Understanding the Linux Kernel

4.2. Interrupts and Exceptions

The Intel documentation classifies interrupts and exceptions as follows:

• Interrupts:

  Maskable interrupts

  Sent to the INTR pin of the microprocessor. They can be disabled by clearing the IF flag of the eflags register. All IRQs issued by I/O devices give rise to maskable interrupts.

  Nonmaskable interrupts

  Sent to the NMI (Nonmaskable Interrupts) pin of the microprocessor. They are not disabled by clearing the IF flag. Only a few critical events, such as hardware failures, give rise to nonmaskable interrupts.

• Exceptions:

  Processor-detected exceptions

  Generated when the CPU detects an anomalous condition while executing an instruction. These are further divided into three groups, depending on the value of the eip register that is saved on the Kernel Mode stack when the CPU control unit raises the exception:

  Faults

  The saved value of eip is the address of the instruction that caused the fault, and hence that instruction can be resumed when the exception handler terminates. As we shall see in Section 7.4 in Chapter 7, resuming the same instruction is necessary whenever the handler is able to correct the anomalous condition that caused the exception.

  Traps

  The saved value of eip is the address of the instruction that should be executed after the one that caused the trap. A trap is triggered only when there is no need to reexecute the instruction that terminated. The main use of traps is for debugging purposes: the role of the interrupt signal in this ...