Newbie question: is this the speed of the blitter? Or am I doing something wrong?

pushead · 14 April 2021, 18:09

Hi everyone,

In my quest for learning assembly coding on the Amiga I've noticed a thing that I don't know if is a correct observation or me doing something wrong.

I know that the blitter can be used for clearing the screenbuffer. But I would like to measure how long the blitter actutally takes in clearing a 10240 bytes screenbuffer (320 x 256 bitplane). I'm not looking for an exact timing here, just a simple visual representation using colored rasterlines when the blitter is in operation.

In the picture attached you see (I beleive) a blue color when the blitter is working on clearing the 10240 bytes. But does this look correct? I thought that the blitter perhaps would be even faster? In the Amiga HW ref. guide it says that the blitter is able to move 4 megabytes pr. second? But in the pitcure it looks like it takes quite some time to clear "just" 10k of data?

To measure the blitter speed I've done the following:

1) Wait for rasterline $80
2) Make sure that blitter is not busy
3) Set background color to blue
4) Clear 10240 bytes with only D channel enabled
5) Wait for blitter to finish
6) Set background color to black again

The code is like this:

HTML Code:

	move.w	$dff002,d0		; Read DMACONR
	or.w	#$8200,d0		
	move.w	d0,Old_DMACON

	move.w	#$7fff,$dff096

	move.w	#$8640,$dff096


MainLoop:

	bsr	WaitForRaster		; Wait for rasterline $80
	bsr	WaitForBlitterReady	; Blitter not busy?
	
	move.w	#$0009,$dff180		; Set background color to blue
	
	bsr	ClearScreenBuffer	; Clean 10240 bytes

	bsr	WaitForBlitterReady	; Blitter finished?
			
	move.w	#$0000,$dff180		; Set background color to black
	
	btst	#6,$bfe001
	bne.s	MainLoop

	move.w	Old_DMACON,$dff096
	
	rts

***************************************************************
* Subroutines
***************************************************************

WaitForRaster:

	move.l	$dff004,d0
	lsr.l	#8,d0
	and.w	#$01ff,d0
	cmp.w	#$80,d0
	bne.b	WaitForRaster
	rts

WaitForBlitterReady:

	btst	#6,$dff002
	bne.b	WaitForBlitterReady
	rts
	
ClearScreenBuffer:

	move.w	#$0100,$dff040			; BLTCON0
	move.w	#$0000,$dff042			; BLTCON1
	move.l	#ScreenBuffer,$dff054		; D pointer
	move.w	#$00,$dff066			; D modulo
	move.w	#(256*64)+20,$dff058		; BLTSIZE
	rts
	

Old_DMACON:
	dc.w	$0000

	SECTION ScreenBuffer, DATA_C

ScreenBuffer:
	blk.b	10240,$ff

Please see the attached image for the result of this code.

a/b · 14 April 2021, 19:35

4MB/s = 80 KB/frame
1/8 frame for 10KB or ~40 rasterlines, and since not all dma slots are available you are not far from theorethical maximum.

ross · 14 April 2021, 19:46

With all DMAs off excluding blitter (yes, BLTPRI does practically nothing in this situation because of idle cycles) you can do (227/2) writes per line.

(10240/2)/(227/2)=...

EDIT:
This is because with only blitter D channel active you can access alternating bus cycles (D-D-D-...).
But you can interleave it with a CPU clear operation:
https://eab.abime.net/showthread.php?t=103515

pushead · 14 April 2021, 22:37

Ok! Thanks for explaining! Then this works as it should!

Rotareneg · 15 April 2021, 02:52

And you can visualize blitter (and other DMA access) in WinUAE: http://eab.abime.net/showpost.php?p=...7&postcount=67

14 April 2021, 18:09	#1
pushead Registered User Join Date: May 2019 Location: Norway Posts: 32	Newbie question: is this the speed of the blitter? Or am I doing something wrong? Hi everyone, In my quest for learning assembly coding on the Amiga I've noticed a thing that I don't know if is a correct observation or me doing something wrong. I know that the blitter can be used for clearing the screenbuffer. But I would like to measure how long the blitter actutally takes in clearing a 10240 bytes screenbuffer (320 x 256 bitplane). I'm not looking for an exact timing here, just a simple visual representation using colored rasterlines when the blitter is in operation. In the picture attached you see (I beleive) a blue color when the blitter is working on clearing the 10240 bytes. But does this look correct? I thought that the blitter perhaps would be even faster? In the Amiga HW ref. guide it says that the blitter is able to move 4 megabytes pr. second? But in the pitcure it looks like it takes quite some time to clear "just" 10k of data? To measure the blitter speed I've done the following: 1) Wait for rasterline $80 2) Make sure that blitter is not busy 3) Set background color to blue 4) Clear 10240 bytes with only D channel enabled 5) Wait for blitter to finish 6) Set background color to black again The code is like this: HTML Code: move.w $dff002,d0 ; Read DMACONR or.w #$8200,d0 move.w d0,Old_DMACON move.w #$7fff,$dff096 move.w #$8640,$dff096 MainLoop: bsr WaitForRaster ; Wait for rasterline $80 bsr WaitForBlitterReady ; Blitter not busy? move.w #$0009,$dff180 ; Set background color to blue bsr ClearScreenBuffer ; Clean 10240 bytes bsr WaitForBlitterReady ; Blitter finished? move.w #$0000,$dff180 ; Set background color to black btst #6,$bfe001 bne.s MainLoop move.w Old_DMACON,$dff096 rts *************************************************************** * Subroutines *************************************************************** WaitForRaster: move.l $dff004,d0 lsr.l #8,d0 and.w #$01ff,d0 cmp.w #$80,d0 bne.b WaitForRaster rts WaitForBlitterReady: btst #6,$dff002 bne.b WaitForBlitterReady rts ClearScreenBuffer: move.w #$0100,$dff040 ; BLTCON0 move.w #$0000,$dff042 ; BLTCON1 move.l #ScreenBuffer,$dff054 ; D pointer move.w #$00,$dff066 ; D modulo move.w #(256*64)+20,$dff058 ; BLTSIZE rts Old_DMACON: dc.w $0000 SECTION ScreenBuffer, DATA_C ScreenBuffer: blk.b 10240,$ff Please see the attached image for the result of this code. Attached Thumbnails

14 April 2021, 19:46	#3
ross Defendit numerus Join Date: Mar 2017 Location: Crossing the Rubicon Age: 53 Posts: 4,468	With all DMAs off excluding blitter (yes, BLTPRI does practically nothing in this situation because of idle cycles) you can do (227/2) writes per line. (10240/2)/(227/2)=... EDIT: This is because with only blitter D channel active you can access alternating bus cycles (D-D-D-...). But you can interleave it with a CPU clear operation: https://eab.abime.net/showthread.php?t=103515 Last edited by ross; 14 April 2021 at 20:53.

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14 April 2021, 19:35	#2
a/b Registered User Join Date: Jun 2016 Location: europe Posts: 1,039	4MB/s = 80 KB/frame 1/8 frame for 10KB or ~40 rasterlines, and since not all dma slots are available you are not far from theorethical maximum.

14 April 2021, 22:37	#4
pushead Registered User Join Date: May 2019 Location: Norway Posts: 32	Ok! Thanks for explaining! Then this works as it should!

15 April 2021, 02:52	#5
Rotareneg Registered User Join Date: Sep 2017 Location: Kansas, USA Posts: 324	And you can visualize blitter (and other DMA access) in WinUAE: http://eab.abime.net/showpost.php?p=...7&postcount=67

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