Invalid bytes read from disk using simple boot sector in QEmu
So, I am trying to learn how to make a small OS, mainly using this tutorial.
I'm on page 28 of this tutorial trying to make a little disk loader.
However, when I ran qemu-system-i386 boot_sector.bin
, which worked for the previous boot sector examples, the hex output I get is 0xDADA 0xZRWV
, the first is correct and the second is not.
My code looks like this.
Boot_sector.asm
; Very simple boot sector to read sectors from disk
jmp main ; ensures that we start where we want to
%include "disk_load.asm"
BOOT_DRIVE:
db 0
[org 0x7c00] ; tell the assembler where this code will be loaded
main:
mov bp, 0x8000 ; Make sure the stack is out of our way
mov sp, bp
mov bx, 0x9000
mov dh, 2 ; Want to read 2 sectors
call disk_load
mov dx, [0x9000]
call hex_print ; Should output 0xDADA
mov dx, [0x9000 + 512]; Should output 0xFACE
call hex_print
hang:
jmp hang ; Continually jump to current location
; Add padding and magic BIOS number (to let BIOS know this is a boot sector)
times 510-($-$$) db 0 ; pad with zeroes
dw 0xaa55 ; magic number
times 256 dw 0xdada ; second sector
times 256 dw 0xface ; third sector
Disk_load.asm:
%include "bios_print.asm"
ERROR_MSG:
db "Disk read error!", 0
SECTOR_ERROR_MSG:
db "Wrong number of sectors read!", 0
disk_load:
push dx ; Store the number of sectors we wanted to read
mov ah, 0x02 ; BIOS read sector function
mov al, dh ; Number of sectors to be read
mov ch, 0x00 ; Cylinder 0
mov dh, 0x00 ; Head 0
mov cl, 0x02 ; Sector 2 (just after the boot sector)
int 0x13
jc disk_error ; if carry flag is set, report an error
pop dx
cmp dh, al
jne sector_error ; if we didn't read as many sectors as we wanted.
ret
disk_error:
mov bx, ERROR_MSG
call bios_print
jmp $
sector_error:
mov bx, SECTOR_ERROR_MSG
call bios_print
jmp $
And then bios_print.asm just has functions for printing the hexadecimal code etc:
bios_print:
pusha
mov si, bx
loop:
lodsb
or al, al
jz done
mov ah, 0x0e
int 0x10
jmp loop
done:
popa
ret
HEX_OUT:
db '0x0000', 0
hex_print:
pusha
mov cx, 4 ; Counter
char_loop:
dec cx
mov ax, dx
shr dx, 4
and ax, 0xf ; Mask to get the last part of ax
mov bx, HEX_OUT
add bx, 2 ; Skip the '0x' in HEX_OUT
add bx, cx
; If the character is a number, we just plop that out, but if it
; is a letter, we have to convert it to ASCII by adding 7 (because
; ASCII starts at 17, and the numbers end at 10)
cmp ax, 0xa
jl set_char
add byte [bx], 7
jl set_char
set_char:
add byte [bx], al ; Add the value of the byte to the char stored at bx
cmp cx, 0
je finished
jmp char_loop
finished:
mov bx, HEX_OUT
call bios_print
popa
ret
I have also tried starting QEmu using the -fda flag that has been suggested in several places, but no luck. Also, if I change the number of sectors to read, say 5 (as in the tutorial), I get a disk read error (raised from the disk_load function).
This annoyed me very much. Thanks in advance for any help.
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