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
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Timer Ports