VIC-20 Cartridge mapping question

From: Carlsson, Anders (
Date: 2000-10-02 16:18:43

Hi folks!

During the past weekend, I borrowed six VIC-20 cartridges from a friend.
These are Sea Wolf, Pinball (Spectacular), (Super) Slot, UMI:s Renaissance
(packaged for UK/Europe by Audiogenic though) and Amazing Maze, a rather
rare cartridge which includes a runs-in-RAM-check.

The last one, Amazing Maze, is a 4K cartridge (A000-AFFF), but when I
searched through the memory, I found something at B000-BFFF as well, and
saved it down to disk. It turned out the first and second images are
identical, down to the single byte.

My question is how this is possible? Is the cartridge ROM designed this
way, or is it VIC which maps a cartridge which doesn't fill a full 8K
block with itself again? Or could it be my RAM expander which plays me
a trick?

The game (with a warranty label from OEM on the back side) btw turned out 
to be quite a lot of BASIC, possibly with some MC routines and should be
the same image already found on Funet.

/Anders Carlsson
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