DosBox 启动 MBR，dl 中的错误值用于 int 13h

问题描述

我有一个自定义 MBR，我用 NASM 将其编译为二进制文件，我可以在 qemu 和 bochs 中正常启动它。

我正在尝试使用 boot mbr.bin 在 DosBox 中启动此 MBR。只要 MBR 很小并且不调用 int 13h 来从磁盘读取扇区，这就能正常工作。

如果我尝试读取下一个扇区，它会失败，从而导致某种磁盘读取错误。我想这与 dl 的值没有设置为“启动驱动器”有关。我尝试将 dl 手动设置为 0 和 80h，但都不起作用。

有谁知道 dl 的值应该是什么，以便使用 int 13h 命令使 boot 在 DosBox 中工作？或者是否有其他方法可以动态获取该值？

编辑；这是初始代码：

CPU 386
BITS 16
org 7C00h

; Calculate the full size of the code from start of segment $$ to the end label at the bottom (end - $$)
%define SECTORS_TO_LOAD ((end - $$) / 512) + ((end - $$) % 512 > 0) - 1

main:
    jmp 0x0000:setcs
    ; Some error messages
    disk_err db "Error reading disk",0
    sector_err db "Error reading sector",0

setcs:
    ; Base setup

    cld

    xor ax,ax      ; Set ES=DS=0 - this can be changed later for rep operations
    mov ds,ax
    mov es,ax

    mov ss,ax
    mov sp,0x7c00

    ; Clear the screen
    mov ah,0x07    ; Function 0x07: scroll window
    mov al,0x00    ; Clear entire window
    mov bh,0x07    ; White on black
    mov cx,0x00    ; Specifies top left of screen as (0,0)
    mov dh,0x18    ; 18h = 24 rows of chars
    mov dl,0x4f    ; 4fh = 79 cols of chars
    int 0x10    ; Video interrupt


    ; Move cursor to position 0,0
    mov ah,0x02    ; Function 0x02: set cursor position
    mov dh,0x00    ; Row 0
    mov dl,0x00    ; Col 0
    mov bh,0x00    ; Page 0
    int 0x10    ; Video interrupt


    ; Load the calculated numbers of sector from the disk
    ; For this function call,dl should be set to the disk number to read from. The BIOS sets dl to
    ; the disk number of the MBR and dl was not changed,so we don't have to set it.

    ;mov dl,0x80

    mov ax,SECTORS_TO_LOAD ; Number of sectors to read
    mov bx,next_sector ; The memory address to read to
    xor ch,ch      ; Cylinder 0
    mov cl,0x02        ; Start at sector 2 (sector 1 is the first part of the MBR)
    xor dh,dh      ; Head 0
    mov ah,0x02        ; Read mode

    int 0x13        ; Read interrupt
    jc .disk_error      ; Check carry bit for error

    cmp al,SECTORS_TO_LOAD ; The interrupt sets 'al' to the number of sectors actually read
    jne .sectors_error

    ; After a succesful load,the layout in memory will be the same as the layout in our ASM files,; so we can jump to a defined label without worrying about offsets and exact addresses.
    jmp after_load

.disk_error:
    push disk_err   ; Print a disk error message
    call print
    cli     ; Halt all interrupts and operations
    hlt

.sectors_error:
    push sector_err ; Same as above,different message
    call print
    cli
    hlt


; The print function prints a string onto the screen in text mode
print:
    push bp     ; Basic stack setup
    mov bp,sp
    pusha       ; Push everything - this function doesn't
            ; return anything

    mov si,[bp+4]  ; Grab the pointer to the data
    mov bh,0   ; Page 0
    mov bl,0   ; Foreground color,irrelevant - in text mode
    mov ah,0x0E    ; Function 0x0E: print character in TTY
.char:
    mov al,[si]    ; Get current char from pointer position
    inc si      ; Keep incrementing si until a null char
    or al,0
    je .return
    int 0x10    ; Video interrupt
    jmp .char
.return:
    popa        ; Restore registers
    mov sp,bp  ; Restore stack
    pop bp
    ret 2       ; Remove param from stack on return

times 510-($-$$) db 0

; MBR signature
db 55h,0AAh

next_sector:

之后，在 next_sector 下，它有一些 incbins 和一些其他代码，包括标签 after_load。

我得到的错误是第一个； “读取磁盘时出错”，显示正常。这样看来es，ds等都设置对了。

注意注释掉的 mov dl,0x80 - 这是我尝试让它工作的尝试之一。

解决方法

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