Path: utzoo!yunexus!ists!jarvis.csri.toronto.edu!mailrus!uunet!cbmvax!daveh
From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Hard disks, DMA vs Non-DMA
Message-ID: <8620@cbmvax.UUCP>
Date: 17 Nov 89 22:55:01 GMT
Article-I.D.: cbmvax.8620
References: <1989Nov16.185706.29328@sjsumcs.sjsu.edu>
Organization: Commodore Technology, West Chester, PA
Lines: 36

in article <1989Nov16.185706.29328@sjsumcs.sjsu.edu>, 33014-18@sjsumcs.sjsu.edu (Eduardo Horvath) says:

> 	Can you DMA directly into FAST RAM, 

Yes.  In fact, it's greatly preferred and recommended.

> 	If a controller DMA'd into FAST RAM, wouldn't that solve the problem 
>	of contention with the custom chips?

It can solve most of the problem.  There are two components to the DMA transfer.
DMA to Fast memory will solve the second, which is the basic transfer rate for 
whatever block size the controller transfers in one chunk.  The first problem is
what I call "DMA lag", or how long it takes from the time your controller asks
for the bus to when it actually gets the bus.  In order to acquire the bus, the
CPU bus be finished with a bus cycle.  If the CPU is in wait states, waiting for
access to the chip bus, the DMA controller will have to wait for the CPU to
finish it's cycle, (eg, wait for the chip bus to be free), before it can take
over the bus.  DMA controllers often transfer a whole block (512 bytes) in
several DMA passes, so it's actually possible to incur this lag several times for
each block, if your CPU is doing lots of stuff with video memory.  Also, if you
have an autoboot controller of any kind that copies it's code to RAM before 
using it, you get slowdowns if your autoboot card is the first one in the machine,
since that code will get copied into chip memory.  So, unless you know your code
is running from ROM, or you have something like an A2620/A2630 that puts autoconfig
RAM in before your device is configured, it's best to put a memory card in before
your device.  Hopefully all-in-one memory/disk cards autoconfig the memory before
the disk.


>       ///     \		Eduardo Horvath		=	Makes a man poor


-- 
Dave Haynie Commodore-Amiga (Systems Engineering) "The Crew That Never Rests"
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                    Too much of everything is just enough