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Path: utzoo!mnetor!seismo!lll-crg!rutgers!cbmvax!daveh
From: daveh@cbmvax.cbm.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: re DMA hard disks
Message-ID: <1051@cbmvax.cbmvax.cbm.UUCP>
Date: Wed, 3-Dec-86 14:47:00 EST
Article-I.D.: cbmvax.1051
Posted: Wed Dec  3 14:47:00 1986
Date-Received: Thu, 4-Dec-86 09:09:57 EST
References: <2135@well.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 58

> 
> Really, is that true?  If you get a hard disk that is DMA it has a DMA chip 
> .. completely seperate from the Amiga DMA circuitry?  So what does that mean for
> the lowly 3.5" floppies?  They don't have their own DMA circuitry so do they
> DMA only to chip memory and not to fast memory?  

True.  If you go down to the lowest levels of 3 1/2" interface software,
you're required to supply a 512 byte disk buffer in Chip memory, and of
course the normal track buffer is as well in chip memory.  The actual 
decoding of the MFM encoded data is done with the Amiga's blitter, which
requires Chip memory.

> Is it definate that when 
> an advertiser says that their hard disk is DMA then they mean that they have 
> one of those full memory DMA's and are not just using the Amiga DMA to Chip
> memory?  How does the sidecar do it? or is the side car just going to be slow?
> 

Any external DMAed device will have to be a DMA device in the conventional
sense; you can't use the Agnus supplied DMA to talk to external DMA devices;
remember, this DMA can only take place over the 512K range of Chip memory,
and is driven by the custom chips.  An external DMA must be driven by an
external DMA controller, though the rest of the system, being quite used
to DMAs happening, will oblige that external DMA.  The DMA can be to the Chip
memory or any fast memory, base on the abilities of the external controller.
The SideCar works though shared memory; an area of the Sidecar memory is
accessable to both the PC and Amiga sides.

> Is the sidecar going to have 2 meg of sockets for Amiga memory and sockets to
> bring the IBM memory up to 640K and an 8087 co-processor socket and is it going
> to reach the market?

Last I saw, the SideCar would accept an internal Amiga side memory expansion
board, though its not just filling sockets.  The basic configurations are 
256K and 512K I think, and the 256K version has the sockets available to add
another 256K for a total of 512K.  There is an 8087 socket, and the SideCar
is already being sold in Canada; don't know how long before its in the US.

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-- 
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Dave Haynie	{caip,ihnp4,allegra,seismo}!cbmvax!daveh

	"Laws to supress tend to strengthen what they would prohibit.
	 This is the fine point on which all the legal professions of
	 history have based their job security."
						-Bene Gesserit Coda

These opinions are my own, though for a small fee they may be yours too.

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