Bootloader无法加载内核且无错误输出的技术求助

问题描述 #

我用汇编编写了一个Bootloader，它无法加载内核，也不会输出任何错误信息。

Bootloader代码 #

org 7C00h ;start at
bits 16 ;16bit real mode

%define endl 0Dh, 0Ah

jmp strict short setup ;go back to your place bitch
nop

; FAT12 HEADER

bdb_oem:                    db 'MSWIN4.1'           ; 8 bytes
bdb_bytes_per_sector:       dw 512
bdb_sectors_per_cluster:    db 1
bdb_reserved_sectors:       dw 1
bdb_fat_count:              db 2
bdb_dir_entries_count:      dw 0E0h
bdb_total_sectors:          dw 2880                 ; 2880 * 512 = 1.44MB
bdb_media_descriptor_type:  db 0F0h                 ; F0 = 3.5" floppy disk
bdb_sectors_per_fat:        dw 9                    ; 9 sectors/fat
bdb_sectors_per_track:      dw 18
bdb_heads:                  dw 2
bdb_hidden_sectors:         dd 0
bdb_large_sector_count:     dd 0

; EBR SECTION

ebr_drive_number:           db 0                    ; 0x00 floppy, 0x80 hdd, useless
ebr_useless:                db 0                    ; reserved, windows stuff, i guess
ebr_signature:              db 29h
ebr_volume_id:              db 12h, 34h, 56h, 78h   ; serial number, value doesn't matter
ebr_volume_label:           db 'GOLD OS    '        ; 11 bytes, padded with spaces
ebr_system_id:              db 'FAT12   '           ; 8 bytes

; END OF HEADER

;MAIN AREA

;setup registers and stack
setup:
    ;setup registers
    mov ax, 0 ;clear ax
    mov ds, ax ;clear ds by copying ax
    mov es,ax ;clear es by copying ax

    ;setup stack
    mov ss, ax ;clear ss by copying ax
    mov sp, 0x7c00 ;set stack to work without overwriting our stuff

    ;set disk number
    mov [ebr_drive_number], dl

    jmp kernel_find

;says shit
;PARAMS: si - String to print.
;RETURNS: nothing
talk:
    ;save registers to stack
    push si
    push ax

.loop:
    lodsb ;load next character in al
    or al, al ;verify if next character is null
    jz .done ;if null then complete

    ;BIOS INTERRUPT, SEE RESOURCES.md
    mov ah, 0eh ;print character
    mov bh, 0 ;page 0
    int 10h ;INTERRUPT VIDEO SERVICE

    jmp .loop ;restart
.done:
    pop ax
    pop si
    ret

;reboot function
reboot:
    jmp 0FFFFh:0 ;jump to beginning of bios

;DISK AREA

;converts LBA(logical block addressing) address to CHS(cylinder, head, sector) address
;PARAMS: ax - LBA Address.
;RETURNS: cx[0-5] - Sector number.
;       : cx[6-15] - Cylinder.
;       : dh - Head.
disk_lba_chs_conversion:

    ;save registers that are used, but are not part of param or input
    push ax
    push dx

    xor dx, dx ;clear dx register
    div word [bdb_sectors_per_track] ;ax = LBA Address / Sectors per Track
                                     ;dx = LBA Address % Sectors per Track
    inc dx ;dx = the previous operation + 1 = sector
    mov cx, dx ;send to the correct output register

    xor dx, dx ;clear dx
    div word [bdb_heads] ;ax = (address / sectors per track) / Heads = cylinder
                         ;dx = (address / sectors per track) % Heads = head

    mov dh, dl ;send value to output location
               ;dl = lower 8 bits of dx
    mov ch, al ;ye same shit
    shl ah, 6 ;shift 6 bits to left
    or cl, ah ;move the data, don't replace everything

