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        Port 070  -  CMOS RAM address register and NMI mask (PS/2)

        This register is used in conjunction with port 070 to read
        and write the Real Time Clock/CMOS RAM bytes.

             1xxx xxxx  NMI Mask
                          When this bit is set to 1, the NMI is masked
                          off (the NMI is disabled). This bit is set to
                          1 by a power-on reset. This is a write-only
                          bit.
             x1xx xxxx  Reserved
             xx11 1111  RT/CMOS RAM Address
                          These bits are used to select RT/CMOS RAM
                          addresses.

        Warning:  The operation following a write to 070 should access
                  port 071; otherwise malfunction and unreliable of the
                  RT/CMOS RAM can occur.