fat12 set cluster

[xyjamepa]

Hi,

I'm trying to write a fat12 driver,but this time I'm writing it using assembly,
right now I don't know how to set an entry in the fat table to a specific value,
would you please give me a code snippt (asm code) about that...

Thanx.

[DT170x]

I thought there is FAT, FAT16, and FAT32

FAT 12

It that odd

[suthers]

I thought there is FAT, FAT16, and FAT32

FAT 12

It that odd

FAT12 definitely exist, trust me, I'm an OSdever
Jules

[DT170x]

Sorry I didn't Know that.

[PatrickV]

I think the number is the number of clusters per file it can handle for example FAT12- 12 Clusters? That i at least what i think it is. Please point out to me if I am wrong

[egos]

Sorry I didn't Know that.

Look at my reset macro to generate sequential cluster chain for a file. Maybe this helps you.

Code: Select all

if id#_size>0
count=(id#_size+511)/512
cur=id#_base/512-(33-2)
repeat count
if %=count
val=0FFFh
else
val=cur+1
end if
if cur and 1
val=val shl 4
end if
disp=(cur*3)/2
load var word from 512+disp
var=var or val
store word var at 512+disp
store word var at 10*512+disp
cur=cur+1
end repeat
end if

Source: mkfloppy.zip

[ru2aqare]

I think the number is the number of clusters per file it can handle for example FAT12- 12 Clusters? That i at least what i think it is. Please point out to me if I am wrong

The number is the amount of bits that are used to store each cluster number in the allocation table. So FAT12 theoretically supports 2^12 clusters (actually, it's around 4086, due to the last ten or so numbers being reserved), FAT16 theoretically supports 2^16 clusters (again, somewhat less), and FAT32 supports 2^28 clusters (although the name says 32, only 28 bits are used for the cluster number, the highest four bits are reserved; and again there are some reserved cluster numbers).

It would be a pretty serious limitation, if FAT12 could only handle files that are 12 clusters long. Even with the maximum cluster size 64KiB (if I remember correctly, FAT12 can't do clusters larger than this), that is only 768KiB - that would be pretty lame.

[AJ]

Hi,

I thought there is FAT, FAT16, and FAT32

As a word of warning, FAT (without a suffix) normally relates to FAT16 - at least on MS fdisk.

Cheers,
Adam

[xyjamepa]

Okay guys lets get back to the main point
some asm code snippt...

[egos]

Okay guys lets get back to the main point
some asm code snippt...

It's so pitiful for me to give you my code

[Dex]

My OS has fat12, fat16, fat32 implemented (in fasm) you are free to use it, but you MUST include the licence, if you use any part of the code.
You can get it here: http://dex4u.com/download.htm

[salil_bhagurkar]

Why don't you go for the actual math instead, and write a driver yourself using this specification from MS... Reading it at the most 10 times, you should be able to understand the FAT in general (and hence FAT12).

[xyjamepa]

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,I wrote "getFat" function by
myself ,but till now I couldn't get this function done.
anyway here's what I have...

Code: Select all

	mov	ax,word [cluster]
	mov	cx,ax
	mov	dx,ax
	shr	dx,0x0001
	add	cx,dx		;sum of (3/2)
	mov	bx,[es:0x500]	;location of fat table in memory
	add	bx,cx		;index into fat
	test	ax,0x0001
	jnz	.odd
.eve:	
	push	bx
	inc	bx
	and	bx,0x0F
	mov	cx,[value]
	and	cx,0xF00
	shr	cx,0x0008
	or	bx,cx
	mov	cx,[value]
	and	cx,0x0FF
	pop	bx
	mov	[bx],cx
.odd:
	and	[bx],0x0F
	mov	cx,[value]
	and	cx,0x00F
	shl	cx,4
	or	[bx],cx
	mov	cx,value
	and	cx,0xFF0
	shr	cx,4
	inc	bx
	mov	bx,cx

Please note that I loaded the fat table into memory at [es:0x500]
Also "value" holds the value I want to set the entry to it

[Troy Martin]

I think the number is the number of clusters per file it can handle for example FAT12- 12 Clusters? That i at least what i think it is. Please point out to me if I am wrong

FAT12 uses a twelve-bit File Allocation Table, allowing 2^12-19 clusters. At 8KB per cluster (16 sectors) that makes a volume max size of 32 MB.

[xyjamepa]

here's what I got till now...

Code: Select all

setcluster:
             mov	ax,word [cluster]   ;current cluster
	mov	cx,ax
	mov	dx,ax
	shr	dx,0x0001
	add	cx,dx		;sum of (3/2)
	mov	bx,[es:0x500]	;location of fat table in memory
	add	bx,cx		;index into fat
	mov       dx, WORD [bx]       ;read two bytes from FAT
	test	ax,0x0001
	jnz	.odd
.eve:	
	and	[value],0xFFF
	and	bx,0xF000
	jmp	done
.odd:
	shl	[value],4
	and	bx,0x000F

done:
	or	bx,[value]
            ret

would any one please check this function and till me what bugs does it have?
Thanx.