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From: csg019@cck.cov.ac.uk (Z*A*P*H*O*D)
Newsgroups: comp.sys.amiga.tech
Subject: MFM coding/protection.
Message-ID: <1990Nov27.113820.28055@cck.cov.ac.uk>
Date: 27 Nov 90 11:38:20 GMT
Organization: Coventry Polytechnic, Coventry, UK
Lines: 241

Hi peeps, i seem to recall someone asking for some code to read disks, and
some info on MFM coding.

Well, we'll start at the begining, the amiga uses 11 sectors per track, 2
sides, and 80 tracks, it stores the information on the disks using a method
of encryption called MFM coding. This method helps to detect errors, and
allows the track to be split into the 11 sectors.

Each sector has a sector header (64 bytes) which is coded slightly differently
from the rest of the track. There are actually 22 sectors on one side of a
DOS disk, but these are combined to produce one sector. The coding of the
sectors requires the first longword of sector 1 to be decoded with the first
long word of the next sector (1 sector = 512 ($200) bytes)). To further
complecate things, each track has a gap that contains no useable data.

To cut a long story short, here is an assemly language program to read DOS
disks using MFM coding. It was written in DEVPAC II but should be compatable
with other assemblers like Argasm (nice little plug there eh FUZZZ???)

Hope this helps the guy from London a bit!

To protect you own software you should write a track with just one long sector
of a strange length and have a custome routine on the bootblock to load it.

I'm not a cracker and i am just just trying to help so no flames!

Well i suppose i'm in for a flaming from all thos Americans that thing that
games should multitask with 400 drives and 9 gigabytes of RAM, but still
here goes.....

* CODE WRITTEN BY ZAPHOD OF INTUITION *
	
	SECTION disk_map,code_c

num_tracks	=	10		Number of tracks -1 to read
step_wait	=	3000		Delay for track movement

*Make program an exception for super mode
start
	move.l	$80,trap_save
	move.l	#program_start,$80
	trap	#0
	move.l	trap_save,$80
	rts

program_start
	bsr	save_status		Set system
	bsr	motor_on	
	bsr	seek_zero	
	move.w	#num_tracks,d4
read_more
	bsr	read_track
	bsr	decode
	bsr	seek_80			Read tracks
	dbra	d4,read_more
	bsr	motor_off
	bsr	restore_status
	rte
	
*Track in until 0 is found
seek_zero
	btst	#4,$bfe001
	beq	found_0
	bset	#1,$bfd100
	bclr	#0,$bfd100
	bset	#0,$bfd100
	bsr	stepper_delay
	bra	seek_zero
found_0
	rts

seek_80
	bclr	#1,$bfd100
	bclr	#0,$bfd100
	bset	#0,$bfd100
	bsr	stepper_delay
	rts

stepper_delay
	move.w	#step_wait,d1
wait
	dbra	d1,wait

wait_command
	btst.b	#5,$bfe001
	bne.s	wait_command
	rts

read_track
*Read block header
	move.w	#$8210,$dff096
	move.w	#$4000,$dff024
	move.w	#$4489,$dff07e
	move.w	#$8400,$dff09e
	move.l	#$60000,$dff020
	move.w	#$8000+16,$dff024
	move.w	#$8000+16,$dff024
	bsr	wait_dma		Wait for block done
	bsr	clear_ints		Clear interupts

*Decode header
	move.l	$60002,d0
	move.l	$60006,d1
	andi.l	#$55555555,d0
	andi.l	#$55555555,d1
	lsl.l	#1,d0
	or.l	d1,d0
	cmp.b	#1,d0			Next sector is gap?
	bne.s	read_track		No, read next sector
	lsr.w	#8,d0			Yep, then get this sector numer
	cmp.b	#10,d0
	bne	not_10
	move.b	#0,d0
	bra.s	doner
not_10
	add.b	#1,d0
	move.b	d0,sector_number			Store it
doner
	move.w	#$8210,$dff096
	move.w	#$4000,$dff024
	move.w	#$4489,$dff07e
	move.w	#$8400,$dff09e
	move.l	#$60000,$dff020
	move.w	#$8000+($220*11),$dff024
	move.w	#$8000+($220*11),$dff024
	bsr	wait_dma
	bsr	clear_ints
	rts

motor_delay
	move.w	#$b000,d0
md
	dbra	d0,md
	rts



decode
	moveq.l	#0,d0
	move.b	sector_number,d0
	move.l	d0,d1
	mulu.w	#512,d1
	move.l	#10,d2
	sub.l	d0,d2

	lea	$60000+58,a0
	lea	$60000+512+58,a2
	lea	$70000,a1
	add.l	d1,a1

dl2
	move.w	#(512/4)-1,d6
dl
	move.l	(a0)+,d0
	move.l	(a2)+,d1
	andi.l	#$55555555,d0
	andi.l	#$55555555,d1
	lsl.l	#1,d0
	or.l	d1,d0
	move.l	d0,(a1)+
	dbra	d6,dl
	add.l	#$240,a0
	add.l	#$240,a2
	dbra	d2,dl2

	cmp.b	#0,sector_number
	beq	dones

	clr.l	d2
	move.b	sector_number,d2
	sub.w	#1,d2
	lea	$70000,a1
dl3
	move.w	#(512/4)-1,d6
dl4
	move.l	(a0)+,d0
	move.l	(a2)+,d1
	andi.l	#$55555555,d0
	andi.l	#$55555555,d1
	lsl.l	#1,d0
	or.l	d1,d0
	move.l	d0,(a1)+
	dbra	d6,dl4
	add.l	#$240,a0
	add.l	#$240,a2
	dbra	d2,dl3
	rts
dones
	move.w	#$fff,$dff180
	rts

save_status
	move.w 	$dff01c,saveints		Save interupt stat
	move.w 	$dff002,savedma			Save dma stat.
	move.w	#$7fff,$dff09c			Clear interupts
	rts

motor_on
	move.b	#%01111111,$bfd100
	move.b	#%11110011,$bfd100
	bsr	motor_delay
	rts

motor_off
	move.b	#$fd,$bfd100
	nop
	nop
	move.b	#%01111111,$bfd100
	move.b	#%11111111,$bfd100
	bsr	motor_delay
	rts

restore_status
	move.w saveints,d0
	or.w #$c000,d0
	move.w d0,$dff01c
	move.w savedma,d0
	or.w #$8100,d0
	move.w d0,$dff096
	rts

wait_dma
	btst	#1,$dff01f
	beq.s	wait_dma
	rts


clear_ints
	move.w	#$4000,$dff024
	move.w	#$7fff,$dff09c
	rts
saveints		dc.w 0
savedma			dc.w 0
trap_save		dc.l 0
sector_number		dc.b 0

-- 
*********///  O O **A member of S.H.I.T. (Super High Intelegence Team)**///***
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