本文介绍了是什么导致该引导加载程序在硬件上失败,但在DOSBOX中失败?它显示所有寄存器的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!

问题描述

我最近编写了一个x86'bootloader'程序,该程序显示BIOS跳转到我的程序后硬件寄存器的值.为了进行测试,我将 AX 寄存器设置为已知值,以确保程序正确运行.

I recently wrote an x86 'bootloader' program that shows the values of the hardware registers after the BIOS jumps to my program. For the purpose of testing, I set the AX register to a known value to ensure that the program runs correctly.

BITS 16
%macro pad 1-2 0
    times %1 - ($ - $$) db %2
%endmacro
[org 0x7C00]
    CLD                 ; clear direction flag (forward direction)
    CLI                 ; clear interrupt flag (disable interrupts, opposite of 65xx)
    
    MOV [0x8000], AX    ; display all registers,
    MOV [0x8004], BX    ;   including stack,
    MOV [0x8008], CX    ;   segment, & extra
    MOV [0x800C], DX    ;   registers

    MOV [0x8010], SP
    MOV [0x8014], BP
    MOV [0x8018], SI
    MOV [0x801C], DI
    
    MOV [0x8020], CS
    MOV [0x8024], SS    ; we also display DS register,
    MOV [0x8028], ES    ;   so we can't modify it or
    MOV [0x802C], DS    ;   we'll loose our data
    
    MOV [0x8030], FS
    MOV [0x8034], GS
    
    MOV AX, 0x0123      ; write 0x0123 to [0x8000]
    MOV [0x8000], AX    ;   for debugging
    
    MOV DI, 0x804C      ; DI is pointer to address 0x804C
                        ; (temporary data)
    MOV AH, 0x02
    MOV BH, 0x00        ; video page 0?
    MOV DX, 0x0401
    INT 0x10            ; move cursor to XY:($01, $04)

    ; display register data
    MOV AL, 'A'
    CALL printXl        ; print 'AX:'
    MOV DX, [0x8000]    ; recall value of AX register
                        ;   (set to 0x0123 for test)
    CALL printascii     ; print 16-bit value @ [0x8000]
    
    ;...                ; omitted code: display other registers
    
    MOV AH, 0x00        ; wait for keyboard press
    INT 0x16
    
    INT 0x18            ; boot Windows

printXl:
    MOV AH, 0x0E
    XOR BX, BX
    INT 0x10            ; display character in 'AL'
    MOV AL, 'X'
    ; falls through
prnt:                   ; referenced in omitted code
    MOV AH, 0x0E
    INT 0x10            ; display character 'X'/'S'
    MOV AL, ':'
    INT 0x10            ; display character ':'
    RET
    
printascii:
    MOV [DI], DX            ; store value for later recall
    MOV AH, 0x0E            ; INT 10,E
    
    MOV SI, hexascii        ; load address of 'hexascii'
    AND DX, 0xF000
    SHR DX, 0x0C            ; shift high nibble to lowest 4 bits
    ADD SI, DX
    CS LODSB                ; AL = CS:[0x1EE + DX >> 12];
    INT 0x10                ; display high nibble of character value
            
    MOV SI, hexascii
    MOV DX, [DI]
    AND DX, 0x0F00
    SHR DX, 0x08
    ADD SI, DX
    CS LODSB
    INT 0x10                ; display low nibble of character value
            
    MOV SI, hexascii
    MOV DX, [DI]
    AND DX, 0x00F0
    SHR DX, 0x04
    ADD SI, DX
    CS LODSB
    INT 0x10                ; display high nibble of character value
            
    MOV SI, hexascii        ;
    MOV DX, [DI]
    AND DX, 0x000F
    ADD SI, DX
    CS LODSB
    INT 0x10                ; display low nibble of character value
            
    RET
pad 0x01EE
hexascii:
    db "0123456789ABCDEF"   ;
    
pad 0x01FE                  ; pad to end of bootsector
    dw 0xAA55               ; bootsector signature

从DOSBOX运行时,我可以正确看到 AX:0123 ,但是从软盘启动时,我会得到 AX:FFFF .我不知道我在做什么错.作为参考,我的PC是 Intel Core 2 Quad .

