Path: utzoo!utgpu!watmath!att!tut.cis.ohio-state.edu!ucbvax!UOTTAWA.BITNET!451061
From: 451061@UOTTAWA.BITNET (Valentin Pepelea)
Newsgroups: comp.sys.amiga.tech
Subject: Re: DMA or polling (was Re: GVP controller)
Message-ID: <8908092130.AA23369@jade.berkeley.edu>
Date: 9 Aug 89 21:15:23 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 50

Steve -Raz- Berry  writes in <120232@sun.Eng.Sun.COM>

> In article <8908072207.AA14796@jade.berkeley.edu> 451061@UOTTAWA.BITNET
>  (Valentin Pepelea) writes:
>
> >The net result is that the processor therefore spends less time on the data
> >transfer and is available more often for other concurrent tasks.
> >Unfortunately this means that there are two transfers occurring, a slow
> >DMA from hard disk to the cache, and a fast CPU transfer from the cache
> >to internal memory. Other controllers such as the A2090 and HardFrame DMA
> >directly from the harddrive into internal memory, thus tying up you CPU
> >much longer.
>
>       Yikes! I'm sorry, but I TOTALLY disagree with you on this one.
> Logicly, if you look at the time to complete a given task, based only
> on the number of bus cycles it takes to transfer a given block of data,
> DMA will always win. Period. Unless of course your DMA circuitry is
> totally braindead.

Clearly you don't understand, or perhaps I did not explain well. The bottleneck
here is the speed at which the hard disk turns, and therefore the rate at which
data is available to the DMA channel. That is why DMAing directly from the hard
disk to internal memory is a loosing proposition. GVP provides a cache into
which it reads from the disk while leaving the Amiga's 680x0 alone. Only then
does it transfer the data from the cache into internal memory at full speed,
without having to wait for the mechanical limitations of the hard disk.

Perhaps it should then DMA from its cache into internal memory, but that is
another question. Even if it did that, it still would get lower diskperf's than
the A2090 or HardFrame. The improvement would be rather limited, and the cost
would be higher. The GVP controller is expensive enough as it is.

> Sorry, this is one EE type that
> just won't believe it. The Amiga is a DMA machine, that is part of
> what gives it it's amazing speed for graphics and sound.

Obviously some EE types are better than others. Good luck on your '030
accelerator design.

> (this is part of my effort to insure a "kinder and gentler" netdome)

Perhaps you meant "thunder.net.dome". Two men enter, one flamed leaves.

Valentin
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