Reading FAT bs

[BurtonBenson]

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[BrightLight]

How are you reading the floppy drive? Do you have a floppy driver/BIOS interrupts? Does your floppy driver work properly?

[~]

first of all I am new to OSDev started a few weeks ago just for fun and because I think it is a very interesting subject. the things I tried so far worked.
I did build a very simple kernel that printed some information on the screen. then I looked up on how to read a FAT table and that is where it did go wrong.

From what I understand is that the first 512bytes form a floppy get loaded to RAM 0x0000:7C00.
But when I read that part some bytes are not as expected.
To read the first FAT I should read 36 bytes form the 4 byte. (0x0000:7C03 - 0x0000:7C27).

This is what i would expect (hex editor on floppy image):
6D 6B 66 73 2E 66 61 74 00 02 01 01 00 02 E0 00 40 0B F0 09 00 12 00 02
This is what i got form RAM:
6D 00 12 00 00 00 02 00 00 00 50 00 00 00 06 00 40 0B F0 09 00 12 00 02

Something definitely messes them um but I can't why

The first hex string says "mkfs.fat" from offset 0. It wouldn't boot properly.

The second one is mostly gibberish.

You must make sure that you can read the floppy normally from DOS, Windows or Linux, and that the following fields have correct standard values at the very start of the Master Boot Record apart from good code to execute.



This is what the boot sector of a floppy contains:

Code: Select all

_00h_jmp: db 0EBh                             ;jmps 003E
          db 03Ch                             ;2  (2)
_02h_nop: nop                                 ;1  (3)

_03h_OEMid               db    "OEMIdent"      ;8  (11)
_0Bh_bytesPerSect        dw         0200h      ;2  (13)
_0Dh_sectsPerClust       db          001h      ;1  (14)
_0Eh_reservedSects       dw         0001h      ;2  (16)
_10h_numOfFATs           db          002h      ;1  (17)
_11h_numRootDirEntries   dw         00E0h      ;2  (19)
_13h_numSectors          dw         0B40h      ;2  (21)
_15h_mediaType           db          0F0h      ;1  (22)
_16h_numFATsectors       dw         0009h      ;2  (24)
_18h_sectorsPerTrack     dw         0012h      ;2  (26)
_1Ah_numHeads            dw         0002h      ;2  (28)
_1Ch_numHiddenSects      dd     00000000h      ;4  (32)
_20h_numSectorsHuge      dd     00000000h      ;4  (36)
_24h_driveNumber         db           00h      ;1  (37)
_25h_reserved            db           00h      ;1  (38)
_26h_signature           db           29h      ;1  (39)
_27h_volumeID            db        "????"      ;4  (43)
_28h_volumeLabel         db "VolumeLabel"      ;11 (54)
_36h_FSType              db    "FAT12   "      ;8  (62)

[iansjack]

The first hex string says "mkfs.fat" from offset 0. It wouldn't boot properly.

The OP clearly said that he was reading from offset 0x3, not 0. You expect to find a string there.

[linuxyne]

Debugging can help. bochsdbg is one option (if the problem is also seen when running as a vm).

[Octocontrabass]

Are you sure your bootloader isn't modifying the data in memory? For example, GRUB uses that portion of RAM to store some temporary variables.