为什么我的根目录没有被加载? (FAT12) [英] Why isn't my root directory being loaded? (FAT12)
问题描述
我正在编写汇编中的Stage 1引导加载程序，以尝试将FAT12文件系统加载到内存中，以便可以加载我的Stage 2引导加载程序.我已经设法将FAT加载到内存中，但是我正在努力将根目录加载到内存中.
I am writing a stage 1 bootloader in assembly with which I am attempting to load the FAT12 filesystem into memory so that I can load my stage 2 bootloader. I have managed to load the FATs into memory, however I am struggling to load the root directory into memory.
我目前正在使用此作为参考，并产生了以下内容:
I am currently using this for reference and have produced the following:
.load_root:
;es is 0x7c0
xor dx, dx ; blank dx for division
mov si, fat_loaded ; inform user that FAT is loaded
call print
mov al, [FATcount] ; calculate how many sectors into the disk must be loaded
mul word [SectorsPerFAT]
add al, [ReservedSectors]
div byte [SectorsPerTrack]
mov ch, ah ; Store quotient in ch for cylinder number
mov cl, al ; Store remainder in cl for sector number
xor dx, dx
xor ax, ax
mov al, ch ; get back to "absolute" sector number
mul byte [SectorsPerTrack]
add al, cl
mul word [BytesPerSector]
mov bx,ax ; Memory offset to load to data into memory after BOTH FATs (should be 0x2600, physical address should be 0xA200)
xor dx, dx ; blank dx for division
mov ax, 32
mul word [MaxDirEntries]
div word [BytesPerSector] ; number of sectors root directory takes up (should be 14)
xor dh, dh ; head 0
mov dl, [boot_device] ; boot device
mov ah, 0x02 ; select read mode
int 13h
cmp ah, 0
je .load_OS
mov si, error_text
call print
jmp $
但是，如果我用gdb检查0xA200处的内存，我只会看到0.我的根目录没有包含一个文件-我已经在根目录中放置了一个名为OS.BIN的文件进行测试.
However, if I inspect the memory at 0xA200 with gdb, I just see 0s. My root directory does contain a file -- I have put a file called OS.BIN in the root directory to test with.
在读取操作之后在gdb中使用info registers会给出以下输出:
Using info registers in gdb after the read operation gives the following output:
eax 0xe 14
ecx 0x101 257
edx 0x0 0
ebx 0x2600 9728
esp 0x76d0 0x76d0
ebp 0x0 0x0
esi 0x16d 365
edi 0x0 0
eip 0x7cdd 0x7cdd
eflags 0x246 [ PF ZF IF ]
cs 0x0 0
ss 0x53 83
ds 0x7c0 1984
es 0x7c0 1984
fs 0x0 0
gs 0x0 0
操作状态为0，读取的扇区数为14，es:bx指向0xA200，但是当我希望看到OS.BIN的数据时，x/32b 0xa200显示32个0.
The status of the operation is 0, the number of sectors read is 14, and es:bx points to 0xA200, but x/32b 0xa200 shows 32 0s, when I would expecting to see the data for OS.BIN.
编辑
我在中断之前做了info registers，输出如下:
EDIT
I did info registers before the interrupt and the output is the following:
eax 0x20e 526
ecx 0x101 257
edx 0x0 0
ebx 0x2600 9728
esp 0x76d0 0x76d0
ebp 0x0 0x0
esi 0x161 353
edi 0x0 0
eip 0x7cc8 0x7cc8
eflags 0x246 [ PF ZF IF ]
cs 0x0 0
ss 0x53 83
ds 0x7c0 1984
es 0x7c0 1984
fs 0x0 0
gs 0x0 0
除了功能请求号已替换为状态码之外，其他与之后相同.
Which is the same as after, except the function request number has been replaced with the status code.
我要去哪里错了?我是从错误的CHS地址读取吗?还是其他一些简单的错误?而我该如何纠正呢?
Where am I going wrong? Am I reading from the wrong CHS address? Or some other simple mistake? And how can I correct this?
