Blitter MFM decoding

Photon · 13 March 2006, 00:31

Just got my trackloader with blitter decoding to work... the thing is, I need TWO 256 word blits to do it, because the blitter shifts to the right and the MFM data needs to be shifted left by one lousy bit

Darn It!

It seems impossible to do it with a single blit, since blitsize width is max. 64 words and shifts don't happen across rows.

Does anyone know if anyone in the history of Amiga has achieved this with a single blit? I'd sure like to see that code...

Codetapper · 13 March 2006, 01:55

Isn't shifting switched around if you use the blitter descending mode? ie. Shifts become to the left?

Photon · 13 March 2006, 09:46

The System Programmer's Guide says the blitter shifts right and in the chapter about descending mode it doesn't mention any change in shift direction. Wish I had my Hardware Reference Manual around to settle the matter, but it's stored somewhere.

musashi5150 · 13 March 2006, 10:41

It does shift the other way in descending mode - I'm sure of it.

Codetapper · 13 March 2006, 11:24

Indeed. From the book Mapping the Amiga:

You should also use descending mode if you need to shift your source data left. You must use descending mode if you plan to perform a blitter fill operation.

BippyM · 13 March 2006, 15:12

I think I might have that book.. Is it by Abacus?

If so I'll dig it out later and scan anything required.. Just say da word guys

redblade · 13 March 2006, 22:55

dee jay may of typed it in ages ago.

alexh · 14 March 2006, 00:07

Quote:

Descending mode is turned on by setting bit one of BLTCON1 (defined as BLITREVERSE). If you use descending mode the address pointers will be decremented by two (bytes) instead of incremented by two for each word fetched. In addition, the modulo values will be subtracted rather than added. Shifts are then towards the left, rather than the right, the first word mask masks the last word in a row (which is still the first word fetched) and the last word mask masks the first word in a row.

Hope that helps.

Photon · 15 March 2006, 00:53

Sorry for no reply guys, been away (project deadline at Uni). I actually tried descending mode right after posting and it worked straight away

Just goes to show that you need thorough manuals when you try to do something special

Anyway, here's the code for it:

Code:

**** MFM DECODE BLIT ****

;a0=destination (chipmem)
;a1=start of sector data in MFM-buffer
;a5=$dff000
;d7.l=$55555555

	lea $200-2(a0),a0
	lea $200-2(a1),a1
	lea $200(a1),a2
	moveq #0,d5

.wbl1:   btst #6,2(a5)
	bne.s .wbl1

	move.w d5,$62(a5)	;bltbmod
	move.l d5,$64(a5)	;bltadmod
	move.l a2,$4c(a5)	;bltbptH
	move.l a1,$50(a5)	;bltaptH
	move.l a0,$54(a5)	;bltdptH

	move.l d7,$44(a5)	;bltaflwm - MASK BEFORE SHIFT!!
	move.l #$1df80002,$40(a5)	;bltcon0&1, shift a by 1, descending

	move.w d7,$70(a5)	;bltcdat - mask for b

	move.w #256*64+1,$58(a5)	;bltsize=256w=512b=1 sector

My loader is really fast now

Does anyone know a megademo with a fast loader, so I can compare? :P (And I don't mean those cheatz0r demos that load only one side of each track

)

bobbybearing · 15 March 2006, 09:01

see Trashcan/Wizzcat. fast loader

CFou! · 15 March 2006, 11:48

Quote:

Originally Posted by Photon

Just got my trackloader with blitter decoding to work... the thing is, I need TWO 256 word blits to do it, because the blitter shifts to the right and the MFM data needs to be shifted left by one lousy bit

Darn It!

It seems impossible to do it with a single blit, since blitsize width is max. 64 words and shifts don't happen across rows.

Does anyone know if anyone in the history of Amiga has achieved this with a single blit? I'd sure like to see that code...

Thalamus's track loader for BOROBODURhttp://hol.abime.net/186 use blitter to decode MFM (track length $1800 without checksum in my memory)

If You are interesting in it, I can extract the code

Photon · 15 March 2006, 19:54

Quote:

Originally Posted by bobbybearing

see Trashcan/Wizzcat. fast loader

That's one of those cheatz0r demos I was talking about. I can modify mine to only load 1 sector per cylinder, then it will tick like crazy!!!

Photon · 15 March 2006, 19:58

Quote:

Originally Posted by CFOU!

