How to write a Delay in Assembler

Steve · 26 May 2003, 14:46

I've decided to play around with the LED to make it flash on and off using bit 1 of the CIA register $BFE001. It works great in debug when you step through the code but the problem is that the LED flashes too fast when you run it properly. It just flickers away instead of flashing. So basically how would you go about putting a decent delay in so that it flashes on then waits half a second before flashing off again. I tried a simple loop that counted down from 50 to 0 but that made no difference. I then tried 50000 but it still didn't slow the thing down. Darn assembler, it's too fast.

Any ideas? BTW I'm still a bit of a beginner with assembler so keep it simple.

AmiGer · 26 May 2003, 14:52

Maybe this:

Bynary count:
clr.b d0

loop:
move.b d0,(a2) ;supposing a2 is address where LEDs are represented
add.b #1,d0
move.l #100,d2
bsr pause
bra loop

;Pause function (assuming 4 us for each instr.)
;holds execution for as many ms as is the value of d2

pause:
move.l #11,d3
innerloop:
sub.l #1,d3
bne innerloop ; 8 * 11 = 84
sub.l #1,d2
nop
bne pause ; 84 + 16 = 100
ret

source:
http://www.experts-exchange.com/Prog..._20078051.html

Toni Wilen · 26 May 2003, 15:09

<code snipped removed>

urghh...

Steve · 26 May 2003, 16:13

Quote:

Originally posted by Toni Wilen
urghh...

I'll try that code out Amiger but it looks a bit long-winded. I was hoping for some sort of Wait instruction. I guess there isn't one.

LaundroMat · 26 May 2003, 17:44

Use VBlanks, but don't ask me how (it's been over 10 years...)

Basically a vblank occurs when the monitor strobe reaches the last line on the screen. Make your code wait until it reaches the last line (guess: line 255) and then return to the start of the loop. This will guarantee perfect timing.

Note that Vblanks occur every 50th of a second, so maybe you should just keep track of vblanks and blink the led after 50 vblanks (ie after 1 second).

Ofcourse, if the goal is to have other code running at the same time, you could either put all the code within a vblank loop, or use interrupts (ie assign some code to an interrupt that keeps track of the time, and makes the led blink whenever a second has passed).

Codetapper · 26 May 2003, 22:03

Why you would want to do this is beyond me but here you go:

move.l #10-1,d0 ;flash 10 times
.flash bchg #1,$bfe001
bsr _wait
dbf d0,.flash
rts

_wait move.l #50-1,d0 ;waits 1 second on PAL
.wait bsr _waitvb
dbf d0,.wait
rts

_waitvb ;waits 1/50th of a second
.1 btst #0,(_custom+vposr+1)
beq .1
.2 btst #0,(_custom+vposr+1)
bne .2
rts

Formatting is a bit up the spout as the system removes all the indenting - hope you can follow it!

CodyJarrett · 26 May 2003, 22:17

You can use the CODE tag to preserve formatting:

Code:

	move.l #10-1,d0		 ;flash 10 times
.flash	bchg	#1,$bfe001
	bsr	_wait
	dbf	d0,.flash
	rts

_wait	move.l #50-1,d0		;waits 1 second on PAL
.wait	bsr	_waitvb
	dbf	d0,.wait
	rts

_waitvb				;waits 1/50th of a second
.1	btst #0,(_custom+vposr+1)
	beq  .1
.2	btst #0,(_custom+vposr+1)
	bne  .2
	rts

LaundroMat · 26 May 2003, 23:22

Not trying to vaunt my skills here, but dang that's still easy to understand.
Is other asm code (86x, PPC) as easy tor read/write, or is this only on Amiga (tm) again?

Steve · 27 May 2003, 00:23

Brilliant. It works perfectly. I had to change a few things to get it to compile under Devpac 3 but it worked in the end. I also reduced the wait from 50 frames down to 30 frames to speed up the blinking.

Quote:

Originally posted by Codetapper
Why you would want to do this is beyond me but here you go:

No real reason for doing it. I just thought it would be a nice thing to try out. I'm still a beginner with assembler so I'm just tinkering around with the inner workings of the Amiga and just seeing what the registers and other things do.

