PIT timer

The Programmable Interval Timer Chip (PIT) is a type of chip used in MS-DOS and 6502 era systems that can be given an amount of time (typically in microseconds, milliseconds or some other timing unit), once that amount of time has concluded, the chip will generate an interrupt to the processor. This was necessary as multitasking was too computationally expensive at the time and was very primitive, oftentimes this chip was used to aid in said multitasking.

Byte	Purpose/Function
0x00	PIT data HI
0x01	PIT data LO
0x02	PIT command
0x03	PIT source remember (remembers the source of the interrupt)
0x04	PIT status
0x05	reserved

CMDs

CMD #	Type	Description
0x00	NULL	Does nothing
0x01	WRITE	Restarts the clock with the new timing from the data register