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Posted By

TLC
on 2007-01-08
15:07:17
 Re: Plus/4 Expansion port...

Hi Christian! Well... I do have a 64K memory expansion module (like that) somewhere, but never checked it closely (I thought I'd need to break the case open just to take a close look at the board... which I didn't want). So I haven't even known that they're just that plain simple inside.

As for the freezer module... I was lucky enough to browse the ex-Funet CBM archive (now hosted at zimmers.net) someday and stumbled upon this: http://www.zimmers.net/anonftp/pub/cbm/schematics/cartridges/plus4/index.html . Jerzy Sobola has reverse engineered the cartridge and drew a schematic / exported the EPROM dump.

I checked the sch, but for me it's a little bit confusing. It's clear that the Eprom can be "seen" at Cartridge 1 low, regardless to other conditions. This makes it able to "boot". There's a circuit, that would also assert this chip regardless to the RAM/ROM banking settings and C1low, as soon as 1.) the pushbutton is pressed, 2.) IRQ is high, 3.) AEC is high, 4.) A15 is high. Upon 1.), 2.) and 3.), plus 4.) R/W' is high, A9 of the address bus would also be forced to low. ...I just can't complete the story. What is all this hassle about?... I can understand pieces, but not the whole thing. For example, pulling the address line (which is fully legal, as the circuit pulls it down by an OC output) would be suitable (mostly) for hijacking a currently running code. Also, forcing in the EPROM might be suitable to do some "bootstrapping", then initializing the RAM/ROM setting and finally bank in C1low. ...It's just not fully logical. For example, why messing with A9 at all?... Also, it could be possible to freeze something using this method, but not restarting it, as far as I'm concerned. ...Should be the conclusion, that the sch is not fully precise?...



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