS interrupt', 0xA, 0xD, 0
MSG_A20_SUCCESS db 'Successfully enabled A20 line', 0xA, 0xD, 0
MSG_A20_FAILURE db 'Failed to enable A20 line', 0xA, 0xD, 0
times 510-($-$$) db 0
dw BOOT_ID
The problem is the function a20_is_enabled which is supposed to check if the A20 line is enabled after a20_enable_bios has activated it via a BIOS interrupt (I know this is not foolproof, more code will follow here). When I debug the code everything seems to be fine until call a20_is_enabled. The processor does then indeed perform a near call to to correct address here but no return address is pushed onto the stack (which I have verified with gdb). So when ret is executed in a20_is_enabled, the instruction pointer is set to some garbage address. Why is this?
EDIT: note that there is not ORG 0x7C00 at the beginning of my assembly code. This is because I first create an elf file so that I can debug my code using gdb and that doesn't play well with ORG, So I actually do this:
nasm -f elf32 -g -F dwarf boot.asm -o boot.o
ld -Ttext=0x7c00 -melf_i386 boot.o -o boot.elf
objcopy -O binary boot.elf boot.bin
Normally one might close this question as it is caused by a typographical error but the error isn't necessarily obvious at first. One has to pay close attention in a debugger observing the instructions that are being executed.
This had me scratching my head since when I looked in the debugger the sequence:
push ds
push si
push es
push di
; Snip other code
pop di
pos es
pop si
pop ds
ret
only showed the processor executing 3 POPs and a ret when there are clearly 4 POP instructions. Because the processor isn't doing enough POPs the return address is incorrect and ret returns to the wrong part of memory and causes unexpected behavior.
The problem is rather trivial and because of a stroke of bad luck an instruction is produced without error but isn't the instruction you want. If you look closely this is the culprit:
pos es
There is a typo. POS should be POP. My brain didn't catch it at first. pos is being treated as a label and es is a segment override so can appear on a line by itself. This caused the instruction es pop si to be produced.
Clearly the fix is to change it to:
pop es