Custom memory card to support macro expander on Apple 2e

• The Apple 2e is an 8-bit machine with 16-bit addresses. The resulting space crunch results in considerable "virtual memory" thrashing when running my macro expander.
• The memory card I built provides convenient access to an additional 32K list cells, each of which is 8 bytes wide. It all fits on a standard sized Apple 2 daughterboard, which fits in an I/O slot and is accessed through 16 bytes of memory mapped I/O.
• The user program puts the desired cell number into the upper 2 bytes of the I/O space, and the circuitry provides read/write access to the cell through the lower 8 bytes of the I/O space.
• I wrote the memory test program for this card, which found a problem. After changing one wire, everything worked perfectly. This was good, since I had no logic analyzer available at the time.