http://www.xpl0.org/rr6/

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.