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        Port 043  -  8253-5 Mode Control

        Only mode 2 and 3 used.

                11xx xxxx  00=channel 0
                           01=channel 1
                           10=channel 2
                xx11 xxxx  00=latch present value
                           01=read/write only MSB
                           10=read/write only LSB
                           11=read/write LSB, the MSB
                xxxx 111x  000=mode 0: countdown with optional inhibit
                                       (level output)
                           001=mode 1: countdown with optional restart
                                       (level output)
                           010=mode 2: generate one pulse out of N
                                       (used for DMA memory refresh
                                       by channel 1)
                           011=mode 3: generate square wave
                                       (used for channels 0 and 2)
                           100=mode 0: countdown with optional inhibit
                                       (pulse output)
                           101=mode 1: countdown with optional restart
                                       (pulse output)
                xxxx xxx1  0=binary, 1=BCD counter decrementing
                           binary counting always used in PC