Interrupt  08h Timer
(0:0020h) 55ms timer "tick" taken 18.2 times per second. Updates BIOS clock and 
(IRQ0)    turns off diskette drive motors after 2 seconds of inactivity.  
 
entry   AH      08h 
return  absolute addresses: 
	40:6C   number of interrupts since power on (4 bytes) 
	40:70   number of days since power on (1 byte) 
	40:67   day counter on all products after AT 
	40:40   motor control count - gets decremented and shuts off diskette  
		motor if zero 
note    Int\1C invoked by int 08h as a user interrupt.
(internal) - DOUBLE FAULT (80286/80386 protected mode) 
	Called when multiple exceptions occur on one instruction, or an 
	exception occurs in an exception handler. If an exception occurs in the 
	double fault handler, the CPU goes into SHUTDOWN mode (which circuitry 
	in the PC/AT converts to a reset). 

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