Adding extended memory to a homebrew computer

You can never have too much memory. Or perhaps you have enough now, but at some point you’re going to want more.

In simple microcomputers, the amount of memory space you have available is dictated by the processor chip’s architecture — and specifically the width of its address bus. For example, the 65C02 at the heart of the Zolatron 64 has a 16-bit address bus that enables you to talk to 2¹⁶ address locations, otherwise known as 65,536 or 64K.

When I designed the Zolatron’s memory map, I kept the bottom 32K addresses for RAM, the top 16K for ROM and, just under the ROM, an 8K allocation for addressing I/O devices. That left an 8K section just above the main memory that I originally marked as ‘do something with this someday’. That day came quicker than I expected.