reading file from floppy/fat12

[McZ]

I have fighting alot with my damn bootsector thing... i have successfully loaded the rootdir and located the file (well at least I don't get any error doing this) but it fail reading it later..
I do as it say I should read the fat12 in the FAQ (if I got my assembly code right was a while since I coded assembly)

this is a pice of my code

;---( Read file )---
;
; Input
; AX - The first cluster entry value
; BX - FileBuffer (where to store the file)
ReadFile:

cmp ax, 0x0fff ; end of file?
jge EOF   ; well if it is jump to EOF

call LBAtoCHS   ; convert AX to CHS values
mov al, 1   ; sectors to read
call ReadChunk ; read the sector that this value is ( LBAtoCHS already set CHS values needed in correct registers and BX is the FILE_BUFF and al is sectors to read)
add bx, 0x200   ; increase bx with 512 bytes becouse we just read in that into it

push ax   ; push AX onto stack
pop dx      ; retrive AX from stack but into DX
add dx, dx   ; multiply with 2 ( or actually dx + dx -> dx=5 5+5 )
add dx, ax   ; multiply with 3 ( or actuallu dx + dx + ax -> AX=5 5+5+5 )
rcr dx, 1   ; this would divide with 2 (or multiply with 1.5)
      ; and it will also put a bit in carry flag if
pushf      ; save carry flag for a while
mov ax, FAT_BUFF
add ax, dx   ; now this would be the next value from the fat
popf
jc integer
shr ax, 4   ; it wasn't a whole integer
jmp ReadFile

integer:
and ax, 0x0fff
jmp ReadFile

Eof:
ret

bochs says I try to read a sector that is far beoyond the limit of the floppy size

[oswizard]

Code: Select all

add dx, ax   ; multiply with 3 ( or actuallu dx + dx + ax -> AX=5 5+5+5 )
rcr dx, 1   ; this would divide with 2 (or multiply with 1.5)
      ; and it will also put a bit in carry flag if 
pushf      ; save carry flag for a while
mov ax, FAT_BUFF
add ax, dx   ; now this would be the next value from the fat
popf
jc integer
shr ax, 4   ; it wasn't a whole integer
jmp ReadFile

With "add ax, dx", you are moving into AX the address of the proper value within the FAT, not the value at that address. Before the "popf", insert a "mov ax, word [ax]" - that will get the value at address AX.

Good luck,
Mike

[McZ]

...

With "add ax, dx", you are moving into AX the address of the proper value within the FAT, not the value at that address. Before the "popf", insert a "mov ax, word [ax]" - that will get the value at address AX.

Good luck,
Mike

so if I understand correct the lines between the pushf and popf would be

mov ax, FAT_BUFF ; get the base address
add ax, dx ; make the addres point at the new value
mov ax, word[ax] ; retrives the new value into ax

would that be the same as this:
add dx, FAT_BUFF
mov ax, word [dx]
;-- or --
mov ax, word [FAT_BUFF + dx]

[ASHLEY4]

Go here and get bootprog, it as full nasm code, so you can compear it to yours.
http://alexfru.chat.ru/epm.html

\\\\||////
(@@)
ASHLEY4.

[oswizard]

so if I understand correct the lines between the pushf and popf would be

mov ax, FAT_BUFF ; get the base address
add ax, dx ; make the addres point at the new value
mov ax, word[ax] ; retrives the new value into ax

would that be the same as this:
add dx, FAT_BUFF
mov ax, word [dx]
;-- or --
mov ax, word [FAT_BUFF + dx]

Yes. Both would work.

Good luck,
Mike