我目前正在修复我为加载自定义实模式 x86 内核（SYS.BIN）而编写的引导加载程序。我设法让它读取根目录和 FAT，并从文件系统加载一个小型内核，所有这些都在引导扇区内。但是，我开始使用更大的内核对其进行测试，并且似乎引导加载程序不会加载多个集群。我根据另一个类似的引导加载程序检查了我的代码，在加载多集群文件时，它似乎在有效地做同样的事情。主要区别在于我将第一个 FAT 加载到 segment 中0x3000，将根目录加载到 segment 中0x3800，以便内核可以访问它们。（我完全搞砸了分割吗？）
我可能应该提到我正在通过使用 NASM 编译来测试它，将生成的BOOT.BIN文件写入原始 32M 映像的第一个扇区，将其安装到循环设备上，复制SYS.BIN并创建该循环设备的新映像，这然后我将 QEMU 作为硬盘放入。我确信它只是加载文件的第一个集群。
特别是，我相信导致问题的代码可能在这里：
.load_cluster:
mov si, msg_load_cluster
call print_str ; Print message
mov ax, word [cluster] ; Our cluster number
sub ax, 0x0002 ; Clusters begin at #2
mul byte [sectors_cluster] ; Multiply by number of sectors
mov dx, ax ; Save in DX
call calc_root_start ; Start of root directory
add ax, 0x20 ; Root directory is 32 sectors
add ax, dx ; Add to the number of sectors
call calc_chs_ls ; Convert this Logical sector to CHS
mov ax, 0x2000
mov es, ax ; Load the kernel into this segment
mov bx, word [buffer_pointer] ; At this offset
mov ah, 0x02 ; Read disk sectors
mov al, byte [sectors_cluster] ; 1 cluster
int 0x13 ; BIOS disk interrupt
jnc .next_cluster ; If no error, set up for the next cluster
call reset_disk ; Otherwise, reset the disk
mov ah, 0x02 ; Read disk sectors
mov al, byte [sectors_cluster] ; 1 cluster
int 0x13 ; Try again
jc reboot ; If failed again, reboot
.next_cluster:
mov ax, 0x3000
mov ds, ax ; Segment where the FAT is loaded
mov si, word [cluster] ; Our cluster number
shl si, 0x1 ; There are two bytes per entry in FAT16
mov ax, word [ds:si] ; DS:SI is pointing to the FAT entry
mov word [cluster], ax ; The entry contains our next cluster
cmp ax, 0xFFF8 ; Is this the end of the file?
mov ax, 0x0200
mul word [sectors_cluster]
add word [buffer_pointer], ax ; Advance pointer by one cluster
jb .load_cluster ; If not, load next cluster
这是我的完整代码，包括 BPB：
BITS 16
jmp strict short main
nop
; BIOS Parameter Block
; This was made to match up with the BPB of a blank 32M image formatted as FAT16.
