Incorrect Bytes Being Read from disk using a Simple Boot sector, in QEmu

Question

So I am trying to learn how to make a little OS, primarily using this tutorial thing.

I am at page 28 of that tutorial, trying to do the little disk-loader.

However, when I run qemu-system-i386 boot_sector.bin, which worked for the previous boot sector examples, the hex output I get is 0xDADA 0xZRWV, the first of which is correct, while the second is not.

My code is as follows.

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

The 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 the functions to print the hex and so on:

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 running QEmu using the -fda flag, which was suggested in a few places, but no luck. Also, if I change the number of sectors to read to, say, 5 (as in the tutorial) I get a Drive read error (raised from the disk_load function).

This has got me very annoyed. Thanks in advance for any help.

Answer 1

-_- I just worked out what the issue was.

After calling hex_print, I was not resetting HEX_STRING.

I fixed this by simply copying the location of HEX_STRING back out to BX, and then iterating through BX, indirectly setting each byte of HEX_STRING to ASCII 0.

This has fixed the problem.