我目前正在研究用 x86 NASM 程序集编写的引导加载程序，旨在从 FAT16 格式化磁盘加载内核（R.BIN）。它并没有这样做，在合并了用于调试的短消息（A、B、C、D、J、！；涉及它们的行标有星号）之后，我发现引导加载程序一直到点它完美地跳转到加载的内核，尽管它没有跳转到内核，而是似乎再次加载自己。
为了测试引导加载程序，我有一个已安装的空白映像，我一直在使用以下命令写入引导加载程序：（
dd if=LOADER.BIN of=/dev/loop0
我还复制了 R.BIN 到它）之后，我将保存这个新映像，并在虚拟盒子。
这是我的完整代码：
BITS 16
jmp main ; Jump to main bootloader
nop ; Pad out remaining bytes until boot descriptor
; Disk descriptor
OEM_name db "MARSHMAL" ; Disk label
bytes_sector dw 0x0200 ; Bytes per sector
sectors_cluster db 0x01 ; Sectors per cluster
sectors_record dw 0x0001 ; Sectors reserved for boot record
fats db 0x02 ; Number of file allocation tables
max_root_entries dw 0x0200 ; Max number of root entries
sectors dw 0x0B40 ; Number of sectors
medium_type db 0xF0 ; Type of medium (removable or fixed?)
sectors_fat dw 0x0009 ; Sectors per file allocation table
sectors_track dw 0x0012 ; Sectors per track
heads dw 0x0002 ; Number of heads
hidden_sectors dd 0x00000000 ; Number of sectors before partition
total_sectors dd 0x00000000 ; Number of sectors in medium (zero because 2B != 0)
drive_number db 0x00 ; Drive number (for BIOS int 0x13)
drive_signature db 0x00 ; NOT USED
ext_signature db 0x29 ; Extended boot signature
volume_serial dd 0x00000000 ; Volume's serial number
volume_label db "MARSHMALLOW"; Volume label
fs_type db "FAT16 " ; Filesystem type
main:
mov ax, 0x07C0
add ax, 0x0220
mov ss, ax
mov sp, 0x1000 ; 4K of stack
mov ax, 0x07C0
mov ds, ax
mov byte [drive_num], dl ; Save boot drive number
mov bx, ds
mov es, bx ; Set ES to Data Segment
mov bx, disk_buffer ; Set BX to disk buffer
mov ax, 0x13 ; Start of root = sectors_record + fats * sectors_fat = 1 + 2 * 9 = logical 19
call ls_hts ; Convert logical 19 to head, track and sector
mov al, 0x0E ; Number of sectors in root = max_root_entries * 32 / bytes_sector = 224 * 32 / 512 = 14
mov si, a ; Read root dir message*
call print_str ; Print!*
.read_disk:
int 0x13 ; BIOS disk interrupt
jnc .search_init ; If successful, get ready to search the disk
call reset_disk ; Otherwise, reset the disk
jmp .read_disk ; And retry
.search_init:
mov si, success ; Success message*
call print_str ; Print!*
mov ax, ds
mov es, ax ; Move data segment to extra segment
mov di, disk_buffer ; Location of disk buffer (ES:DI will be the location of the root entry we will be checking)
mov si, r_name ; Location of filename of R (DS:SI will be the location of the string to compare to the root entry)
mov bx, 0x00 ; Start at root entry 0
push si ; Push*
mov si, b ; Search message*
call print_str ; Print!
pop si ; Pop*
.check_entry:
mov cx, 0x0B ; Compare the first 11 bytes
push si ; Push filename location to stack
rep cmpsb ; Compare the two strings
pop si ; Restore filename location to SI
je .found_entry ; If equal, we found the root entry!
add di, 0x15 ; Otherwise, move to next entry
inc bx ; Number of next entry
cmp bx, max_root_entries ; Have we gone through all root entries?
jg .missing ; If so, R is missing
jmp .check_entry ; Otherwise, look at this next entry
.found_entry:
mov si, success ; Success message*
call print_str ; Print!*
mov ax, word [es:di+0x0F]
mov word [cluster], ax ; Move starting cluster number to our spot in memory
mov bx, disk_buffer ; ES:BX points to disk buffer
mov ax, 0x01 ; 1st FAT begins at logical sector 1
call ls_hts ; Convert to head, track and sector
mov al, sectors_fat ; Read all sectors in FAT
mov si, c ; Read FAT message*
call print_str ; Print!*
.read_fat:
int 0x13 ; BIOS disk interrupt
jnc .read_cluster ; If successful, load the first cluster of the file
call reset_disk ; Otherwise, reset the disk
jmp .read_fat ; And try again
.read_cluster:
mov si, d ; Attempt to read cluster message*
call print_str ; Print!*
mov ax, 0x2000
mov es, ax ; Segment into which we will load R
mov bx, word [buffer_pointer] ; Spot into which we will load this cluster
mov ax, word [cluster] ; Cluster to read
add ax, 0x1F ; Convert to logical sector
call ls_hts ; Convert to head, track and sector
mov al, sectors_cluster ; Read the number of sectors in 1 cluster
int 0x13 ; BIOS disk interrupt
jnc .find_next_cluster ; If successful, find the next cluster
call reset_disk ; Otherwise, reset the disk
jmp .read_cluster ; And try again
.find_next_cluster:
mov si, success ; Success message*
call print_str ; Print!*
mov ax, word [cluster] ; Location of current cluster
mov bx, 0x02 ; There are two bytes per entry in FAT16
mul bx ; The memory location of CLUSTER should fit in AL
mov si, disk_buffer ; Location of start of FAT
add si, ax ; Add the number of bytes until current cluster
mov ax, word [ds:si] ; Number of next cluster
mov word [cluster], ax ; Store this
cmp ax, 0xFFF8 ; Check whether this next cluster is an end-of-file marker
jae .jump ; If it is, we have fully loaded the kernel
jge .jump
add word [buffer_pointer], 0x0200 ; Otherwise, increment the buffer pointer a sector length
jmp .read_cluster ; And load it into memory
.jump:
mov si, loaded ; Loaded kernel message
call print_str ; Print!
