Online HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens
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
[helppc.bosbyte.nl] BIOS timer interrupt