Expanding the internal memory of the C128?

I am the author/perpetrator of that particular memory expansion.

In the 1990s I performed the original modification (for a modest fee,) on several C-128s besides my own. The only complaint I was ever made aware of was that it didn't work with any software that relocated Zero page and/or the stack. The so-far-untested c256v2 design theoretically corrects that limitation.

I demonstrated the 512K modification at a Bronx, New York User Group, running the Spinning Globe demo with the pictures residing in internal memory, instead of in an REU. (I no longer think 512K was a Good Idea.)

The only point where I thought delays might pose a problem was where I replaced U9, a quad OR gate that sorts out /CAS0 and /CAS1 from the MMU, /CASENB from the PLA, and /CAS from the VIC with a 74LS138 1-of-8 decoder. 74LS_ worked in MY '128 -- even at 2MHz -- but when modifying other's computers I used a 74F_ part, just to be safe.

OF course no C= software accesses the extra memory: it's not supposed to exist. But Basic and the Kernal assume Blocks 2 and 3 are real, and a User with the chops can access them easily using only standard Basic commands and Kernal calls.

The heaviest hit power-wise is the second MMU. (150mA, max.) I made sure to use CMOS DRAMs for the added RAM, and only four of them, as 64Kx4s. I optimistically (or naively,) assumed that the PSUs for both the low profile '128 and '128D had sufficient current capacity. I never experienced any power problems.

I don't know how well or poorly my expansion worked in CP/M. At the time, I didn't care, and still don't.

I found Richard C.'s detailed, original Twin Cities 128 magazine articles in the form of .PDF's at


Truly,
Robert Bernardo
Fresno Commodore User Group - www.dickestel.com/fcug.htm
Southern California Commodore & Amiga Network - www.portcommodore.com/sccan
Nov. 6-7 Commodore Los Angeles Super Show 2021 - www.portcommodore.com/class