mov dl, byte [drive_num] ; Make the boot drive number accessible to R
jmp 0x2000:0x0000 ; Jump to R's location!
.missing:
mov si, m_r_missing ; Display the missing message
call rsod ; Display it in a Red Screen of Death
reset_disk:
pusha ; Push register states to stack
mov ax, 0x00 ; RESET disk
mov dl, byte [drive_num] ; Boot drive number
int 0x13 ; BIOS disk interrupt
jc .disk_fail ; If failed, fatal error and reboot
popa ; Restore register states
ret ; And retry
.disk_fail:
mov si, m_disk_error ; Display the disk error message
call rsod ; Display it in a Red Screen of Death
print_str: ; Prints string pointed to by REGISTER SI to cursor location (si=str)
pusha ; Push register states to stack
mov ah, 0x0E ; BIOS will PRINT
.repeat:
lodsb ; Load next character from SI
cmp al, 0x00 ; Is this a null character?
je .ret ; If it is, return to caller
int 0x10 ; Otherwise, BIOS interrupt
jmp .repeat ; Do this again
.ret:
mov ah, 0x00 ; Read keyboard buffer
int 0x16 ; BIOS keyboard interrupt
popa ; Restore register states
ret ; Return to caller
ls_hts: ; Convert logical sector to head, track, and sector configuration for int 0x13 (AX = logical sector)
mov dx, 0x00 ; Upper word of dividend is 0
div word [sectors_track] ; Divide to find the number of tracks before this
mov cl, dl ; The remainder is the number of the sector within the track
add cl, 0x01 ; Sectors start at 1, not 0
mov dx, 0x00 ; Upper word of dividend is 0
div word [heads] ; Divide by number of heads/sides
mov dh, dl ; The remainder is the head number (it should only take up the lower half of DX)
mov ch, al ; The quotient is the track number (it should only take up the lower half of CX)
mov dl, byte [drive_num] ; Boot drive number
mov ah, 0x02 ; READ disk sectors
ret ; Return to caller
rsod: ; Red Screen of Death (SI = line to print)
mov al, 0x20 ; SPACE
mov bh, 0x00 ; Page 0
mov bl, 0x40 ; Red background
mov cx, 0x50 ; Enough to fit the screen width
.repeat:
mov ah, 0x09 ; Write character and attribute
int 0x10 ; BIOS VGA interrupt
mov ah, 0x03 ; Get cursor position
int 0x10 ; BIOS VGA interrupt
cmp dh, 0x1A ; Have we gone all the way down the screen?
jge .write ; If we have, return to caller
inc dh ; Otherwise, next row down
mov ah, 0x02 ; Set cursor position
int 0x10 ; BIOS VGA interrupt
jmp .repeat ; Do this again for the next line
.write:
mov ah, 0x02 ; Set cursor position
mov dh, 0x01 ; Row 1
mov dl, 0x03 ; Col 3
int 0x10 ; BIOS VGA interrupt
push si ; Push line to stack
mov si, fatal ; Prepare to display "FATAL" message
call print_str ; Print!
pop si ; Restore line and prepare to print it
call print_str ; Print!
mov si, press_a_key ; Prepare to display prompt
call print_str ; Print!
int 0x19 ; Reboot
data:
r_name db "R BIN" ; Filename of R
cluster dw 0x0000 ; Cluster that we are working with
buffer_pointer dw 0x0000 ; Pointer to offset of buffer
drive_num db 0x00 ; Boot drive number
fatal db "FATAL: ", 0x00 ; Fatal error message
press_a_key db "! Press a key", 0x00; Instruct the user to press a key and reboot
m_r_missing db "R missing", 0x00 ; Missing message
m_disk_error db "Disk failed", 0x00 ; Disk error message
a db "A", 0x00 ; About to read root dir*
b db "B", 0x00 ; About to search root dir*
c db "C", 0x00 ; About to read FAT*
d db "D", 0x00 ; About to attempt cluster read*
success db "!", 0x00 ; Success!*
loaded db "J", 0x00 ; Loaded R message*
times 510-($-$$) db 0x00 ; Pad remainder of boot sector
sig dw 0xAA55 ; Boot signature
disk_buffer: ; Space in memory for loading disk contents
正如我所说，引导加载程序似乎正在按计划执行所有操作，直到跳转到内核，当它在调试消息开始时重新开始并再次循环。