Programmable Interval Timer (8254 Compatible)

I/O      R/W
Address  Mode Description
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0040h    R/W  PIT counter 0
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0041h    R/W  PIT counter 1
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0042h    R/W  PIT counter 2
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0043h    W    PIT control word, where:
              bits 7-6   = 00 select counter 0
                         = 01 select counter 1
                         = 10 select counter 2
                         = 11 read back command
              bits 5-4   = 00 counter latch command
                         = 01 read/write counter bits 0-7 only
                         = 10 read/write counter bits 8-15 only
                         = 11 read/write counter bits 0-7 first, then bits 8-15
              bits 3-1   = 000 mode 0 select
                         = 001 mode 1 select
                         = x10 mode 2 select
                         = x11 mode 3 select
                         = 100 mode 4 select
                         = 101 mode 5 select
              bit 0      = 0 binary counter 16 bits
                         = 1 BCD counter
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