RE: C128 Ram Banks 2 and 3

From: Marko Mäkelä (
Date: 2001-02-22 09:09:34

On Thu, 22 Feb 2001 wrote:

> I assume then that there was never an official CBM way to make the
> expansion even though they design protected signal names?

I don't quite understand what you mean.  Should "they" be "their"?

Anyway, I assume that the MMU chip internally generates the CAS signals
for RAM banks 2 and 3, but for some reason they didn't want to manufacture
a bigger chip so that these signals could be brought out from the chip. (I
think that the MMU is 48 pins already.)  The MMU registers do store the
extra bits.

On an unexpanded machine, selecting bank 2 is equivalent to selecting bank
0, and 3 is the same as 1.  The .sfx (self-extracting lharchive) file
header detects C64 or C128 mode by switching to bank 2, crashing an
expanded machine.  I don't know if other programs misuse this feature to
detect the C128 mode.


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