    ; restore registers
    pop dx
    pop ax

    ret



floppy_error:
    mov si, ferror_read_failed
    call fatal_reboot

;reads the disk
;PARAMS: ax - LBA Address.
;      : cl - nuber of sectors to read.
;      : dl - drive number.
;      : es:bx - memory address to store the data.
;RETURNS: (relative) - data read.
disk_read:

    pusha                        ; save registers we will modify

    push cx                             ; temporarily save CL (number of sectors to read)
    call disk_lba_chs_conversion                   ; compute CHS
    pop ax                              ; AL = number of sectors to read
    
    mov ah, 02h
    mov di, 3                           ; retry count

.retry:
    pusha                               ; save all registers, we don't know what bios modifies
    stc                                 ; set carry flag, some BIOS'es don't set it
    int 13h                             ; carry flag cleared = success
    popa
    jnc .done                           ; jump if carry not set

    ; read failed
    call disk_reset

    dec di
    test di, di
    jnz .retry

.fail:
    ; all attempts are exhausted
    jmp floppy_error

.done:
    popa                           ; restore registers
    ret

disk_reset:
    pusha
    mov ah, 0
    stc
    int 13h
    jc floppy_error
    popa
    ret

;KERNEL LOAD AREA

KERNEL_LOAD_SEGMENT equ 0x2000
KERNEL_LOAD_OFFSET equ 0

kernel_cluster: dw 0

;just gets the size of the root directory
kernel_find:

    ;get root directory address
    mov ax, [bdb_sectors_per_fat]
    mov bl, [bdb_fat_count]
    xor bh, bh
    mul bx
    add ax, [bdb_reserved_sectors]
    push ax ; 保存根目录起始LBA

    ;get size of root directory
    mov ax, [bdb_dir_entries_count] ; 修复：用目录条目数代替FAT扇区数
    shl ax, 5 ; 每个条目32字节，左移5等价于*32
    xor dx, dx
    div word [bdb_bytes_per_sector]
    test dx, dx
    jz .rdsdone
    inc ax
    .rdsdone:

    ;read root diretory
    mov cl, al
    pop ax
    mov dl, [ebr_drive_number]
    mov bx, buffer
    call disk_read


    ;find the kernel
    xor bx, bx
    mov di, buffer

    .kernelsearch:
    mov si, file_kernel
    mov cx, 11
    push di
    repe cmpsb
    pop di
    je .kernelfound

    add di, 32
    inc bx
    cmp bx, [bdb_dir_entries_count]
    jl .kernelsearch
    mov si, ferror_no_kernel
    call talk
    call fatal_reboot

    .kernelfound:
    mov ax, [di + 26]
    mov [kernel_cluster], ax

    ;get FAT table
    mov ax, [bdb_reserved_sectors]
    mov bx, buffer
    mov cl, [bdb_sectors_per_fat]
    mov dl, [ebr_drive_number]
    call disk_read

    mov bx, KERNEL_LOAD_SEGMENT
    mov es, bx
    mov bx, KERNEL_LOAD_OFFSET

    ; 重新计算根目录起始LBA和大小，用于簇转LBA
    mov ax, [bdb_sectors_per_fat]
    mov bl, [bdb_fat_count]
    xor bh, bh
    mul bx
    add ax, [bdb_reserved_sectors]
    mov cx, ax ; cx = 根目录起始LBA
    mov ax, [bdb_dir_entries_count]
    shl ax,5
    xor dx,dx
    div word [bdb_bytes_per_sector]
    test dx,dx
    jz .skip_inc
    inc ax
.skip_inc:
    mov dx, ax ; dx = 根目录占用扇区数

