Now that we understand what the 80×86 microprocessors do with interrupts and exceptions at the hardware level, we can move on to describe how the Interrupt Descriptor Table is initialized.

Remember that before the kernel enables the interrupts, it must load the initial address of the IDT table into the idtr register and initialize all the entries of that table. This activity is done while initializing the system (see Appendix A).

The int instruction allows a User Mode process to issue an interrupt signal that has an arbitrary vector ranging from 0 to 255. Therefore, initialization of the IDT must be done carefully, to block illegal interrupts and exceptions simulated by User Mode processes via int instructions. This can be achieved by setting the DPL field of the particular Interrupt or Trap Gate Descriptor to 0. If the process attempts to issue one of these interrupt signals, the control unit checks the CPL value against the DPL field and issues a “General protection " exception.

In a few cases, however, a User Mode process must be able to issue a programmed exception. To allow this, it is sufficient to set the DPL field of the corresponding Interrupt or Trap Gate Descriptors to 3 — that is, as high as possible.

Let’s now see how Linux implements this strategy.