制作带MBR引导扇区的FAT分区：FAT16软盘镜像挂载失败求助

Let's break down why your floppy image isn't mounting as FAT16—there are a couple of critical structural issues in your BPB layout that are throwing off filesystem parsers:

1. Misaligned BPB Due to Short Jump Instruction #

The FAT specification requires a 3-byte jump instruction at the start of the bootsector, followed immediately by the 8-byte OEM name at offset 0x003. Your code uses a 2-byte jmp short start, which shifts the entire BPB (and all subsequent fields) by 1 byte. Filesystem tools expect BPB fields at specific offsets, so this misalignment makes them read invalid values entirely.

Fix: Add a nop after the short jump to fill the 3-byte requirement:

BITS 16
jmp short start
nop ; <-- Add this to make the jump instruction 3 bytes total
iOEM db "ShapeOS" ; Now starts at offset 0x003, as required

2. Incorrect Volume Label Length #

The acVolumeLabel field must be exactly 11 bytes long (padded with spaces if needed). Your current "MYVOLUME " only uses 9 bytes, which causes the following acFSType field to be misaligned. Parsers rely on these fixed-length fields to identify the filesystem type correctly.

Fix: Extend the volume label to 11 bytes:

acVolumeLabel db "MYVOLUME   " ; 8 letters + 3 spaces = 11 bytes total

3. (Minor) Uninitialized Boot Drive Field #

While this won't break mounting, you should capture the boot drive number from the DL register (BIOS passes this value to the bootloader on startup) to make your code more robust for real hardware:

start:
cli
mov [iBootDrive], dl ; Save BIOS-provided drive number
mov ax, 0x07C0
; ... rest of your setup code ...

Modified Working Bootsector Code #

Here's the corrected version of your code with all fixes applied:

BITS 16
jmp short start
nop ; Fix 1: 3-byte jump instruction
iOEM db "ShapeOS"
iSectSize dw 0x200 ; bytes / sector
iClustSize db 1 ; 1 sector per cluster (for simplicity)
iResCnt dw 1 ; number of reserved sectors
iFatCnt db 2 ; # of fat copies
iRootSize dw 224 ; size of root dir (number of entries)
iTotalSect dw 2880 ; total sectors
iMedia db 0xF0 ; media descriptor
iFatSize dw 9 ; size of each FAT
iTrackSect dw 9 ; sectors per track
iHeadCnt dw 2 ; number of r/w heads
iHiddentSect dd 0 ; number of hidden sectors
iSect32 dd 0 ; number of > 32MB sectors
iBootDrive db 0 ; holds drive of bootsector
iReserved db 0 ; empty reserved attribute
iBootSign db 0x29 ; extended bootsig
iVolID db "seri" ; disk serial
acVolumeLabel db "MYVOLUME   " ; Fix 2: 11-byte volume label
acFSType db "FAT16 " ; fs type (8 bytes)
start:
cli
mov [iBootDrive], dl ; Fix 3: Capture boot drive number
mov ax, 0x07C0
add ax, 288
mov ss, ax
mov sp, 4096
mov ax, 0x07C0
mov ds, ax
sti
call clear_screen
mov si, intro
call puts16
mov si, loadmsg
call puts16
mov si, failed
call puts16
jmp $
intro db 'Shaper Bootloader, written by KingLuigi4932 and Kerndever', 0xD, 0xA, 0
loadmsg db 'Loading kernel... ', 0
failed db 'Failed!', 0xD, 0xA, 0
puts16:
mov ah, 0Eh
.repeat:
lodsb
cmp al, 0
je .done
int 10h
jmp .repeat
.done:
ret
clear_screen:
mov AX, 1003h
mov BL, 00h
int 10h ; Clear screen
;; Set mode to clear screen for all bioses
mov AH, 00h
int 10h
;; Fill screen with blue background
mov AH, 09h
mov AL, 20h
mov BH, 00h
mov BL, 1Eh
mov CX, 2000h
int 10h
ret
times 510-($-$$) db 0
dw 0xAA55

Testing the Fixed Image #

Rebuild your bootloader and rewrite the image with your existing commands:

nasm -f bin bootloader.asm -o bootloader.bin
dd if=/dev/zero of=fat.img bs=512 count=2880
dd status=noxfer conv=notrunc if=bootloader.bin of=fat.img

You should now be able to mount the image (e.g., with mount -o loop fat.img /mnt/floppy on Linux) as a valid FAT16 filesystem.

内容的提问来源于stack exchange，提问作者user8735693