80386 Programmer's Reference Manual -- Opcode LGDT
LGDT/LIDT -- Load Global/Interrupt Descriptor Table Register
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;
DescriptionThe 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.
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 ExceptionsInterrupt 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
NoteThese instructions are valid in Real Address Mode to allow power-up initialization for Protected Mode
Virtual 8086 Mode ExceptionsSame exceptions as in Real Address Mode; #PF(fault-code) for a page fault