OEM db "HDOSALPH" ; OEM ID
bytes_sector dw 0x0200 ; Number of bytes per sector (DO NOT CHANGE)
sectors_cluster db 0x04 ; Number of sectors per cluster
reserved dw 0x0001 ; Number of sectors reserved for bootsector
fats db 0x02 ; Number of FAT copies
root_entries dw 0x0200 ; Max number of root entries (DO NOT CHANGE)
sectors dw 0x0000 ; Number of sectors in volume (small)
media_type db 0xF8 ; Media descriptor
sectors_fat dw 0x0040 ; Number of sectors per FAT
sectors_track dw 0x0020 ; Number of sectors per Track (It's a LIE)
heads dw 0x0040 ; Number of heads (It's a LIE)
sectors_hidden dd 0x00000000 ; Number of hidden sectors
sectors_large dd 0x00010000 ; Number of sectors in volume (large)
drive_num db 0x80 ; Drive number
db 0x00 ; Reserved byte
extended_sig db 0x29 ; Next three fields are available
serial dd 0x688B221B ; Volume serial number
label db "NATE " ; Volume label
filesystem db "FAT16 " ; Volume filesystem type
; Main bootloader code
main:
mov ax, 0x07C0 ; Segment we're loaded at
mov ds, ax
add ax, 0x0020 ; 32-paragraph bootloader
mov ss, ax
mov sp, 0x1000 ; 4K stack
mov byte [boot_drive_num], dl ; Save boot drive number
mov ah, 0x08 ; Read disk geometry
int 0x13 ; BIOS disk interrupt
mov dl, dh
mov dh, 0x00
inc dl
mov word [heads], dx ; The true number of heads
mov ch, 0x00
and ch, 0x3F
mov word [sectors_track], cx ; The true number of sectors per track
.load_fat:
mov si, msg_load
call print_str ; Print message
mov ax, 0x3000
mov es, ax ; Load FAT into this segment
mov bx, 0x0000
mov ax, word [reserved] ; First sector of FAT 1
call calc_chs_ls ; Convert to CHS address
mov ax, word [sectors_fat] ; Read the entire FAT
mov ah, 0x02 ; Read disk sectors
int 0x13 ; BIOS disk interrupt
jnc .load_root ; If no error, load the root directory
jmp reboot ; Otherwise, reboot
.load_root:
mov si, msg_load
call print_str ; Print message
mov ax, 0x3800
mov es, ax ; Load root directory into this segment
call calc_root_start ; First sector of root directory
call calc_chs_ls ; Convert to CHS address
mov ah, 0x02 ; Read disk sectors
mov al, 0x20 ; Root directory is 32 sectors (512/512 = 1)
int 0x13 ; BIOS disk interrupt
jnc .search_init ; If no error, begin searching
call reset_disk ; Otherwise, reset the disk
mov ah, 0x02 ; Read disk sectors
mov al, 0x20 ; Root directory is 32 sectors (512/512 = 1)
int 0x13 ; BIOS disk interrupt
jc reboot ; If error, reboot
.search_init:
mov si, msg_search_root
call print_str ; Print message
mov ax, 0x07C0
mov ds, ax ; The segment we are loaded at
mov ax, 0x3800
mov es, ax ; The segment the root directory is loaded at
mov di, 0x0000 ; Offset 0
mov cx, word [root_entries] ; Number of entries to look through
.check_entry:
push cx ; Save this to stack
mov cx, 0x000B ; Compare the first 11 bytes
mov si, kern_filename ; This should be the filename
push di ; Save our location
repe cmpsb ; Compare!
pop di ; Restore our location
pop cx ; Restore the remaining entries
je .found_entry ; If the filenames are the same, we found the entry!
add di, 0x0020 ; Otherwise, move to next entry
loop .check_entry ; And repeat
jmp reboot_fatal ; If we've gone through everything, it's missing
.found_entry:
mov ax, word [es:di+0x1A]
mov word [cluster], ax ; The starting cluster number
.load_cluster:
mov si, msg_load_cluster
call print_str ; Print message
mov ax, word [cluster] ; Our cluster number
sub ax, 0x0002 ; Clusters begin at #2
mul byte [sectors_cluster] ; Multiply by number of sectors
mov dx, ax ; Save in DX
call calc_root_start ; Start of root directory
add ax, 0x20 ; Root directory is 32 sectors
add ax, dx ; Add to the number of sectors
call calc_chs_ls ; Convert this Logical sector to CHS
mov ax, 0x2000
mov es, ax ; Load the kernel into this segment
mov bx, word [buffer_pointer] ; At this offset
mov ah, 0x02 ; Read disk sectors
mov al, byte [sectors_cluster] ; 1 cluster
int 0x13 ; BIOS disk interrupt
jnc .next_cluster ; If no error, set up for the next cluster
call reset_disk ; Otherwise, reset the disk
mov ah, 0x02 ; Read disk sectors
mov al, byte [sectors_cluster] ; 1 cluster
int 0x13 ; Try again
jc reboot ; If failed again, reboot
.next_cluster:
mov ax, 0x3000
mov ds, ax ; Segment where the FAT is loaded
mov si, word [cluster] ; Our cluster number
shl si, 0x1 ; There are two bytes per entry in FAT16
mov ax, word [ds:si] ; DS:SI is pointing to the FAT entry
mov word [cluster], ax ; The entry contains our next cluster
cmp ax, 0xFFF8 ; Is this the end of the file?
