Re: Help: info needed for 9-007-001 SVA Apple Disk Controller

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

Steven Hirsch wrote:
> e p chandler wrote:
> > On Sep 22, 7:18 am, Steven Hirsch <snhir...@gmail.com> wrote:
> > > A2Aviator wrote:
> > > > http://twitpic.com/ijlx2
> > > > 4th row, last picture-
> > > > http://apple2.org/images/Interface_Cards/Floppy_Controllers/
> > > That looks a lot like a Lobo-branded card I have here.  I know there 
> > > are a
> > > couple of driver diskettes for it and may even be able to find them :-).
> > >
> > > The SVA Megaflex was a totally different device.  It was physically 
> > > smaller
> > > and supported DD 8" diskettes.  The data rate on these was just fast 
> > > enough
> > > (500Kbps) to make conventional polled I/O impossible, so vendors 
> > > resorted to a
> > > number of techniques.  The Megaflex established two almost-identical 
> > > polling
> > > loops one page apart in memory.  After issuing a read command to the 
> > > FDC, the
> > > unit entered an endless loop.  When the data-ready line went true, it 
> > > pulled
> > > the A7 line in between instruction fetches.  The CPU started executing a
> > > second loop from which it immediately branched out and starting 
> > > reading bytes.
> > >   Repeat, rinse...
> > >
> > > Other DSDD controllers used buss-master DMA (Vista A800) or played 
> > > tricks with
> > > halting the processor (Taurus).
> > >
> > > Steve
> >
> > That's almost as nasty as the code that WOZ wrote for the original
> > Apple parallel printer card. Instead of reading a handshaking line
> > through a data register, his code used that line to change the
> > addressing on the card's ROM. In one state it was an endless loop, in
> > the other state, it processed the data.
>
> That's essentially the same as the Megaflex (or, actually, the Megaflex 
> is the same as Woz's since his came first).  The Megaflex used static 
> RAM to run the on-board code.  I can't remember if there was a boot ROM 
> or not.  Will have to dig it out one of these days.

And note the similarity to the way the Disk ][ Controller state machine
works by using the read data line as an address bit.

After all, "State machines is state machines."

-michael

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Home page:  http://home.comcast.net/~mjmahon/

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