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up: Chapter 17 -- 80386 Instruction Set
prev: LEAVE High Level Procedure Exit
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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