Re: DMA 4 meg v. 8 meg

["Michael J. Mahon" <mjmahon@aol.com>]

Charlie wrote:
> On 5/5/2011 4:03 PM, Michael J. Mahon wrote:
>
> > Charlie<charlieDOTd@verEYEzon.net>  wrote:
> >
> > > On 5/5/2011 1:21 AM, David Empson wrote:
> > >
> > > > Michael Black<et472@ncf.ca>   wrote:
> > > >
> > > > > On Wed, 4 May 2011, KP wrote:
> > > > >
> > > > > > I have a 4 meg RAM card in the memory slot of my ROM 03 IIgs.  I have
> > > > > > often heard that only the first 4 megs of expansion-card RAM is DMA-
> > > > > > compatible.  Can somebody explain what DMA actually is?
> > > > > >
> > > > > > Does this mean that AppleWorks v5.1 will only recognize 5 megs (1 meg
> > > > > > motherboard RAM + 4 meg expansion card RAM) even if I put an 8 meg
> > > > > > expansion RAM card in the memory card slot?
> > > > > >
> > > > > > Does this mean that the second 4 meg of expansion card RAM is useful
> > > > > > only as a RAM disk, and not as memory for programs to access, in 
> > > > > > order
> > > > > > to store long documents that the user is editing, graphics being
> > > > > > opened from disk, sounds being edited, HyperStudio stacks, etc.?
> > > > > I see the specific question wasn't answered.
> > > > >
> > > > > It won't mean a thing for your purposes.
> > > > >
> > > > > Unless you have something that takes control of the Apple bus in 
> > > > > order to
> > > > > directly control the RAM, lack of DMA won't matter to you.
> > > >
> > > >
> > > > And for a IIgs, the only cards I can think of which use DMA are the
> > > > Apple High-Speed SCSI card, and the RamFAST SCSI card. If you don't 
> > > > have
> > > > either of them, you don't need to worry about DMA.
> > > >
> > > > Is anyone else aware of any other DMA-capable cards that are likely to
> > > > be used in a IIgs?
> > >
> > >
> > > The Carte Blanche card.  As for how likely they are to be used on a 
> > > IIgs,
> > > I might be the only one.
> >
> >
> > I thought the JAT firmware only eavesdropped--no DMA...
>
>
> That's true in the video module.  There is also a DMA module in the JAT 
> firmware code.  What it is used for would be better answered by Alex 
> Freed, but it could be to load files from the SD card.

But that has nothing to do with the Apple II bus.

In fact, it would be a *very bad* idea to create any Apple peripheral
that relied upon DMA to function.  Since DMA discipline is so poorly
supported by most cards, DMA-using cards are almost all incompatible
with each other.

Everyone avoided it like the plague, except for Z80 cards and the like,
which got away with it only because they were the only cards in the
machine using DMA.

-michael

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