Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!bagate!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: (Video) Hardware Idiots ?
Message-ID: <22368@cbmvax.commodore.com>
Date: 12 Jun 91 19:33:56 GMT
References: <1991Jun9.0 <1991Jun9.060440.29078@leland.Stanford.EDU> <1991Jun10.065629.21255@marlin.jcu.edu.au> <1991Jun10.074421.6782@mintaka.lcs.mit.edu>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 35

In article <1991Jun10.074421.6782@mintaka.lcs.mit.edu> rjc@wookumz.gnu.ai.mit.edu (Ray Cromwell) writes:
>In article <1991Jun10.065629.21255@marlin.jcu.edu.au> cpca@marlin.jcu.edu.au (Colin Adams) writes:

>Assuming those 5 million instructions were all move's theoretically
>the 030 could transfer 20 megs of data/sec if there was no bus
>contention and using fast ram. (5 MIPS * 4 bytes per longword = 20meg)

Strangely enough, the A3000's fast RAM bus runs at 20 MB/s (megabytes per
second), nonburst, 33 MB/s burst.  A 25MHz 68030 running with 0 wait states
(you never use burst at 0 wait states, the '030 cache is too small for that
to always win) manages 50 MB/s.  Obviously, if you're talking about data
transfers, the best you do is half of that, since the CPU need to read in,
then write out, any particular datum.  

>Chip ram cuts this figure in half (the processor only gets access
>every other cycle), and a 16bit bus cuts it in half again. 

Essentially wrong on both counts.  Forget about "every other cycle", that only
has meaning to Agnus.  Think "bandwidth".  Agnus will support 7.16MHz 68000
accesses at full speed, which turns out to be roughly 3.5 MB/s.  An A2500 can
only hit chip RAM at 3.5 MB/s, but since an A3000 has a 32 bit Chip RAM bus,
it's 68030 can hit chip RAM at 7 MB/s.  Since the blitter, if allowed, can also
manage 7MB/s (twice the RAM speed but half the bus width), there's no clear
winner in any race, it depends totally on what you're doing.

> And I wouldn't want 100% of my CPU being eaten up to copy data, 

That's true; people seem amazed when the CPU does something faster than the
blitter.  "Why not have the CPU do this?", they ask.  They are missing the 
big picture.

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
   {uunet|pyramid|rutgers}!cbmvax!daveh      PLINK: hazy     BIX: hazy
	"This is my mistake.  Let me make it good." -R.E.M.