Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!bagate!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.acorn,eunet.micro.acorn
Subject: Re: Archimedes Suggestions
Message-ID: <19337@cbmvax.commodore.com>
Date: 27 Feb 91 06:42:17 GMT
References: <1991Feb13.161140.7781@vax1.tcd.ie> <5345@acorn.co.uk>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Distribution: comp
Organization: Commodore, West Chester, PA
Lines: 33

In article <5345@acorn.co.uk> jroach@acorn.co.uk (Jonathan Roach) writes:
>In article <1991Feb13.161140.7781@vax1.tcd.ie> hughesmp@vax1.tcd.ie writes:

>>On the Amiga, you
>>can have up to 5 bitplanes and 4 channels of music going, without impinging
>>on the speed of the machine at all.

>which is actually not true. As I understand it the Amiga has the screen and
>DMA buses separated such that memory expansion beyond a certain point can't
>be DMAed, but doesn't have its bandwidth swallowed by DMA either, thus
>enabling the CPU to run at full speed in that area. If the CPU is running in
>the DMAable area, then its speed will be significantly hit as it contends
>for the memory's time with the screen and sound. 

That's essentially true.  Everything governed by the Amiga custom chips is
lock-stepped to the video cycle.  Video memory runs two cycles for each
single CPU cycle.  Resources with their own dedicated DMA slots, such as
audio and floppy disk, never contend with the CPU or any other resource.
Video display fetch, blitter, and video coprocessor activity, on the other
hand, is dynamically assigned.  Video display fetch can replace sprite 
fetch cycles, thus eliminating sprites in return for a larger display.  It
can also displace CPU access, in return for a deeper display.  The CPU can
run free out of "Fast" memory, which is on its own bus and these days many
times faster than video memory anyway, but the CPU can stall in access to
custom chip memory in certain cases, which can degrade system performance.

>--Jonathan


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	"What works for me might work for you"	-Jimmy Buffett