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      The six registers mapped into data memory are:

            Register    Address     Definition
            DRR(15-0)   0     Serial port data receive register
            DXR(15-0)   1     Serial port data transmit register
            TIM(15-0)   2     Timer register
            PRD(15-0)   3     Period register
            IMR(5-0)    4     Interrupt mask register
            GREG(7-0)   5     Global memory allocation register

      The memory-mapped registers may be accessed in the same manner as
      any other data memory location, with the exception that block moves
      using the BLKD (block move from data memory to data memory)
      instruction cannot be performed from the memory-mapped registers.