Re: overriding motherboard ROM with a peripheral card in a platinum iie

[jonnosan <jonnosan@gmail.com>]

On Mar 21, 1:01 pm, David Wilson <mcs6...@gmail.com> wrote:
> On Mar 21, 9:57 am, Mike Willegal <m...@willegal.net> wrote:
>
> > One other thought.
>
> > Inhibit only controls enable of the onboard ROM D000 to DFFF.
>
> This is true for the Apple II and II+.
>
> The Apple IIe is different - according to Jim Sather (page 5-24
> Understanding the Apple IIe) the INHIBIT' line disables all main &
> auxiliary RAM as well as the ROMs.

Indeed - and this is why (for example) the original Wildcard doesn't
work in a iie - when you press the button to capture an image, it
generates and NMI, and also asserts INHIBIT', so that control
transfers to code in the ROM on the wildcard.  But the code in the ROM
assumes it can still access RAM while INHIBIT' is active, so it fails
in a iie.

What I was thinking of doing was making a home-brew copycard, using
the senior prom image, and AVR and a pushbutton, such that the
pressing the button would switch the rom images and trigger an NMI.

I have previously done something similar to what Silicon Sam
suggested, i.e. made a 27128 with a switch that toggles A13 on and off
to flip between 2 ROM images. But that won't let me synch the flip
between the two images with generating an NMI.

I think the easiest thing to do will be to forget trying to assert
INHIBIT on a peripheral card, and just replace the standard
motherboard ROM with 27128 with A13 hooked up directly to my AVR
circuit.