Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!purdue!haven!grebyn!ckp
From: ckp@grebyn.com (Checkpoint Technologies)
Newsgroups: comp.sys.amiga.tech
Subject: DMA in VM
Message-ID: <14059@grebyn.com>
Date: 29 Nov 89 14:45:10 GMT
Reply-To: ckp@grebyn.UUCP (Checkpoint Technologies)
Organization: Grebyn Timesharing, Vienna, VA, USA
Lines: 23


	In the great DMA vs non-DMA debate, there is one question which
still nags at me. When AmigaDOS sprouts virtual memory, and there is a
possibility that a disk buffer may be scattered in several discontiguous
physical memory pages, which of the available disk controllers will be
able to handle this kind of buffering effectively? The answer, I fear,
is "the non-DMA ones".

	Do any of the DMA controllers have the ability to support
a discontiguous IO buffer? I know the Commodore A2090 can't. I don't
know about the A2091 or the A590. The HardFrame may; I know it uses
the Motorola 68430 DMA chip, and I also know that it's big brother
the 68450 4-channel DMA chip supports chained DMA operations, but I
don't know if Motorola endowed the 68430 with this feature.

	If the DMA controller hardware can't handle discontiguous IO
buffers, then they must DMA into a contiguous buffer and CPU-copy the
results to the task buffer. This would be a big win for non-DMA
controllers, which would not need this intermediate RAM buffer.

	Oh, and it would be a dirty shame if programs weren't able to
allocate virtual memory for their disk buffers, IMHO. It would make
virtual memory less useful.