Re: Combined VIC and PET cartridge

From: Ruud Baltissen (
Date: 2001-05-21 13:54:13

Hallo Marko,

> My question for PET experts is: Are there free I/O block selection signals
> on the expansion port?

The VIA's, CRT and PIA resides on $E8xx.
The only lines available are those covering $4xxx until $Bxxx in steps of
I had a quick look at the various docs of Olaf but couldn't find anything
that said that $E900-$EFFF is used by ROM or whatever.
$9xxx and $Axxx are free for onboard ROM-expansion.

So IMHO you have two solutions:
- you use $9xxx and/or $Axxx
- you use $E9xx..$EFFF (if available), but complete decoding needed

> And regarding the 6545 CRTC: are the registers R18
> through R31 unassigned?

Seen from the CPU, the 6545 only uses two registers. So the fact that the
INTERNAL registeres R18/31 are used or not, doesn't matter.

Groetjes, Ruud

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