Timer 8253                      040 - 043

The chip has three timers (16 - bit counters) :

Timer 0 (040) : Time-of-day clock. Final count invokes (IRQ 0) INT 8
                Also used for timing Disk I/O.

Timer 1 (041) : RAM refresh. Connected to DMA.

Timer 2 (042) : Connected to speaker. Active when bit 0 of port 61h is
                set to 0.

Input clock rate for all channels is 1.19318 MHz (PC,XT,AT)

Command register (043) :
                Bit 0     : 0 if binary, 1 if BCD
                Bit 3 - 1 : mode 0 - 5 (000 - 101)
                Bit 5 - 4 : kind of operation
                    00 : move counter value into latch.
                    01 : read/write high byte only.
                    10 : read/write low byte only.
                    11 : read/write high byte, then low byte.
                Bit 7 - 6 : channel number, 0 - 2 (00 - 10)