Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!sun-barr!ames!elroy!usc!pollux.usc.edu!addison
From: addison@pollux.usc.edu (Richard Addison)
Newsgroups: comp.sys.amiga.tech
Subject: Re: AmigaOS is real-time?
Summary: Nit clarification (copper list revisited)
Keywords: interrupts,copper,display
Message-ID: <17463@usc.edu>
Date: 27 May 89 05:02:09 GMT
References: <16100@louie.udel.EDU> <9881@watcgl.waterloo.edu> <16182@louie.udel.EDU> <17441@usc.edu> <16286@louie.udel.EDU>
Sender: news@usc.edu
Reply-To: addison@pollux.usc.edu (Richard Addison)
Organization: University of Southern California, Los Angeles, CA
Lines: 40

In article <16286@louie.udel.EDU> new@udel.EDU (Darren New) writes:
>In article <17441@usc.edu> addison@pollux.usc.edu (Richard Addison) writes:
>>Note that the copper takes care of maintaining the display (yes, even
>>interlaced) even when the 68000 doesn't respond to interrupts.
>
>I thought you had to re-stuff the start-of-copper-list out to the
>copper address registers during the vblank. Or can the copper
>do that to itself? (I don't think so, but it's been a while
>since I looked at that stuff...) -- Darren

Well, I went and double checked myself, and here's the low down:

The copper's "first location register" is set to the copper list startup
instructions (which sets the background color and initializes the sprite
pointers):

	Custom.cop1lc = GfxBase->copinit;

This copper list startup code strobes Custom.copjmp2 after waiting for the
top of the video field in order to jump to the "second location register."
Note that this will be either:

	Custom.cop2lc = GfxBase->LOFlist;

or:

	Custom.cop2lc = GfxBase->SHFlist;

The tail end of these two copper lists stuffs Custom.cop2lc with the address
of the opposite copper list and then waits for an impossible display
position.  All is not lost, though, because the vertical sync pulse comes
along to trigger a jump to Custom.cop1lc!

Thus, once it is set up, the copper takes care of the display for you!

PS.  In case anyone cares, I ordered custom license plates for my car
today:  GFXBASE

Richard Addison
"...and watch out for the Blue Meanies!"