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

[e p chandler <epc8@juno.com>]

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.

- Elliot