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

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

jonnosan wrote:
> Howdy all.
>
> I'd like to understand how to have a peripheral card override the ROM
> on the motherboard, to allow access to alternative ROM images (e.g.
> Senior PROM / EDM).
>
> What I want to do is have a peripheral card with an EPROM in it, and
> an 'on/off' switch, and when the switch is 'on', any attempt to access
> to either the Applesoft or Monitor ROM code goes to the code in the
> EPROM instead.
>
> I'd initally thought I could  hook up d0..d7 and a0..a13 on a 27128 to
> the corresponding bus signals, and then connect a14,a15 and the 'on/
> off' switch via AND gates, and then NOT the result to the EPROM OE' so
> OE' only goes active when both A14 & A15 are high (i.e. the address
> requested is in the range $C000..$FFFF). I'd then also connect OE' to
> INHIBIT' (so that when the EPROM gets activated, the motherboard ROM
> is disabled)
>
> But I think this would cause problems during access to any of the
> other things that can also reside in the memory range $C000.$FFFFF,
> i.e. any of the softswitchs or RAM banks.
>
> So how would a peripheral card detect when access is attempted to
> firmware ROM, so it can override it?

With a little more complex decoding, you can derive a signal that
is true only in the Dxxx..Fxxx range.

For example:  A15 * A14 * /A13 * /A12

As to the practicalities of using /INHIBIT, that's another matter.

-michael

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