Re: Ultimax questions

On 20/10/2013 14:45, Michał Pleban wrote:
> But first, you need to start supporting 16kB cartridges which you don't
> at the moment, and the MAX BASIC cartridge is 16kB. Actually this is
> quite easy. The only problem is that you need to AND the /ROML and /ROMH
> lines and feed this as the /CS line to your EPROM. You can use two
> diodes and a resistor to do that - so no additional chip needed. Then,
> either /ROML or /ROMH needs to go to A13 line of the EPROM and you're
> good to go.
I have around 20 spare 27C020 2mb eproms which I was planning on using 
which would hold 32 x 8k images. With this method I would only fit 16 
images and as there's only 3 16k MAX cartridges, it's rather a waste. 
I'm wondering if there is a way of using /ROML to override the latched 
values on A16, A15 and A14 so all the $8000-$9FFFF images only use the 
bottom 4 x 8k images on the eprom?

> Exactly what I had in mind.
>
> There is one major drawback here - this technique will work only on a
> real MAX. On C64 these lines have different meanings; in fact, instead
> of CLK there is a DOT CLOCK line which will very quickly set the shift
> regiser to all 1's (unless you play with $DF00, but you wouldn't). I
> need to think on how to modify this for using in the C64, if that's even
> possible.
I would really like it to work on a 64 as well. Primarily because I do 
not want to be experimenting with my MAX during development any more 
than necessary. If I blow up a few chips in a 64, it's not a big problem 
- but if I damage my MAX, it would upset me.

How about AND'ing pin 10, which is /IO2 on the 64 and /CIA on the MAX, 
with A7? On the assumption that none of the MAX ROM's don't write to any 
of the CIA's mirrored registers during their normal operation we could 
use a write to $DC80 and $DF80 on the MAX and 64 respectively?

Rob


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