    .kernelload:
    mov ax, [kernel_cluster]
    sub ax, 2 ; 簇号从2开始，减去2得到偏移
    mov bl, [bdb_sectors_per_cluster]
    xor bh, bh
    mul bx
    add ax, cx ; 加上根目录起始LBA
    add ax, dx ; 加上根目录占用扇区数，得到内核数据的起始LBA

    mov cl, 1
    mov dl, [ebr_drive_number]
    call disk_read

    add bx, [bdb_bytes_per_sector]

    mov ax, [kernel_cluster]
    mov cx, 3
    mul cx
    mov cx, 2
    div cx

    mov si, buffer
    add si, ax
    mov ax, [ds:si]

    or dx, dx
    jz .even

    .odd:
    shr ax, 4
    jmp .continue

    .even:
    and ax, 0x0FFF

    .continue:
    cmp ax, 0x0FF8
    jae .finishreading 

    mov [kernel_cluster], ax
    jmp .kernelload

    .finishreading:
    mov dl, [ebr_drive_number]
    mov ax, KERNEL_LOAD_SEGMENT

    jmp KERNEL_LOAD_SEGMENT:KERNEL_LOAD_OFFSET

    mov si, ferror_kernel_jump_failed
    call fatal_reboot
        
    ret
;STRING AREA

file_kernel: db "KERNEL  BIN"

ferror_no_kernel: db "FATAL ERROR: Kernel Not Found", endl, 0

ferror_read_failed: db "FATAL ERROR: Disk read operation Failed", endl, 0 ;message for when it can't read the disk.

ferror_kernel_jump_failed: db "FATAL ERROR: Jump to Kernel Failed"

; MISC AREA

;reboots in case of fatal error
;PARAMS: si - error string
fatal_reboot:
    call talk ;here just to gain a little bit of space
    ;BIOS INTERRUPT, SEE RESOURCES.md
    mov ah, 0 ;wait for keypress
    int 16h ;INTERRUPT KEYBOARD SERVICE
    jmp reboot
    ret


times 510-($-$$) db 0 ;nullify the rest of the 512 bytes we can use
dw 0AA55h ;bootloader signature, DO NOT TOUCH, DON'T EVEN THINK ABOUT IT.
;everything beyond this point will not be protected and can be overwritten by the stack, write at your own risk.
buffer: ;label to unused space

内核代码（编译为kernel.bin） #

%define endl 0Dh, 0Ah

jmp main
;says shit
;PARAMS: si - String to print.
;RETURNS: nothing
talk:
    ;save registers to stack
    push si
    push ax

.loop:
    lodsb ;load next character in al
    or al, al ;verify if next character is null
    jz .done ;if null then complete

    ;BIOS INTERRUPT, SEE RESOURCES.md
    mov ah, 0eh ;print character
    mov bh, 0 ;page 0
    int 10h ;INTERRUPT VIDEO SERVICE

    jmp .loop ;restart
.done:
    pop ax
    pop si
    ret

main:
    mov si, str_hello
    call talk
    jmp $ ; 添加死循环，防止执行完后跑飞

str_hello: db "do u hav som ppsi", endl, 0 ; 修复：把endl放在终止符前，确保换行被打印

已尝试的操作 #

• 检查并更换过验证有效的Makefile
• 替换过已知可用的kernel_find代码段
• 问题依旧存在

关键错误修复点 #

1. 根目录大小计算错误：原代码错误使用bdb_sectors_per_fat计算根目录大小，应改为bdb_dir_entries_count，根目录大小由条目数量（每个条目32字节）和扇区大小决定。
2. LBA转CHS函数的寄存器恢复错误：原函数最后错误修改了dl寄存器（驱动器号），导致后续磁盘操作使用错误驱动器，正确恢复应为pop dx和pop ax，不修改dl。
3. 簇转LBA计算错误：原代码硬编码add ax,31，应使用标准公式：LBA = (簇号 - 2) * 每簇扇区数 + 根目录起始LBA + 根目录占用扇区数，确保准确定位内核数据扇区。
4. 内核字符串格式错误：原内核将endl放在终止符0之后，导致换行符无法被打印，需调整顺序为db "内容", endl, 0。
5. 内核缺少死循环：原内核执行完打印后会执行内存中随机代码，添加jmp $让内核停在打印后的状态。

内容的提问来源于stack exchange，提问作者Golden Star