Each 8520 has 16 registers that you may read or write.  Here is the list
of registers and the access address of each within the memory space
dedicated to the 8520:

                       Register
   RS3  RS2  RS1  RS0  #(hex)  NAME      MEANING
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    0    0    0    0     0     pra       Peripheral data register A 
    0    0    0    1     1     prb       Peripheral data register B 
    0    0    1    0     2     ddra      Data  direction register A 
    0    0    1    1     3     ddrb      Direction register B 
    0    1    0    0     4     talo      Timer A  low register
    0    1    0    1     5     tahi      Timer A  high register
    0    1    1    0     6     tblo      Timer B  low register
    0    1    1    1     7     tbhi      Timer B  high register
    1    0    0    0     8     todlow    Event LSB 
    1    0    0    1     9     todmid    Event 8-15 
    1    0    1    0     A     todhi     Event MSB 
    1    0    1    1     B               No connect
    1    1    0    0     C     sdr       Serial data register 
    1    1    0    1     D     icr       Interrupt control register 
    1    1    1    0     E     cra       Control register A 
    1    1    1    1     F     crb       Control register B 
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