Timer Ports

  Ports 40H-43H control the PC timer circuits.  There are three timer
  channels:

   0) Channel 0 is used as a general system timer.  It handles the periodic
      "timer-tick".  It is initialized by the BIOS to invoke INT 08H
      (IRQ 0) 18.206 times per second (once every 54.936 milliseconds).

   1) Channel 1 is used in a dedicated manner to support the memory refresh
      function.  The output is used to generate a direct-memory access
      (DMA) cycle request on DMA channel 0.  This DMA channel is used to
      refresh the systems RAM by performing a dummy memory-read every so
      often (once every 15 microseconds on the original PC).

      This is critical to system operation, so we are usually advised to
      keep "hands off".  However, I have seen experiments in which this
      timer was slowed down a bit in order to increase overall system
      performance by a few percent.  Too slow of a value eventually causes
      fatal memory errors.

   2) Channel 2 is used to drive the audio speaker on the system board (it
      originally handled cassette-tape I/O, but that is surely moot).  The
      output of channel 2 can be directed to control the speaker via I/O
      port 61H (see PPI Ports and Speaker Control).

      Channel 2 is available for application-controlled timing tasks;
      however, it is a global resource.  Rather than using direct port I/O
      you are advised to use fns such as INT 1aH and INT 15H 86H for timed
      operations.

Port  Description
▀▀▀▀  ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
 40H  Read/Write: Count value for counter 0
                  Port 43H bits 4-5 control the way the counter is set.
 41H  Read/Write: Count value for counter 1
 41H  Read/Write: Count value for counter 2
──── ───────────────────────────────────────────────────────────────────────
 43H  Write: set channel's mode of operation
      ╓7┬6┬5┬4┬3┬2┬1┬0╖
      ║ch#│r/l│mode │ ║
      ╙─┴─┴─┴─┴─┴─┴─┴╥╜ bits mask
       └╥┘ └╥┘ └─╥─┘ ╚═►  0: 01H 0=process count as binary
        ║   ║    ║               1=process counts as BCD▲
        ║   ║    ╚═════►1-3: 0eH select timer mode:
        ║   ║                    000 = mode 0: interrupt on terminal count
        ║   ║                    001 = mode 1: programmable one-shot
        ║   ║                    x10 = mode 2: rate generator
        ║   ║                    x11 = mode 3: square-wave rate generator
        ║   ║                    100 = mode 4: software-triggered strobe
        ║   ║                    101 = mode 5: hardware-triggered strobe
        ║   ╚══════════►4-5: 30H select read/load sequence:
        ║                        00 = latch counter for stable read
        ║                        01 = read/load most significant byte only
        ║                        10 = read/load least significant byte only
        ║                        11 = read/load LSB then MSB
        ╚══════════════►6-7: c0H specify counter to affect:
                                 00 = counter 0, 01= counter 1
                                 10 = counter 2, 11= counter 3

See Also: Speaker Control
          PPI Ports
          I/O Port Map
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