我目前正在研究用x86 NASM程序集编写的引导程序，该引导程序旨在从FAT16格式化的磁盘加载内核（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，！；涉及它们的标有星号的行）之后，我发现引导加载程序一直贯穿到它可以完美地跳转到已加载的内核，尽管它不会跳转到内核，而是看起来可以再次自行加载。
要测试引导加载程序，我有一个已装载的空白映像，该映像已通过使用以下命令写入引导加载程序：
dd if=LOADER.BIN of=/dev/loop0
（我也将R.BIN复制到了它）
之后，我将保存此新图像，并在Virtualbox中对其进行测试。
这是我完整的代码：
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
正如我说的那样，引导加载程序似乎正在按计划进行所有操作，直到跳转到内核为止，直到它在调试消息的开头重新开始并再次循环。
答案 0 :(得分：2)
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
最好写repe cmpsb以便更好地表明您的平等性！
在 .check_entry 中，您始终将21添加到DI中。这是不对的！如果repe cmpsb报告不等于，那么DI可能在任何地方。您需要将DI放回到32字节记录的开头，然后再添加32以转到下一个根记录。
.check_entry:
mov cx, 0x0B ; Compare the first 11 bytes
push si di ; Push filename location to stack
repe cmpsb ; Compare the two strings
pop di si ; Restore filename location to SI
je .found_entry ; If equal, we found the root entry!
add di, 0x20 ; Otherwise, move to next entry
inc bx ; Number of next entry
cmp bx, max_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, [es:di+0x1A]
mov [cluster], ax
请注意更新您在此后创建的根记录中的所有引用。
在上面，我已经将[es:di+0x0F]更改为现在正确的格式[es:di+0x1A]。
答案 1 :(得分：1)
cmp bx, max_root_entries ; Have we gone through all root entries? mov al, sectors_fat ; Read all sectors in FAT mov al, sectors_cluster ; Read the number of sectors in 1 cluster
以上所有都是错误的。您正在使用NASM，因此需要写方括号才能从内存中获取内容。没有括号，您将获得地址本身而不是内容。
cmp bx, [bpbRootEntries]
...
我的FAT文件系统规范提到：
不存在少于4085个簇的FAT16卷... ...如果您尝试制作违反此规则的FAT卷，则Microsoft操作系统将无法正确处理它们，因为他们会认为该卷具有与您认为的类型不同的FAT。
您使用的磁盘具有2880个扇区，因此群集太少，无法识别为FAT16。也许您的Linux操作系统会很好地识别它，但也许您的mount命令应该使用fat=16之类的选项。
那些相同的2880个扇区，并且看到bpbSectorsPerCluster = 0x01，将要求FAT分别为12个扇区。尽管如此，我仍然看到bpbFATSize16 = 0x0009，这是我期望的FAT 12 值。
引导扇区和BPB 结构具有程序应使用的大量信息，而不是依赖无法正确初始化的常量。 不过，为了简单起见，我将继续使用这些（校正后的）常量！
FirstFATSecNum = bpbHiddenSectors + bpbReservedSectors
= 0x00000000 + 0x0001
= 1
FirstRootDirSecNum = FirstFATSecNum + bpbNumberOfFATs * bpbFATSize16
= 1 + 0x02 * 0x0009
= 19
RootDirSectors = ((bpbRootEntries * 32) + (bpbBytesPerSector - 1)) / bpbBytesPerSector
= ((0x0200 * 32) + (0x200 - 1)) / 0x200
= 32
FirstDataSecNum = FirstRootDirSecNum + RootDirSectors
= 19 + 32
= 51
请务必注意，RootDir占用32个扇区或16384个字节！您的程序已设置了仅8192字节的DiskBuffer。这是基于您错误的假设，即RootDir包含224个条目，即FAT 12 的正常计数。
可以将转换逻辑扇区号的例程更恰当地命名为“ SetupCHS”。这更好地表达了它所做的不仅仅是简单的转换，而且还强调了圆柱，标头和扇形之间的相对重要性。例如将其与HMS进行比较，以小时，分钟和秒为单位。因此，从最高有效（C）到最低有效（S）。
; IN (ax) OUT (cx,dx) MOD (ax)
SetupCHS:
cwd
div word [bpbSectorsPerTrack]
