Relay Computer

Summary: 216 relays, Harvard architecture, 6-bit data, 8-bit program.

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Relay computer. Harvard architecture, similar to PIC 16C5x.
Solves Towers of Hanoi puzzle, etc.

The RR6 utilizes Harvard architecture, with data and programs stored in separate memory. The entire data memory consists of seven 6-bit registers:

R0/AC is the accumulator, which is used with all 2-operand instructions such as ADD and XOR.
R1, R2, R3 are storage registers.
R4 also operates the 7-segment display (center bar shows Carry status).
R5 is not implemented (because it wouldn't fit).
R6/LR also holds the return address (link) for subroutine calls.
R7/PC is the program counter, which can be written and thus form a jump table. Reading from R7/IN reads the front panel switches.

The program memory (PROM: Programmable Read-Only Memory) consists of 32 8-bit words implemented with 32 DIP switches.