Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!wugate!uunet!cbmvax!jesup
From: jesup@cbmvax.UUCP (Randell Jesup)
Newsgroups: comp.arch
Subject: Re: *big iron*
Message-ID: <8124@cbmvax.UUCP>
Date: 7 Oct 89 02:29:44 GMT
References: <7981@cbmvax.UUCP> <11538@burdvax.PRC.Unisys.COM> <22488@cup.portal.com <1016@cavs.syd.dwt.oz>
Reply-To: jesup@cbmvax.UUCP (Randell Jesup)
Organization: Commodore Technology, West Chester, PA
Lines: 35

>>Yup. That is because of intelligent channel processors that do DMA to multi-
>>ported
>>memory. The same thing SCSI can do. Except with one user, we only need one
>>channel
>>(or one for each file). But with UNIX we could use a few more.
>>
>	One small point to add, the amiga does DMA to dual port ram.  All DMA
>goes through a bank of ram called chip ram ( because the graphics coprocessors
>also use this area) while everything else is run is fast ram.  This is a big 
>help as the amiga does have a multitasking operating system ( usually single
>user though.)

	A correction: some amiga disk controllers DMA to dual-ported memory.
Some DMA directly to system memory.  A few don't use DMA at all (but are
slightly cheaper).

	DMA to DP memory has some advantages, but speed isn't usually one of
them on the Amiga.  This is because the data has to cross the bus at least
one extra time, and of course the processor load increases.

	DMA straight to system memory (essentially any memory in the system)
is faster if done right.  FIFO's are important here to avoid DMA overruns.
This, combined with the Amiga FastFileSystem allows data to often be DMA'd
directly into the application's destination for the read (or from it for
a write).  This improves performance even more.

	"chip ram" is ram that the graphics/audio/floppy/etc coprocessors
can access directly.  This is currently 1Meg (used to be 512K).  Expansion
devices (like HD controllers) can DMA to any memory they want to (including
"chip ram").

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"