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Table des matières
Int 08h - System timer (Hardware Handler)
Generated 18.2 times per second by channel 0 of the 8253/8254 system timer, this interrupt is used to keep the time-of-day clock updated programs which need to be invoked regularly should use INT 1C unless they need to reprogram the timer while still keeping the time-of-day clock running at the proper rate
default handler is at F000h:FEA5h in IBM PC and 100%-compatible BIOSes may be masked by setting bit 0 on I/O port 21h
CPU-generated (80286+) - DOUBLE EXCEPTION DETECTED
Called when multiple exceptions occur on one instruction, or an exception occurs in an exception handler called in protected mode if an interrupt above the defined limit of the interrupt vector table occurs
- return address points at beginning of instruction with errors or the beginning of the instruction which was about to execute when the external interrupt caused the exception
- 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);
this “triple fault” is a faster way of returning to real mode on many 80286 machines than the standard keyboard controller reset
Request/Call:
no input
related memory
| 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
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