Clearing COLOR00 on A500 vs A1200

MethodGit · 24 October 2018, 16:24

I only noticed this fairly recently, but whereas on a stock A500 configuration clearing DFF180 on startup just works without issue, on a standard A1200 configuration initiating the same command doesn't stick and the screen will immediately restore the original background colour. After looking it up, it seems you have to mess about with the DMACON interrupt (DFF096) before doing the clearing - in particular the Copper DMA bit is what determines it. I just move #$80 to DFF096 which appears to do the trick.

But why is this necessary on an A1200 but not an A500? What changed between them (apart from the obvious)? Is it specifically related to the processor, the chipset or the Kick ROM?

StingRay · 24 October 2018, 16:27

I'd be interested to see the code you've been trying.

meynaf · 24 October 2018, 16:27

Quick guess : kickstart 1.x does not use the copper to setup colors (at least not that one).

ross · 24 October 2018, 17:09

There is only one 'substantial' difference between system copper list in KS1.x and KS20+.
In KS13 COLOR00 is set only through gb_LOFlist/gb_SHFlist, on KS20+ also through gb_copinit.
This does not justify a different behavior in case the COPJMPx addresses are not touched (and your COPEN tampering in DMACON proves this).
So the error is somewhere in your code.

24 October 2018, 16:24	#1
MethodGit Junior Member Join Date: Dec 2002 Location: The Streets Age: 39 Posts: 2,731	Clearing COLOR00 on A500 vs A1200 I only noticed this fairly recently, but whereas on a stock A500 configuration clearing DFF180 on startup just works without issue, on a standard A1200 configuration initiating the same command doesn't stick and the screen will immediately restore the original background colour. After looking it up, it seems you have to mess about with the DMACON interrupt (DFF096) before doing the clearing - in particular the Copper DMA bit is what determines it. I just move #$80 to DFF096 which appears to do the trick. But why is this necessary on an A1200 but not an A500? What changed between them (apart from the obvious)? Is it specifically related to the processor, the chipset or the Kick ROM?

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24 October 2018, 16:27	#2
StingRay move.l #$c0ff33,throat Join Date: Dec 2005 Location: Berlin/Joymoney Posts: 6,863	I'd be interested to see the code you've been trying.

24 October 2018, 16:27	#3
meynaf son of 68k Join Date: Nov 2007 Location: Lyon / France Age: 51 Posts: 5,323	Quick guess : kickstart 1.x does not use the copper to setup colors (at least not that one).

24 October 2018, 17:09	#4
ross Defendit numerus Join Date: Mar 2017 Location: Crossing the Rubicon Age: 53 Posts: 4,468	There is only one 'substantial' difference between system copper list in KS1.x and KS20+. In KS13 COLOR00 is set only through gb_LOFlist/gb_SHFlist, on KS20+ also through gb_copinit. This does not justify a different behavior in case the COPJMPx addresses are not touched (and your COPEN tampering in DMACON proves this). So the error is somewhere in your code.

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