When running from DOSBOX, I correctly see AX:0123, but when booting from my floppy disk, I get AX:FFFF. I have no idea what I'm doing wrong. For reference, my PC is a Intel Core 2 Quad.

推荐答案

引用布伦丹的答案:

此问题的解决方案是使用ROM-BIOS设置的初始堆栈,该堆栈应至少几十个字节是安全的,然后至关重要的是将前几个寄存器的值存储到所占用的空间中由我们自己的引导扇区加载程序.此空间为我们保留,并且不能由ROM-BIOS设置的初始堆栈覆盖.将堆栈切换到一个已知良好的区域后,我们也可以使用其他内存,尽管在本示例中我们不需要这样做.这是NASM来源(test.asm):

The solution to this problem is to use the initial stack as set up by the ROM-BIOS, which should be safe for at least a few dozen bytes, and crucially to then store the first few registers' values into the space occupied by our own boot sector loader. This space is reserved for us and must not be overwritten by the initial stack as set up by the ROM-BIOS. After switching the stack to a known-good area we are allowed to use other memory too, though we do not need that for this example. Here's the NASM source (test.asm):

%if 0

Boot sector loader which displays register values
 by C. Masloch, 2020

Usage of the works is permitted provided that this
instrument is retained with the works, so that any entity
that uses the works is notified of this instrument.

DISCLAIMER: THE WORKS ARE WITHOUT WARRANTY.

%endif


        struc BS
bsJump: resb 3
bsOEM:  resb 8
bsBPB:
        endstruc

        struc EBPB              ;        BPB sec
bpbBytesPerSector:      resw 1  ; offset 00h 0Bh
bpbSectorsPerCluster:   resb 1  ; offset 02h 0Dh
bpbReservedSectors:     resw 1  ; offset 03h 0Eh
bpbNumFATs:             resb 1  ; offset 05h 10h
bpbNumRootDirEnts:      resw 1  ; offset 06h 11h -- 0 for FAT32
bpbTotalSectors:        resw 1  ; offset 08h 13h
bpbMediaID:             resb 1  ; offset 0Ah 15h
bpbSectorsPerFAT:       resw 1  ; offset 0Bh 16h -- 0 for FAT32
bpbCHSSectors:          resw 1  ; offset 0Dh 18h
bpbCHSHeads:            resw 1  ; offset 0Fh 1Ah
bpbHiddenSectors:       resd 1  ; offset 11h 1Ch
bpbTotalSectorsLarge:   resd 1  ; offset 15h 20h
bpbNew:                         ; offset 19h 24h

ebpbSectorsPerFATLarge: resd 1  ; offset 19h 24h
ebpbFSFlags:            resw 1  ; offset 1Dh 28h
ebpbFSVersion:          resw 1  ; offset 1Fh 2Ah
ebpbRootCluster:        resd 1  ; offset 21h 2Ch
ebpbFSINFOSector:       resw 1  ; offset 25h 30h
ebpbBackupSector:       resw 1  ; offset 27h 32h
ebpbReserved:           resb 12 ; offset 29h 34h
ebpbNew:                        ; offset 35h 40h
        endstruc

        struc BPBN              ; ofs B16 S16 B32 S32
bpbnBootUnit:           resb 1  ; 00h 19h 24h 35h 40h
                        resb 1  ; 01h 1Ah 25h 36h 41h
bpbnExtBPBSignature:    resb 1  ; 02h 1Bh 26h 37h 42h -- 29h for valid BPBN
bpbnSerialNumber:       resd 1  ; 03h 1Ch 27h 38h 43h
bpbnVolumeLabel:        resb 11 ; 07h 20h 2Bh 3Ch 47h
bpbnFilesystemID:       resb 8  ; 12h 2Bh 36h 47h 52h
        endstruc                ; 1Ah 33h 3Eh 4Fh 5Ah


