Interrupt descriptor table

The interrupt descriptor table (IDT) is a data structure used by the x86 architecture to implement an interrupt vector table. The IDT is used by the processor to determine the memory addresses of the handlers to be executed on interrupts and exceptions.

The details in the description below apply specifically to the x86 architecture. Other architectures have similar data structures, but may behave differently.

The IDT consists of 256 interrupt vectors and the use of the IDT is triggered by three types of events: processor exceptions, hardware interrupts, and software interrupts, which together are referred to as interrupts:
 * Processor exceptions generated by the CPU have fixed mapping to the first up to 32 interrupt vectors. While 32 vectors (0x00-0x1f) are officially reserved (and many of them are used in newer processors), the original 8086 used only the first five (0-4) interrupt vectors and the IBM PC IDT layout did not respect the reserved range.
 * Hardware interrupt vector numbers correspond to the hardware IRQ numbers. The exact mapping depends on how the Programmable Interrupt Controller such as Intel 8259 is programmed. While Intel documents IRQs 0-7 to be mapped to vectors 0x20-0x27, IBM PC and compatibles map them to 0x08-0x0F. IRQs 8-15 are usually mapped to vectors 0x70-0x77.
 * Software interrupt vector numbers are defined by the specific runtime environment, such as the IBM PC BIOS, DOS, or other operating systems. They are triggered by software using the INT instruction (either by applications, device drivers or even other interrupt handlers). For example, IBM PC BIOS provides video services at the vector 0x10, MS-DOS provides the DOS API at the vector 0x21, and Linux provides the syscall interface at the vector 0x80.

Real mode
In real mode, the interrupt table is called IVT (interrupt vector table). Up to the 80286, the IVT always resided at the same location in memory, ranging from  to , and consisted of 256 far pointers. Hardware interrupts may be mapped to any of the vectors by way of a programmable interrupt controller. On the 80286 and later, the size and locations of the IVT can be changed in the same way as it is done with the IDT (Interrupt descriptor table) in protected mode (i.e., via the LIDT (Load Interrupt Descriptor Table Register) instruction) though it does not change the format of it.

BIOS interrupts
The BIOS provides simple real-mode access to a subset of hardware facilities by registering interrupt handlers. They are invoked as software interrupts with the INT assembly instruction and the parameters are passed via registers. These interrupts are used for various tasks like detecting the system memory layout, configuring VGA output and modes, and accessing the disk early in the boot process.

Protected and long mode
The IDT is an array of descriptors stored consecutively in memory and indexed by the vector number. It is not necessary to use all of the possible entries: it is sufficient to populate the table up to the highest interrupt vector used, and set the IDT length portion of the IDTR accordingly.

The IDTR register is used to store both the linear base address and the limit (length in bytes minus 1) of the IDT. When an interrupt occurs, the processor multiplies the interrupt vector by the entry size (8 for protected mode, 16 for long mode) and adds the result to the IDT base address. If the address is inside the table, the DPL is checked and the interrupt is handled based on the gate type.

The descriptors may be either interrupt gates, trap gates or, for 32-bit protected mode only, task gates. Interrupt and trap gates point to a memory location containing code to execute by specifying both a segment (present in either the GDT or LDT) and an offset within that segment. The only difference between trap and interrupt gates is that interrupt gates will disable further processor handling of maskable hardware interrupts, making them suitable to handle hardware-generated interrupts (conversely, trap gates are useful for handling software interrupts and exceptions). A task gate will cause the currently active task-state segment to be switched, using the hardware task switch mechanism to effectively hand over use of the processor to another program, thread or process.

Official Intel layout
All INT_NUM between 0x0 and 0x1F, inclusive, are reserved for exceptions by Intel. INT_NUM bigger than 0x1F are to be used for interrupt routines.

IBM PC layout
The IBM PC (BIOS and MS-DOS runtime) does not follow the official Intel layout beyond the first five exception vectors implemented in the original 8086. Interrupt 5 is already used for handling the Print Screen key, IRQ 0-7 is mapped to INT_NUM 0x08-0x0F, and BIOS is using most of the vectors in the 0x10-0x1F range as part of its API.

Hooking
Some Windows programs hook calls to the IDT. This involves writing a kernel mode driver that intercepts calls to the IDT and adds in its own processing. This has never been officially supported by Microsoft, but was not programmatically prevented on its operating systems until 64-bit versions of Windows, where a driver that attempts to use a kernel mode hook will cause the machine to bug check.