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Chapter 17 -- 80386 Instruction Set
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LGDT/LIDT -- Load Global/Interrupt Descriptor Table Register
Opcode Instruction Clocks Description
0F 01 /2 LGDT m16&32 11 Load m into GDTR
0F 01 /3 LIDT m16&32 11 Load m into IDTR
Operation
IF instruction = LIDT
THEN
IF OperandSize = 16
THEN IDTR.Limit:Base := m16:24 (* 24 bits of base loaded *)
ELSE IDTR.Limit:Base := m16:32
FI;
ELSE (* instruction = LGDT *)
IF OperandSize = 16
THEN GDTR.Limit:Base := m16:24 (* 24 bits of base loaded *)
ELSE GDTR.Limit:Base := m16:32;
FI;
FI;
Description
The LGDT and LIDT instructions load a linear base address and limit
value from a six-byte data operand in memory into the GDTR or IDTR,
respectively. If a 16-bit operand is used with LGDT or LIDT, the
register is loaded with a 16-bit limit and a 24-bit base, and the
high-order eight bits of the six-byte data operand are not used. If a 32-bit
operand is used, a 16-bit limit and a 32-bit base is loaded; the high-order
eight bits of the six-byte operand are used as high-order base address bits.
The
SGDT and
SIDT instructions always store into all 48 bits of the
six-byte data operand. With the 80286, the upper eight bits are undefined
after
SGDT or
SIDT is executed. With the 80386, the upper eight bits
are written with the high-order eight address bits, for both a 16-bit
operand and a 32-bit operand. If LGDT or LIDT is used with a 16-bit
operand to load the register stored by SGDT or SIDT, the upper eight
bits are stored as zeros.
LGDT and LIDT appear in operating system software; they are not used
in application programs. They are the only instructions that directly load
a linear address (i.e., not a segment relative address) in 80386 Protected
Mode.
Flags Affected
None
Protected Mode Exceptions
#GP(0) if the current privilege level is not 0; #UD if the source operand
is a register; #GP(0) for an illegal memory operand effective address in
the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in
the SS segment; #PF(fault-code) for a page fault
Real Address Mode Exceptions
Interrupt 13 if any part of the operand would lie outside of the effective
address space from 0 to 0FFFFH; Interrupt 6 if the source operand is a
register
Note
These instructions are valid in Real Address Mode to allow
power-up initialization for Protected Mode
Virtual 8086 Mode Exceptions
Same exceptions as in Real Address Mode; #PF(fault-code) for a page
fault
up:
Chapter 17 -- 80386 Instruction Set
prev: LEAVE High Level Procedure Exit
next: LGS/LSS/LDS/LES/LFS Load Full Pointer