INT 8 - System timer

	no input data

	related memory:

	40:6C = Daily timer counter (4 bytes)
	40:70 = 24 hr overflow flag (1 byte)
	40:67 = Day counter on all products after AT
	40:40 = Motor shutoff counter - decremented until 0 then
		shuts off diskette motor

	- INT 1C is invoked as a user interrupt
	- the byte at 40:70 is a flag that certain DOS functions use
	  and adjust the date if necessary.  Since this is a flag and
	  not a counter it results in DOS (not the RTC) losing days
	  when several midnights pass before a DOS call
	- generated 18.2 times per second by the 8253 Programmable Interval
	  Timer (PIT)
	- normal INT 8 execution takes approximately 100 microseconds

	- see	8253