Read FAT16 table problem

[Raddel]

Hello,

I'have a problem during read FAT16 table... this is a piece of my bootsector code

Code: Select all

.....................
READ_FAT:
        mov             ax,4                            ;-- First Sector
        call              LBACHS
   [b] mov             al,250  [/b]                   ;-- Read 1 FAT (Flashdisk 256Mb)
        mov             bx,ds
        mov             es,bx
        mov             di,buffer
        mov             bx,di
        call            READSECTOR
............
                                         

My problem is at bold instruction, i try to read first FAT on disk and i want to read whole one FAT in memory, that is point to ES:BX. But when i check at ES:BX there a invalid value, not a FAT Table. But when I change the value :

Code: Select all

mov al,1

ES:BX have a valid value. I need help who have experience of this thing... Is there limitation at INT 13h to read disk?
Sorry if my english bad.

Best Regards,
Raddel

[thepowersgang]

What interrupt are you calling, what arguments are being passed?
What is the purpose of each instruction in your assembly dump?

[Raddel]

Here is my code READSECTOR and LBA2CHS

Code: Select all

;-- proc LBACHS must be call to calculation C-H-S
;-- then C-H-S will auto input in register before call int 13h
;-- IN          : AL (Number of Sector to read) 
;-- OUT         : Buffer (ES:BX)
READSECTOR:
        pusha   ;-- push all register
        mov             ah,2h
        
        mov             ch,[absTrack]
        mov             cl,[absSector]
        mov             dh,[absHead]
        mov             dl,[bootdev]
        
        stc
        int             13h
        popa    ;-- pop all register
        ret
                
;-- IN              : AX (LBA)
;   OUT             : Absolute Track (Cylinder), Head, Sector
LBACHS:
        pusha   ;-- push all register
        xor             dx,dx
        
        ;-- Calculation Absolute Sector
        div             word[SectorPerTrack]
        inc             dl
        mov             byte[absSector],dl
        
        ;-- Calculation Absolute Head and Track
        xor             dx,dx
        div             word[Heads]
        mov             byte[absHead],dl
        mov             byte[absTrack],al
        
        popa    ;-- pop all register
        ret
        
        absTrack        db 0
        absHead         db 0
        absSector       db 0              

I have problem when read sector if al = 250, the data is not a valid data, i have check to dump buffer at memory.. But if i read sector set al=1 then buffer have a valid data. How it can be? I need to read FAT table buffered at memory, but it seems gonna wrong when i set al=250. Why 250? because SectorPerFAT=250. Anyone have idea about this?

[egos]

My problem is at bold instruction, i try to read first FAT on disk and i want to read whole one FAT in memory, that is point to ES:BX. But when i check at ES:BX there a invalid value, not a FAT Table. But when I change the value :

Code: Select all

mov al,1

ES:BX have a valid value. I need help who have experience of this thing... Is there limitation at INT 13h to read disk?
Sorry if my english bad.

Function 2 is able to read the sectors on "one track". You can try to use function 42h before using function 2. I think to cache only actual fragments of FAT is better way. I'm loading whole FAT for floppies only.

[azblue]

Here is my code READSECTOR and LBA2CHS

I have problem when read sector if al = 250, the data is not a valid data, i have check to dump buffer at memory.. But if i read sector set al=1 then buffer have a valid data. How it can be? I need to read FAT table buffered at memory, but it seems gonna wrong when i set al=250. Why 250? because SectorPerFAT=250. Anyone have idea about this?

250 Sectors per FAT does not mean the FAT starts at sector 250; if I remember correctly it starts after the reserved sectors.

[Raddel]

@azblues : I have calculate start of LBA before call READSECTOR. al=250 is a number of sector, that's not mean the start of sector, the start of sector read from sector 4.

@Egos: May be you right, i need to extended read. Could you have some example code for that?

If just a disk 1.44Mb it has a little size of FAT, And how i can read FAT Table that have size of partition above 1GB?
Does anyone now the trick about read FAT table in memory?

[egos]

Could you have some example code for that?

Function 42h is easy to use, but don't forget to check support for it by calling function 41h.

If just a disk 1.44Mb it has a little size of FAT, And how i can read FAT Table that have size of partition above 1GB?
Does anyone now the trick about read FAT table in memory?

As I said above you can cache only actual fragments of FAT е.g. only one sector of FAT and use it until you need to load other one.

[Raddel]

I see, thank you about the information .