Question

所以我正在尝试学习如何制作一个小操作系统，主要是使用this教程。

我在该教程的第28页，尝试使用小型磁盘加载器。

但是，当我运行qemu-system-i386 boot_sector.bin时，对于之前的引导扇区示例，我得到的十六进制输出是0xDADA 0xZRWV，第一个是正确的，而第二个不是。

我的代码如下。

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   $

然后bios_print.asm只具有打印十六进制的功能，依此类推：

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

我也尝试使用-fda标志运行QEmu，这是在一些地方建议的，但没有运气。另外，如果我将要读取的扇区数更改为5（如教程中所示），则会出现Drive read错误（从disk_load函数引发）。

这让我非常生气。提前感谢您的帮助。

Answer 1

-_-我刚刚解决了问题所在。

致电hex_print后，我没有重置HEX_STRING。

我通过简单地将HEX_STRING的位置复制回BX，然后迭代BX，间接地将HEX_STRING的每个字节设置为ASCII 0来修复此问题。

这解决了这个问题。

在QEmu中使用简单引导扇区从磁盘读取不正确的字节

1 个答案: