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Vol. 3 6-13
INTERRUPT AND EXCEPTION HANDLING
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 inter-
rupt 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
performance 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 6-1,
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. The
same apply for the Intel 64 architecture. This includes implicit
referencing of the destination code segment through the GDT or LDT
and accessing the stack.