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From: Richard Atkinson
Date: 1998-09-16
Subject: 256K RAM expansions

I've been thinking about RAM expansions again, and I had an idea for the simplest 256K expansion possible... (or is it?)

This expansion uses only commonly availble parts (no MOS or CSG stuff!)
and applies to any machine with 4164 DRAMs and a spare decoded address line. The idea is that an 8 bit latch (eg. 74373) stores the selected banks, and an 8-1 multiplexor (eg. 74151) is used to generate the A8
address line for 41256 DRAMs. A14, A15 and the row/column select line (eg.
/MUX in plus/4) are the control signals for the multiplexor, so that you have 4 banks of 16K, each of which can have one of four possible memory areas switched in.

Any suggestions? For example, does anyone know of a 74 series 8 bit register, so that the current bank values can be read, as well as written to? If my idea works, I will expand a plus/4 and a C64 to 256K and maybe even a C128 to 512K. Then I'll need to convert those expansion-aware programs to work with my system... (argh, software!)

Richard.