Thalamus's track loader for BOROBODURhttp://hol.abime.net/186 use blitter to decode MFM (track length $1800 without checksum in my memory)

If You are interesting in it, I can extract the code

Well I might be interested in booting it to hear if it's faster... unless it only loads one side of the cylinder of course

musashi5150 · 16 March 2006, 09:47

Fairlight's 242 demo tracked in pretty fast as I remember too

IFW · 16 March 2006, 11:24

The disk rotates at 300rpm.
Reading an entire track is one revolution.
You have 5 revolutions per second.
All this means no matter what you do, it won't get any faster than 5 tracks per second.

13 March 2006, 00:31	#1
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,699	Blitter MFM decoding Just got my trackloader with blitter decoding to work... the thing is, I need TWO 256 word blits to do it, because the blitter shifts to the right and the MFM data needs to be shifted left by one lousy bit Darn It! It seems impossible to do it with a single blit, since blitsize width is max. 64 words and shifts don't happen across rows. Does anyone know if anyone in the history of Amiga has achieved this with a single blit? I'd sure like to see that code... Last edited by Photon; 13 March 2006 at 01:54.

15 March 2006, 00:53	#9
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,699	Sorry for no reply guys, been away (project deadline at Uni). I actually tried descending mode right after posting and it worked straight away Just goes to show that you need thorough manuals when you try to do something special Anyway, here's the code for it: Code: ** MFM DECODE BLIT ** ;a0=destination (chipmem) ;a1=start of sector data in MFM-buffer ;a5=$dff000 ;d7.l=$55555555 lea $200-2(a0),a0 lea $200-2(a1),a1 lea $200(a1),a2 moveq #0,d5 .wbl1: btst #6,2(a5) bne.s .wbl1 move.w d5,$62(a5) ;bltbmod move.l d5,$64(a5) ;bltadmod move.l a2,$4c(a5) ;bltbptH move.l a1,$50(a5) ;bltaptH move.l a0,$54(a5) ;bltdptH move.l d7,$44(a5) ;bltaflwm - MASK BEFORE SHIFT!! move.l #$1df80002,$40(a5) ;bltcon0&1, shift a by 1, descending move.w d7,$70(a5) ;bltcdat - mask for b move.w #25664+1,$58(a5) ;bltsize=256w=512b=1 sector My loader is really fast now Does anyone know a megademo with a fast loader, so I can compare? :P (And I don't mean those cheatz0r demos that load only one side of each track ) Last edited by Photon; 15 March 2006 at 10:48.*

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13 March 2006, 01:55	#2
Codetapper 2 contact me: email only! Join Date: May 2001 Location: Auckland / New Zealand Posts: 3,187	Isn't shifting switched around if you use the blitter descending mode? ie. Shifts become to the left?

13 March 2006, 09:46	#3
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,699	The System Programmer's Guide says the blitter shifts right and in the chapter about descending mode it doesn't mention any change in shift direction. Wish I had my Hardware Reference Manual around to settle the matter, but it's stored somewhere.

13 March 2006, 10:41	#4
musashi5150 move.w #$4489,$dff07e Join Date: Sep 2005 Location: Norfolk, UK Age: 43 Posts: 2,351	It does shift the other way in descending mode - I'm sure of it.

13 March 2006, 11:24	#5
Codetapper 2 contact me: email only! Join Date: May 2001 Location: Auckland / New Zealand Posts: 3,187	Indeed. From the book Mapping the Amiga: You should also use descending mode if you need to shift your source data left. You must use descending mode if you plan to perform a blitter fill operation.

13 March 2006, 15:12	#6
BippyM Global Moderator Join Date: Nov 2001 Location: Derby, UK Age: 48 Posts: 9,355	I think I might have that book.. Is it by Abacus? If so I'll dig it out later and scan anything required.. Just say da word guys

13 March 2006, 22:55	#7
redblade Zone Friend Join Date: Mar 2004 Location: Middle Earth Age: 40 Posts: 2,130	dee jay may of typed it in ages ago.

15 March 2006, 09:01	#10
bobbybearing Zone Friend Join Date: Oct 2003 Location: France Age: 51 Posts: 161	see Trashcan/Wizzcat. fast loader

16 March 2006, 09:47	#14
musashi5150 move.w #$4489,$dff07e Join Date: Sep 2005 Location: Norfolk, UK Age: 43 Posts: 2,351	Fairlight's 242 demo tracked in pretty fast as I remember too

16 March 2006, 11:24	#15
IFW Moderator Join Date: Jan 2003 Location: ... Age: 52 Posts: 1,838	The disk rotates at 300rpm. Reading an entire track is one revolution. You have 5 revolutions per second. All this means no matter what you do, it won't get any faster than 5 tracks per second.

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