Here is the finished executable program which makes you think your Amiga has crashed:

Thanks for your help.

Codetapper · 27 May 2003, 03:14

Quote:

Originally posted by LaundroMat
Not trying to vaunt my skills here, but dang that's still easy to understand.
Is other asm code (86x, PPC) as easy tor read/write, or is this only on Amiga (tm) again?

Well I've done precious little hacking on the PC but I find it a complete pig. All backwards, seems to be tonnes of alternative instructions and the lack of registers for storing values in really makes the code unreadable.

I cannot follow C64 code either, it looks terrible.

68000 is just so elegant if well written...

BTW my example above was just typed out quickly without testing it, so it behaves a little strangely as d0 is destroyed in the subroutines - should use a different register like d1/d2 or save the value... This should fix it:

Code:

_wait   move.l  d0,-(sp)                ;save d0 
        move.l  #50-1,d0                ;waits 1 second on PAL 
.wait   bsr     _waitvb 
        dbf     d0,.wait 
        move.l  (sp)+,d0                ;restore d0
        rts

AmiGer · 27 May 2003, 08:03

Quote:

Originally posted by Steve
Here is the finished executable program

Whow, 324 bytes length. I don't remember a windows program being such short...

andreas · 22 July 2003, 02:39

Quote:

source:
http://www.experts-exchange.com/Prog..._20078051.html

Quote:

Originally posted by AmiGer
bne innerloop ; 8 * 11 = 84

EXPERTS' exchange? LOL?!
Definitely no experts in (simple) mathematics

(but I think they're going to beat me in calculus without problems

)

AmiGer · 22 July 2003, 08:16

Maths isn't easy to understand sometimes... I think there's a tricky solution way for this formula...

Seems to be a competent web page I found there, sorry, Steve...

Steve · 22 July 2003, 14:24

Quote:

Originally posted by AmiGer
Seems to be a competent web page I found there, sorry, Steve...

To be honest I hadn't even looked at that link.

fstarred · 26 April 2018, 14:59

This is my 2 cents code, it waits for 3 seconds:

Code:

START:
	move.l	#(50*3)-1,d0	; blank occurs every 1/50th seconds,
				; therefore must multiply with x seconds
				; (value in the example is set to 3)
LOOP:
.v_blank
	move.l	$dff004,   d1
	and.l	#$0001ff00,d1
	cmp.l	#255<<8,d1
	bne.s	.v_blank
.v_blank_after
	move.l	$dff004,   d1
	and.l	#$0001ff00,d1
	cmp.l	#255<<8,d1
	beq.s	.v_blank_after
	dbra	d0,LOOP
	rts

PeterK · 26 April 2018, 16:48

Poor DOS, nobody likes dos.library

Code:

dos.library/Delay                                           dos.library/Delay

    NAME
	Delay -- Delay a process for a specified time

    SYNOPSIS
	Delay( ticks )
	       D1

	void Delay(ULONG)

    FUNCTION
	The argument 'ticks' specifies how many ticks (50 per second) to
	wait before returning control.

    INPUTS
	ticks - integer

    BUGS
	Due to a bug in the timer.device in V1.2/V1.3, specifying a timeout
	of zero for Delay() can cause the unreliable timer & floppy disk
	operation.  This is fixed in V36 and later.

phx · 26 April 2018, 22:43

It always depends on what you want to do.

OS-friendly code: use timer.device or dos.library/Delay.
Take over the system: wait for raster-lines, VERTB-interrupts or use the CIA timers

In any case, NEVER write a busy loop, which depends on CPU speed!

rare_j · 27 April 2018, 02:26

In this case, the delay is almost 15 years.

PeterK · 27 April 2018, 02:31

Wow, didn't notice that ...

OmegaMax · 22 May 2018, 03:37

26 May 2003, 23:22	#8
LaundroMat Junior Member Join Date: Apr 2001 Location: In the cellar. With your mum. Age: 49 Posts: 404	Holy cow Not trying to vaunt my skills here, but dang that's still easy to understand. Is other asm code (86x, PPC) as easy tor read/write, or is this only on Amiga (tm) again?

