Re: RFC : Apple II DMA for free.

["bieling@terra.es" <bieling@terra.es>]

On Mar 26, 3:08 am, "mdj" <mdj....@gmail.com> wrote:
> Hi Jorge,
>
> It's a cool idea. A way to handle asynchronous I/O on the Apple II.
>
> I guess what's missing is the lack of a good use-case. I've run over a
> couple in my mind, but the main issue is that in order to take
> advantage of asynchronous I/O, you need appropriate software.
>
> One example might be say, a serial interface card that used this
> technique for data transfer. In theory, such a card could use your
> technique to fill/empty FIFO buffers during normal execution. This
> might turn out to be advantagous in say a terminal emulator, where the
> software can simply update the screen and read from the keyboard, and
> I/O happens transparently.
>
> The question you have to ask though, is would this technique provide
> sufficient bandwidth as to allow the Apple II to do serial comms at a
> faster speed than if this process was done synchronously? My guess
> would be yes on an unaccelerated II, no on an accelerated II (where
> the technique would be useless anyway)
>
> Perhaps a more compelling example would an operating system that
> supported pre-emptive multitasking. The big win here would be using
> the technique on a storage controller, which could potentially do it's
> transfers asynchronously and interrupt the OS when it was finished.
> This would allow much greater utilisation of the 6502 for actual
> computing tasks, but again, being compatible with existing
> accelerators is, I think, an unsurmountable challenge.
>
> Like any new hardware design or technique, you're reliant on the
> 'killer app' to make it worthwhile, so I guess the challenge is now on
> for all of us to come up with a way to do so :-)
>
> Cheers,
>
> Matt

This would have served as a great debugging aid. You could read/set
softswitches, vectors, page 0 locs, monitor a certain mem loc,
whatever, 100% transparently. You could interface a button to switch
the video modes, to step the disk II heads, to beep the speaker, to
mod the reset vector, the interrupt vector... to hack it all around
without the 6502 even noticing...!

I'd like to discover what the real achievable bandwidth would have
been.
A counter connected to this FREE/BUSY signal reseted every second
would tell... (i.e. a freq. counter)

And, yes, a zipchip would render it completely useless.. 8-(

Jorge.