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Posted By

siz
on 2015-12-21
08:15:46
 Re: unused I/O location for debug purposes

You have to do your own address decoding outside the computer. (Except for the expansion ROM lines) Basically you catch all write attempts in the 64K address range.
For I/O purposes the $fd00-$feff range is reserved so they are safe for read. And you have to be prepared for other expansions present which are mentioned above.
You can use ROM expansions also as does BSZ's custom extension used for the Evo Lution demo: it maps RAM in the place of the ROM as video memory. In this case your write attempts are processed by your expansion AND the underlying RAM too.
And also there is the possibility which is used by the 64K RAM expansions which "replaces" the internal RAM (or more like overrides) with an external one. I'm still not sure how does that work.

But for your debugging purposes $fe00-$fe7f range seems to be safe to use.



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