设置GDT时Qemu无限重启

问题描述

我主要使用 C++ 制作操作系统，但对于引导加载程序，我使用的是 FASM。当我尝试设置 GDT 时，Qemu 会清除屏幕并在顶部重新打印“SeaBIOS”。它会一直循环下去，直到我关闭它。这是它的gif：

我尝试使用 -nographic 运行它，但它在 Windows 控制台中执行相同的操作。

哦，是的，操作系统/版本信息。
窗户：20H2
FASM：1.73.25
Qemu：5.1.0

这是我的代码：

gdt_start:
    dd 0x00
    dd 0x00

gdt_code:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10011010b
    db 11001111b
    db 0x0

gdt_data:
     dw   0xffff
     dw   0x0
     db   0x0
     db   10010010b
     db   11001111b
     db   0x0

gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1
    dd gdt_start

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

switch_to_pm:
    cli
    lgdt [gdt_descriptor]
    mov eax,cr0
    or eax,0x1
    mov cr0,eax
    jmp CODE_SEG:init_pm

use32

init_pm:
    mov ax,DATA_SEG
    mov ds,ax
    mov ss,ax
    mov es,ax
    mov fs,ax
    mov gs,ax
    
    mov ebp,0x90000
    mov esp,ebp
    
    call BEGIN_PM

BEGIN_PM:
    mov ebx,MSG_PROT_MODE
    call print_string_pm
    call KERNEL_OFFSET
    jmp $

更新：完整代码：

; boot.asm

org 0x7c00
KERNEL_OFFSET equ 0x1000

mov [BOOT_DRIVE],dl
mov bp,0x9000
mov sp,bp

mov bx,MSG_REAL_MODE
call print
call print_nl

call load_kernel

call switch_to_pm

jmp $

print:
    pusha

start:
    mov al,[bx]
    cmp al,0
    je done
    
    mov ah,0x0e
    int 0x10
    
    add bx,1
    jmp start
    
done:
    popa
    ret

print_nl:
    pusha
    
    mov ah,0x0e
    mov al,0x0a
    int 0x10
    mov al,0x0d
    int 0x10
    
    popa
    ret

load_kernel:
    mov bx,MSG_LOAD_KERNEL
    call print
    call print_nl
    
    mov bx,KERNEL_OFFSET
    mov dh,1
    mov dl,[BOOT_DRIVE]
    call disk_load
    ret

disk_load:
    pusha
    push dx
    
    mov ah,0x02
    mov al,dh
    mov cl,0x02
    
    mov ch,0x00
    mov dh,0x00
    
    int 0x13
    jc disk_error
    
    pop dx
    cmp al,dh
    jne sectors_error
    popa
    ret

disk_error:
    mov bx,DISK_ERROR
    call print
    call print_nl
    mov dh,ah
    call print_hex
    jmp disk_loop

sectors_error:
    mov bx,SECTORS_ERROR
    call print

disk_loop:
    jmp $

print_hex:
    pusha
    mov cx,0

hex_loop:
    cmp cx,4
    je end_hex
    
    mov ax,dx
    and ax,0x000f
    add al,0x30
    cmp al,0x39
    jle step2
    add al,7

step2:
    mov bx,HEX_OUT + 5
    sub bx,cx
    mov [bx],al
    ror dx,4
    
    add cx,1
    jmp hex_loop

end_hex:
    mov bx,HEX_OUT
    call print
    popa
    ret

HEX_OUT:
    db '0x0000',0

gdt_start:
    dd 0x00
    dd 0x00

gdt_code:
    dw 0xffff
    dw 0x0
    db 0x0
    db 10011010b
    db 11001111b
    db 0x0

gdt_data:
     dw   0xffff
     dw   0x0
     db   0x0
     db   10010010b
     db   11001111b
     db   0x0

gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1
    dd gdt_start

CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

switch_to_pm:
    cli
    lgdt [gdt_descriptor]
    mov eax,MSG_PROT_MODE
    call print_string_pm
    call KERNEL_OFFSET
    jmp $

VIDEO_MEMORY equ 0xb8000
WHITE_ON_BLACK equ 0x0f

print_string_pm:
    pusha
    mov edx,VIDEO_MEMORY

print_string_pm_loop:
    mov al,[ebx]
    mov ah,WHITE_ON_BLACK
    
    cmp al,0
    je print_string_pm_done
    
    mov [edx],ax
    add ebx,1
    add edx,2
    
    jmp print_string_pm_loop

print_string_pm_done:
    popa
    ret

BOOT_DRIVE db 0
MSG_REAL_MODE db "Started in 16-bit real mode",0
MSG_LOAD_KERNEL db "Loading kernel into memory",0
DISK_ERROR db "Disk read error",0
SECTORS_ERROR db "Incorrect number of sectors read",0
MSG_PROT_MODE db "Loaded 32-bit protected mode",0

times 510-($-$$) db 0
dw 0xAA55

; loader.asm

format ELF

extrn main

public _start

_start:
    call main
    jmp $

// kernel.cpp
void main() {
    char* video_memory = (char*) 0xb8000;
    *video_memory = 'X';
}

rem compile.bat

@echo off

<NUL set /p="Cleaning binaries..."
    del *.bin > NUL
    del *.o > NUL
    del *.elf > NUL
echo Done

<NUL set /p="Compiling boot.asm..."
    fasm boot.asm > NUL
echo Done

<NUL set /p="Compiling loader.asm..."
    fasm loader.asm > NUL
echo Done

<NUL set /p="Compiling kernel.c..."
    wsl gcc -m32 -ffreestanding -c kernel.cpp -o kernel.o
echo Done

<NUL set /p="Linking..."
    wsl objcopy kernel.o -O elf32-i386 kernel.elf
    wsl /usr/local/i386elfgcc/bin/i386-elf-ld -o kernel.bin -Ttext 0x1000 loader.o kernel.elf
    type boot.bin kernel.bin > os_image.bin
echo Done

<NUL set /p="Running..."
    qemu-system-i386 os_image.bin
echo Done

解决方法

问题不在于代码，而在于您如何构建它。这个序列实际上创建了一个名为 kernel.bin 的 ELF 可执行文件：

wsl objcopy kernel.o -O elf32-i386 kernel.elf
wsl /usr/local/i386elfgcc/bin/i386-elf-ld -o kernel.bin -Ttext 0x1000 loader.o kernel.elf

应该是：

wsl /usr/local/i386elfgcc/bin/i386-elf-ld -o kernel.elf -Ttext 0x1000 loader.o kernel.o
wsl objcopy -O binary kernel.elf kernel.bin

此更改将目标文件链接到 ELF 可执行文件 kernel.elf，然后将 ELF 可执行文件转换为可由引导加载程序加载到地址 0x01000 的二进制文件 kernel.bin。