mov ax, 0x0200
mul word [sectors_cluster]
add word [buffer_pointer], ax ; Advance pointer by one cluster
jb .load_cluster ; If not, load next cluster
.jump:
mov si, msg_ready
call print_str ; Otherwise, we are ready to jump!
mov ah, 0x00 ; Wait and read from keyboard
int 0x16 ; BIOS keyboard interrupt
mov dl, byte [boot_drive_num] ; Provide the drive number to the kernel
jmp 0x2000:0x0000 ; Jump!
; Calculation routines
calc_root_start: ; Calculate the first sector of the root directory
push dx ; Push register states to stack
mov ax, word [sectors_fat] ; Start with the number of sectors per FAT
mov dh, 0x00
mov dl, byte [fats]
mul dx ; Multiply by the number of FATs
add ax, word [reserved] ; Add the number of reserved sectors
pop dx ; Restore register states
ret ; Return to caller
calc_chs_ls: ; Setup Cylinder-Head-Sector from LBA (AX)
mov dx, 0x0000
div word [sectors_track]
mov cl, dl
inc cl ; Sector number
mov dx, 0x0000
div word [heads]
mov dh, dl ; The remainder is the head number
mov ch, al ; The quotient is the cylinder number
mov dl, byte [boot_drive_num] ; Drive number
ret ; Return to caller
; Other routines
print_str: ; Print string in SI
pusha ; Push register states to stack
mov ax, 0x07C0
mov ds, ax ; Segment in which we are loaded
mov ah, 0x0E ; Teletype output
mov bh, 0x00 ; Page 0
.char:
lodsb ; Load next character
cmp al, 0x00 ; Is it a NULL character?
je .end ; If so, we are done
int 0x10 ; Otherwise, BIOS VGA interrupt
jmp .char ; Repeat
.end:
mov ah, 0x03 ; Get cursor position
int 0x10 ; BIOS VGA interrupt
mov ah, 0x02 ; Set cursor position
inc dh ; One row down
mov dl, 0x00 ; Far left
int 0x10 ; BIOS VGA interrupt
popa ; Restore register states
ret ; Return to caller
reset_disk: ; Reset the disk
push ax ; Push register states to stack
mov si, msg_retrying
call print_str ; Print message
mov ah, 0x00 ; Reset disk
mov dl, byte [boot_drive_num]
int 0x13 ; BIOS disk interrupt
jc reboot_fatal ; If there was an error, reboot
pop ax ; Otherwise, restore register states
ret ; Return to caller
reboot_fatal: ; Display FATAL
mov si, msg_fatal
call print_str ; Print message
reboot: ; Prompt user to press a key and reboot
mov si, msg_reboot
call print_str ; Print message
mov si, msg_ready
call print_str ; Print message
mov ah, 0x00 ; Wait and read from keyboard
int 0x16 ; BIOS keyboard interrupt
int 0x19 ; Reboot
; Data
data:
cluster dw 0x0000
buffer_pointer dw 0x0000
boot_drive_num db 0x00
msg_retrying db "RE", 0x00
msg_fatal db "FATL", 0x00
msg_reboot db "X", 0x00
msg_search_root db "Srch", 0x00
msg_load_cluster db "Clstr", 0x00
msg_ready db "GO", 0x00
msg_load db "Press a key", 0x00
kern_filename db "SYS BIN"
times 510-($-$$) db 0x00 ; Pad remainder of bootsector with zeroes
boot_sig dw 0xAA55 ; Boot signature
在此先感谢您的帮助。
更新：我在 BOCHS 调试器中运行了这个，似乎程序正在将单词加载cluster为0x0003under .load_cluster，但随后0x0000在.next_cluster几条指令下加载。