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26 May 2003, 14:46	#1
Steve I Identify as an Ewok Join Date: Jul 2001 Location: North Lincolnshire Age: 45 Posts: 2,356	How to write a Delay in Assembler I've decided to play around with the LED to make it flash on and off using bit 1 of the CIA register $BFE001. It works great in debug when you step through the code but the problem is that the LED flashes too fast when you run it properly. It just flickers away instead of flashing. So basically how would you go about putting a decent delay in so that it flashes on then waits half a second before flashing off again. I tried a simple loop that counted down from 50 to 0 but that made no difference. I then tried 50000 but it still didn't slow the thing down. Darn assembler, it's too fast. Any ideas? BTW I'm still a bit of a beginner with assembler so keep it simple.

26 May 2003, 14:52	#2
AmiGer Registered User Join Date: Sep 2002 Location: Germany Posts: 349	Maybe this: Bynary count: clr.b d0 loop: move.b d0,(a2) ;supposing a2 is address where LEDs are represented add.b #1,d0 move.l #100,d2 bsr pause bra loop ;Pause function (assuming 4 us for each instr.) ;holds execution for as many ms as is the value of d2 pause: move.l #11,d3 innerloop: sub.l #1,d3 bne innerloop ; 8 * 11 = 84 sub.l #1,d2 nop bne pause ; 84 + 16 = 100 ret source: http://www.experts-exchange.com/Prog..._20078051.html

26 May 2003, 15:09	#3
Toni Wilen WinUAE developer Join Date: Aug 2001 Location: Hämeenlinna/Finland Age: 49 Posts: 26,505	<code snipped removed> urghh...

26 May 2003, 17:44	#5
LaundroMat Junior Member Join Date: Apr 2001 Location: In the cellar. With your mum. Age: 49 Posts: 404	Use VBlanks, but don't ask me how (it's been over 10 years...) Basically a vblank occurs when the monitor strobe reaches the last line on the screen. Make your code wait until it reaches the last line (guess: line 255) and then return to the start of the loop. This will guarantee perfect timing. Note that Vblanks occur every 50th of a second, so maybe you should just keep track of vblanks and blink the led after 50 vblanks (ie after 1 second). Ofcourse, if the goal is to have other code running at the same time, you could either put all the code within a vblank loop, or use interrupts (ie assign some code to an interrupt that keeps track of the time, and makes the led blink whenever a second has passed).

26 May 2003, 22:03	#6
Codetapper 2 contact me: email only! Join Date: May 2001 Location: Auckland / New Zealand Posts: 3,182	Why you would want to do this is beyond me but here you go: move.l #10-1,d0 ;flash 10 times .flash bchg #1,$bfe001 bsr _wait dbf d0,.flash rts _wait move.l #50-1,d0 ;waits 1 second on PAL .wait bsr _waitvb dbf d0,.wait rts _waitvb ;waits 1/50th of a second .1 btst #0,(_custom+vposr+1) beq .1 .2 btst #0,(_custom+vposr+1) bne .2 rts Formatting is a bit up the spout as the system removes all the indenting - hope you can follow it!

22 July 2003, 08:16	#13
AmiGer Registered User Join Date: Sep 2002 Location: Germany Posts: 349	Maths isn't easy to understand sometimes... I think there's a tricky solution way for this formula... Seems to be a competent web page I found there, sorry, Steve...

26 April 2018, 22:43	#17
phx Natteravn Join Date: Nov 2009 Location: Herford / Germany Posts: 2,496	It always depends on what you want to do. OS-friendly code: use timer.device or dos.library/Delay. Take over the system: wait for raster-lines, VERTB-interrupts or use the CIA timers In any case, NEVER write a busy loop, which depends on CPU speed!

27 April 2018, 02:26	#18
rare_j Zone Friend Join Date: Apr 2005 Location: London Posts: 1,176	In this case, the delay is almost 15 years.

27 April 2018, 02:31	#19
PeterK Registered User Join Date: Apr 2005 Location: digital hell, Germany, after 1984, but worse Posts: 3,365	Wow, didn't notice that ...

22 May 2018, 03:37	#20
OmegaMax Knight Of The Kingdom Join Date: Feb 2016 Location: It's a bald world! Posts: 179

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