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        Port 3CF, 00  -  Set/Reset register

        1111 xxxx  Reserved
        xxxx 1xxx  Set/Reset Map 3
        xxxx x1xx  Set/Reset Map 2
        xxxx xx1x  Set/Reset Map 1
        xxxx xxx1  Set/Reset Map 0

        Bits 3-0:  When write mode 0 is selected, the system writes the
                   value of each set/reset bit to its respective memory
                   map. For each write operation, the set/reset bit, if
                   enabled, is written to all eight bits within that map.
                   Set/Reset operation can be enabled on a map-by-map
                   basis through the Set/Reset Register.