Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Accessing the Chips directly
Message-ID: <15683@cbmvax.commodore.com>
Date: 7 Nov 90 19:54:08 GMT
References: <1220@iceman.jcu.oz>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 43

In article <1220@iceman.jcu.oz> cpca@iceman.jcu.oz (C Adams) writes:
>I was wondering whether C= supported directly writing to the chips
>registers.  I know you would have to do a Forbid()/Permit() and
>make sure blitter DMA was finished etc. to be multitasking friendly,
>but does C= state that any new chip set will be compatible with
>the old one.

In general, you shouldn't write to any machine register.  There are some
hardware specific things you can do in a quasi-system-supported manner, via
hardware "resource" descriptions and calls like OwnBlitter().  

>ie. would my program break on an Amiga 4000 if I wrote to the 
>chips for speed.

The underlying chip registers don't change unless there's a good reason for
them to change.  But if there's a good reason, then they change.  The most
recent such change was rather trivial -- the A3000 got a different realtime
clock, so that we could have a small amount of battery backed RAM for the
software folks to play with.  But it's certainly possible for other things
to change.  One other example is the color table.  If you wanted a particular
color setup on a custom screen, you might try to poke color values into color
registers.  Now, the OS will reset this, but ignore that for a minute.  We
recently introduced the ECS chipset in the A3000.  If you set up a 35ns
pixel display with that 3000, the color mapping physically changes.  As long
as you used the system call for setting colors, though, everything works 
properly.  

So my advice is, use the OS for absolutely everything, and at as high a level
as you can.  Leave the hardware alone, and you can expect to run on the
Amiga 10,000 or whatever.  Bang on the hardware, and you may even break on
the A3000.  If you have any detailed questions on a particular point, ask
away -- the software folks probably have a better idea about what they 
expect to support in future system, and what's properly isolated from the
hardware.

>Colin Adams
>Email: cpca@marlin.jcu.edu.au


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	Standing on the shoulders of giants leaves me cold	-REM