Intel 8253
When setting the PIT, the microprocessor first sends a control message, then a count message to the PIT. The counting process will start after the PIT has received these messages, and, in some cases, if it detects the rising edge from the GATE input signal.
On PCs the address for timer0 (chip) is at port 40h..43h like described and the second timer1 (chip) is at 50h..53h.
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