The Interrupt Descriptor Table (IDT) is an array of 8 byte interrupt descriptors in memory devoted to specifying (at most) 256 interrupt service routines. The first 32 entries are reserved for processor exceptions, and any 16 of the remaining entries can be used for hardware interrupts. The rest are available for software interrupts. source : http://www.acm.uiuc.edu/sigops/roll_your_own/i386/idt.html