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Flag Usage By Exception- or Interrupt-Handler Procedure

已有 1155 次阅读| 2011-2-23 21:37

When accessing an exception or interrupt handler through either an interrupt gate or a trap gate, the processor clears the TF flag in the EFLAGS register after it saves the contents of the EFLAGS register on the stack. (On calls to exception and interrupt handlers, the processor also clears the VM, RF, and NT flags in the EFLAGS register, after they are saved on the stack.) Clearing the TF flag prevents instruction tracing

from affecting interrupt response. A subsequent IRET instruction restores the TF (and VM, RF, and NT) flags to the values in the saved contents of the EFLAGS register on the stack. The only difference between an interrupt gate and a trap gate is the way the processor handles the IF flag in the EFLAGS register.

When accessing an exceptionor interrupt-handling procedure through an interrupt gate, the processor clears the IF flag to prevent other interrupts from interfering with the current interrupt handler.

A subsequent IRET instruction restores the IF flag to its value in the saved contents of the EFLAGS register on the stack. Accessing a handler procedure through a trap gate does not affect the IF flag.