        cpu 8086
        org 7C00h

start:
        jmp short entrypoint
        nop

        times (bsBPB + EBPB_size + BPBN_size) - ($ - $$) db 0

entrypoint:
        pushf
        cli                    ; An interrupt could use too much more stack space
        cld
        push bx
        push ds
        call 0:.next           ; Set CS:IP to match ORG
.next:
        pop bx                 ; BX = IP of return address pushed by call
        sub bx, .next - start  ; calculate original IP on entry to start
        push bx
         push cs
         pop ds                ; DS=0 to match ORG
        mov bx, start
        pop word [bx + reg_ip]      ; store into start + BPB space 
        pop word [bx + reg_cs]
        pop word [bx + reg_ds]
        pop word [bx + reg_bx]
        pop word [bx + reg_fl]
        mov word [bx + reg_sp], sp
        mov word [bx + reg_ss], ss
        mov word [bx + reg_ax], ax
        xor ax, ax
        mov ss, ax
        mov sp, bx              ; set sp immediately after ss
        sti
        mov word [bx + reg_cx], cx
        mov word [bx + reg_dx], dx
        mov word [bx + reg_es], es
        mov word [bx + reg_si], si
        mov word [bx + reg_di], di
        mov word [bx + reg_bp], bp

        mov si, table
        ; bx -> start
loop_table:
        mov al, 32
        call disp_al
        lodsw
        call disp_al
        xchg al, ah
        call disp_al
        cmp al, 32
        jbe .next
        mov al, '='
        call disp_al
        mov ax, [bx]
        inc bx
        inc bx
        call disp_ax_hex
.next:
        cmp si, table.end
        jb loop_table

exit:
        xor ax, ax
        int 16h
        int 19h


disp_al:
        push ax
        push bx
        push bp

        mov ah, 0Eh
        mov bx, 7
        int 10h

        pop bp
        pop bx
        pop ax
        retn

disp_ax_hex:                    ; ax
                xchg al,ah
                call disp_al_hex                ; display former ah
                xchg al,ah                      ;  and fall through for al
disp_al_hex:                    ; al
                push cx
                mov cl,4                          ; ror al,4 would require 186
                ror al,cl
                call disp_al_lownibble_hex      ; display former high-nibble
                rol al,cl
                pop cx
                                                ;  and fall through for low-nibble
disp_al_lownibble_hex:
                push ax                  ; save ax for call return
                and al,00001111b                ; high nibble must be zero
                add al,'0'                      ; if number is 0-9, now it's the correct character
                cmp al,'9'
                jna .decimalnum          ; if we get decimal number with this, ok -->
                add al,7                        ;  otherwise, add 7 and we are inside our alphabet
 .decimalnum:
                call disp_al
                pop ax
                retn


        struc registerstorage
reg_ss: resw 1
reg_bp: resw 1
reg_sp: resw 1
reg_cs: resw 1
reg_ip: resw 1
reg_fl: resw 1
reg_ds: resw 1
reg_si: resw 1
reg_es: resw 1
reg_di: resw 1
reg_ax: resw 1
reg_bx: resw 1
reg_cx: resw 1
reg_dx: resw 1
        endstruc

%if registerstorage_size + start > entrypoint
 %error Entrypoint is not safe
%endif

        align 2
table:
        dw "SS"
        dw "BP"
        dw "SP"
        dw "CS"
        dw "IP"
        dw "FL"
        db 13,10
        dw "DS"
        dw "SI"
        dw "ES"
        dw "DI"
        db 13,10
        dw "AX"
        dw "BX"
        dw "CX"
        dw "DX"
        db 13,10
.end:

        times 510 - ($ - $$) db 0
        dw 0AA55h

nasm test.asm -f bin -o test.bin 组装,然后作为引导扇区加载.示例:

Assemble with nasm test.asm -f bin -o test.bin and then load as a boot sector. Example:

 -boot protocol chain test.bin
 -r
 AX=0000 BX=0000 CX=F000 DX=0000 SP=7BF0 BP=07BE SI=07BE DI=0000
 DS=0000 ES=0060 SS=0000 CS=0000 IP=7C00 NV UP DI PL ZR NA PE NC
 0000:7C00 EB58              jmp     7C5A
 -g
  SS=0000 BP=07BE SP=7BF0 CS=0000 IP=7C00 FL=0046
  DS=0000 SI=07BE ES=0060 DI=0000
  AX=0000 BX=0000 CX=F000 DX=0000
 Boot load called
 -

( -g Boot load之间的部分称为是引导扇区加载器的输出.)

(The part between -g and Boot load called is the output of the boot sector loader.)

这篇关于是什么导致该引导加载程序在硬件上失败,但在DOSBOX中失败?它显示所有寄存器的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!

09-17 15:11