mov cl, dl
inc cx ; Sector number
cwd
div word [bpbNumberOfHeads]
mov dh, dl ; Head number
mov ch, al ; Cylinder number
mov dl, [drive_num] ; Drive number
ret
为清楚起见，您不应在此处的AH中设置功能编号，而应靠近int 0x13指令的位置！
许多BIOS一次无法读取/写入多个扇区，特别是如果它们必须跨越某个边界（无论是另一个磁头还是另一个柱面）。
这就是为什么大多数谨慎的程序员将使用1扇区读/写循环的原因。
; Loading the RootDir
mov bx, DiskBuffer ; ES:BX
mov ax, 19 ; FirstRootDirSecNum
call SetupCHS ; -> CX DX (AX)
mov bp, 32 ; RootDirSectors
.Next:
call ReadOneSector ; -> (AX DI)
add bx, [bpbBytesPerSector]
dec bp
jnz .Next
由于有时出于某些无害的原因磁盘访问可能会失败，因此我们将操作重复几次。 5是一个不错的选择。您的程序选择无限次重复，但我觉得这很可疑。
; IN (es:bx,cx,dx) OUT () MOD (ax,di)
ReadOneSector:
mov di, 5
.ReTry:
mov ax, 0x0201 ; BIOS.ReadSector
int 0x13 ; -> CF
jnc .OK
dec di
jz DiskFail
call ResetDisk
jmp .Retry
.OK:
ret
加载第一个FAT当然是一个类似的过程。
对于加载文件群集，您很幸运，因为每个群集在该磁盘上只有一个扇区。无需循环。
搜索RootDir。 Fifoernik已经告诉您使用rep cmpsb的危险。使用rep而不是repe已经使您认为DI寄存器总是提前11，实际上重复可以更早结束。
这里的其他问题是：
您不测试RootDir条目的第一个字节。它具有必须检查的重要信息。如果它是0，则您已经到达RootDir的末尾，继续评估条目是没有用的。如果它是0xE5，则该条目是免费的，您应该跳过它。
您不测试属性字节。该条目很可能是针对卷ID的目录的。既不是您要查找的文件，请跳过该条目！
下一步适用以上条件：
mov di, DiskBuffer ; ES:DI but knowing that ES=DS
mov bx, [bpbRootEntries]
.CheckEntry:
cmp byte [di], 0
je .FileNotFound
cmp byte [di], 0xE5
je .SkipEntry
test byte [di+11], 00011000b ; DIR | VOL
jnz .SkipEntry ; Is not a file
mov si, r_name
mov cx, 11
push di
repe cmpsb
pop di
je .FileFound
.SkipEntry:
add di, 32
dec bx ; Counting downward is easier
jnz .CheckEntry
.FileNotFound
jmp Missing
.FileFound:
mov ax, [di+0x1A]
mov [cluster], ax
跟随集群链。下面是将簇号N转换为其第一个/唯一扇区的扇区号的公式：
FirstSectorOfCluster = FirstDataSecNum + (N - 2) * bpbSectorsPerCluster
= 51 + (N - 2) * 0x01
= N + 49
由于奇怪的jge .jump，程序的这一部分将永远不会加载文件的1个以上的簇。所有 good 群集编号将为 Greater 。当cmp ax, 0xFFF8指令以有符号方式（jge所做的）解释时，将读取cmp ax, -8。因此，从2到几千的所有簇数都将更大。
提示：为了能够加载大文件（大于64KB），您应该更改ES段寄存器，并将偏移量BX保持为0。
mov ax, 0x2000
xor bx, bx
LoadNextCluster:
add ax, bx
mov es, ax
xor bx, bx
mov ax, [cluster]
add ax, 49
call SetupCHS ; -> CX DX (AX)
call ReadOneSector ; -> (AX DI)
mov si, [cluster]
shl si, 1 ; You don't need MUL to calculate x2
mov ax, [DiskBuffer+si] ; FAT gives number of next cluster
mov [cluster], ax
cmp ax, 0xFFF8 ; End-of-file marker ?
mov ax, es
mov bx, 512 / 16
jb LoadNextCluster
Loaded: ; We have fully loaded the kernel
程序中与BIOS显示功能有关的部分有其自身的一些问题，但我认为目前这不是很重要。
如果您设法编辑所有这些更改，但仍然没有得到结果，则可以对修订后的程序发布后续问题。 （如果您今天做的话，我可以再看看...）
永远不要将从答案中获得的信息整合到原始问题中，除非您当然将其写在单独的部分中，但是仍然要这样做。重大更改要求跟进问题（单独发布）。