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Elf Cube Processor Card

This project is part of the Elf Cube. A 1970's retro computer system based on the RCS CDP1802 8 bit microprocessor.

The processor card hosts the system CPU, one 28C256 EEPROM and two 62256 RAM chips giving a total of 32K ROM and 64K RAM.

The 'Q' signal is presented as a single LED on the edge of the board.

On Reset, memory is mapped as:

0x0000 - 0x7FFF ROM 0x8000 - 0xFFFF RAM (2nd RAM chip)

On executing any instruction in the upper memory area (0x8000 - 0xFFFF), the ROM is mapped out, and replaced with the 1st RAM chip, leaving a 64K RAM system.

Typically the ROM will contain bootstrap code which copies OS/Monitor code into high RAM and executes it there, hence leaving the OS/Monitor running in a full 64K RAM. The ROM is no longer available until the processor is reset.