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Interruption 08h - System timer (Hardware Handler)


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

Notes & Comments

  • 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


back2root/ibm-pc-ms-dos/interrupts/int_08/start.txt · Dernière modification : 2024/04/06 18:39 de frater