fat12 set cluster

[jal]

I already wrote complete fat12 driver in C,but now I'm writing it in asm
but I'm really having problem with this specific function

So basically your problem is that you can't code assembly. It's pretty stupid then abandoning C.


JAL

[xyjamepa]

I'm trying to learn assembly by coding a real mode os,and so far I've learnt alot
and I'm very pleased about that.

[Combuster]

DOS .COM works pretty well too if you want to learn to use basic assembly.

[xyjamepa]

I wrote a lot of Dos programs,but I'm having trouble with this specific fumction
Could we guys stop going off-topic please...

[jal]

I wrote a lot of Dos programs,but I'm having trouble with this specific fumction

I really fail to understand that, if you have a working C-version, you are not able to make an assembly one. What is your problem?


JAL

[DeletedAccount]

http://www.it.lth.se/ssoa/Project1/FAT12Description.pdf . You might find this more readable than the official Microsoft specs

Regards
Thomas

[xyjamepa]

here's what I have,till now I couldn't solve my problem,
There's somthing wrong in my code,would someone please check it...

Code: Select all

setcluster:
        push    cx dx bx
        mov     cx, ax                  ;copy current cluster
        mov     dx, ax                  ;copy current cluster
        shr     dx, 0x0001              ;divide by two
        add     cx, dx                  ;cx=offset into FAT			
        mov     bx,[es:0x500]           ;address of fat table
        add     bx, cx                  ;index into FAT
        mov     dx, WORD [bx]           ;read two bytes from FAT(old value in dx)
        test    ax, 0x0001
	jnz	.odd
.eve:	
	mov     bx,dx
	and     bx,0xF000               ;bx holds the old value 
	and     word[value],0x0FFF      ;next cluster
	
	or      word[value],bx
	mov     dx,word[value]
	mov     word [bx],dx
	jmp	donexx
.odd:
	mov     bx,dx
	and     bx,0x000F               ;bx holds the old value
	shl     word[value],4   	;next_cluster
	or      word[value],bx
	mov     dx,word[value]
	mov     word [bx],dx            ;save the new value
donexx:
        pop     bx dx cx
        ret

Thanx.

[xyjamepa]

right now I'm very sure of this function,90% it's fine and nothing wrong with him,
but till now I couldn't change the value in the fat entry.
Also I'm sure that I load FAT table correctly into memory,because I can read
text files even if they are very long.
guys what do you advice me?how can I figure out what's wrong?
how can I debug this problem?

[xyjamepa]

It's working now...
Here's the working code,for anybody else...

Code: Select all

setcluster:
        push     cx 
        mov     cx, ax                  ;copy current cluster 
        mov     dx, ax                  ;copy current cluster 
        shr     dx, 0x0001              ;divide by two 
        add     cx, dx                  ;cx=offset into FAT                      
        mov     bx,[es:0x500]           ;address of fat table 
        add     bx, cx                  ;index into FAT
        push    bx 
        mov     dx, WORD [bx]           ;read two bytes from FAT(old value in dx) 
        test    ax, 0x0001 
        jnz     .odd 
.eve:    
        mov     bx,dx 
        and     bx,0xF000               ;bx holds the old value  
        and     word[value],0x0FFF      ;next cluster 
         
        or      word[value],bx 
        mov     dx,word[value]
        pop     bx 
        mov     word [bx],dx 
        jmp     donexx 
.odd: 
        mov     bx,dx 
        and     bx,0x000F               ;bx holds the old value 
        shl     word[value],4           ;next_cluster 
        or      word[value],bx 
        mov     dx,word[value] 
        pop     bx
        mov     word [bx],dx            ;save the new value 
donexx:          
        pop      cx  
        ret