Intel IA-32 - Manual (Page 188)

5-12 Vol. 3A
INTERRUPT AND EXCEPTION HANDLING
re-generated when the interrupt handler returns execution to the point in the program or task
where the exceptions and/or interrupts occurred.
5.10 INTERRUPT DESCRIPTOR TABLE (IDT)
The interrupt descriptor table (IDT) associates each exception or interrupt vector with a gate
descriptor for the procedure or task used to service the associated exception or interrupt. Like
the GDT and LDTs, the IDT is an array of 8-byte descriptors (in protected mode). Unlike the
GDT, the first entry of the IDT may contain a descriptor. To form an index into the IDT, the
processor scales the exception or interrupt vector by eight (the number of bytes in a gate
descriptor). Because there are only 256 interrupt or exception vectors, the IDT need not contain
more than 256 descriptors. It can contain fewer than 256 descriptors, because descriptors are
required only for the interrupt and exception vectors that may occur. All empty descriptor slots
in the IDT should have the present flag for the descriptor set to 0.
The base addresses of the IDT should be aligned on an 8-byte boundary to maximize perfor-
mance of cache line fills. The limit value is expressed in bytes and is added to the base address
to get the address of the last valid byte. A limit value of 0 results in exactly 1 valid byte. Because
IDT entries are always eight bytes long, the limit should always be one less than an integral
multiple of eight (that is, 8N – 1).
The IDT may reside anywhere in the linear address space. As shown in Figure 5-1, the processor
locates the IDT using the IDTR register. This register holds both a 32-bit base address and 16-bit
limit for the IDT.
The LIDT (load IDT register) and SIDT (store IDT register) instructions load and store the
contents of the IDTR register, respectively. The LIDT instruction loads the IDTR register with
the base address and limit held in a memory operand. This instruction can be executed only
when the CPL is 0. It normally is used by the initialization code of an operating system when
creating an IDT. An operating system also may use it to change from one IDT to another. The
SIDT instruction copies the base and limit value stored in IDTR to memory. This instruction can
be executed at any privilege level.
If a vector references a descriptor beyond the limit of the IDT, a general-protection exception
(#GP) is generated.
NOTE
Because interrupts are delivered to the processor core only once, an
incorrectly configured IDT could result in incomplete interrupt handling
and/or the blocking of interrupt delivery. IA-32 architecture rules need to be
followed for setting up IDTR base/limit/access fields and each field in the
gate descriptors. This includes implicit referencing of the destination code
segment through the GDT or LDT and accessing the stack.