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

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

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