Re: Microsoft Softcard and IIgs

[mdj <mdj.mdj@gmail.com>]

On Mar 13, 4:55 am, "Michael J. Mahon" <mjma...@aol.com> wrote:

> > Add an accelerator to the host and sky's the limit.  You cannot use an
> > accelerator with a DMA device.  Applicard works with any accelerator on
> > any version of the A2.
>
> For me, this is a huge advantage.  It's too bad that most accelerators
> were not implemented with DMA compatibility (though I know it would have
> complicated them).

For Zip Chips, it always struck me as odd, since the cache writes
through via a buffer. The DMA setup time should have been adequate to
dump the write buffer. All it needed to do then was flag the cache as
dirty.

Of course, a better implementation could have used /INH to make the
cache 'visible' to a DMA peripheral but that requires a slot, or at
the very least, a tap to the relevant signal...

> >>> I have a ROM1 GS and a SoftCard clone. If you want I can test it.
>
> >> 01 and 03 should be identical in the slot operations, but yes, if you
> >> find the spare cycles and feel interested - please do.
>
> >> I will dedicate a day (no plans, though) to plug the analyzer in IIe
> >> and IIgs and see what's wrong. M$ SoftCard is the only DMA card I have
> >> so far.
>
> > There were very few A2 cards that used DMA.  The only others I can think
> > of are the Transwarp I (DMA with a vengeance - once it grabs on, it
> > doesn't ever let go) and the Vista 8" floppy controller.
>
> Actually, any DMA device in the Apple II must "let go" frequently
> to prevent the 6502 from losing its dynamic state.  Woz did some
> experiments and found that typical 6502s could tolerate much longer
> "holds" than were specified, but the time was temperature- (leakage-)
> dependent, so holding it longer than factory specs is iffy.

That only applies to the older NMOS 6502's though, right?

Matt