我正在使用fat_imgen制作磁盘映像.用于创建磁盘映像的命令是fat_imgen -c -f floppy.flp -F -s bootloader.bin，用于将OS.BIN添加到该映像的命令是fat_imgen -m -f floppy.flp -i OS.BIN
I am using fat_imgen to make my disk image. Command for creating the disk image is fat_imgen -c -f floppy.flp -F -s bootloader.bin and command for adding OS.BIN to the image is fat_imgen -m -f floppy.flp -i OS.BIN
我有一个 BIOS参数块(BPB)，它表示使用FAT12的1.44MB软盘:
I have a BIOS Parameter Block (BPB) that represents a 1.44MB floppy using FAT12:
jmp short loader
times 9 db 0
BytesPerSector: dw 512
SectorsPerCluster: db 1
ReservedSectors: dw 1
FATcount: db 2
MaxDirEntries: dw 224
TotalSectors: dw 2880
db 0
SectorsPerFAT: dw 9
SectorsPerTrack: dw 18
NumberOfHeads: dw 2
dd 0
dd 0
dw 0
BootSignature: db 0x29
VolumeID: dd 77
VolumeLabel: db "Bum'dOS ",0
FSType: db "FAT12 "
我还有另一个似乎可以正常工作的功能，可以将FAT12表加载到内存地址0x7c0:0x0200(物理地址0x07e00):
I have another function that appears to work that loads the FAT12 table to memory address 0x7c0:0x0200 (physical address 0x07e00):
;;;Start loading File Allocation Table (FAT)
.load_fat:
mov ax, 0x07c0 ; address from start of programs
mov es, ax
mov ah, 0x02 ; set to read
mov al, [SectorsPerFAT] ; how many sectors to load
xor ch, ch ; cylinder 0
mov cl, [ReservedSectors] ; Load FAT1
add cl, byte 1
xor dh, dh ; head 0
mov bx, 0x0200 ; read data to 512B after start of code
int 13h
cmp ah, 0
je .load_root
mov si, error_text
call print
hlt
推荐答案
在加载FAT之后，我最终取消了加载根目录.最后，我修改了.load_fat例程，以同时加载FATs 和根目录(本质上是在引导扇区之后读取32个扇区，但是仍然可以让我轻松地修改磁盘几何形状).
I ended up scrapping loading the root directory after loading the FATs. In the end, I modified my .load_fat routine to load both FATs and the root directory at the same time (essentially reading 32 sectors after the boot sector, but in a way that still allows me to easily modify the disk geometry).
此代码如下:
.load_fat:
mov ax, 0x07c0 ; address from start of programs
mov es, ax
mov al, [SectorsPerFAT] ; how many sectors to load
mul byte [FATcount] ; load both FATs
mov dx, ax
push dx
xor dx, dx ; blank dx for division
mov ax, 32
mul word [MaxDirEntries]
div word [BytesPerSector] ; number of sectors for root directory
pop dx
add ax, dx ; add root directory length and FATs length -- load all three at once
xor dh,dh
mov dl, [boot_device]
xor ch, ch ; cylinder 0
mov cl, [ReservedSectors] ; Load from after boot sector
add cl, byte 1
xor dh, dh ; head 0
mov bx, 0x0200 ; read data to 512B after start of code
mov ah, 0x02 ; set to read
int 13h
cmp ah, 0
je .load_root
mov si, error_text
call print
hlt
尽管不是我打算解决问题的方式，但它确实可以完成工作，我可以从此继续进行下去.
Though not the way I intended to solve the problem, it does the job and I can move on from this to continue development.
编辑
无论如何，我认为我已经弄清楚了旧代码出了什么问题.我在扇区18之后增加了 cylinder ，当时我应该一直增加头部.原因是CHS，而不是HCS！
I think I worked out where the old code was going wrong, anyway. I was incrementing the cylinder after sector 18, when I should have been incrementing the head. It's CHS, not HCS, for a reason!
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