Interruption 08h - System timer (Hardware Handler)

Request/Call:

no input

address	description
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