Load Global/Interrupt Descriptor Table (lgdt, lidt)

lgdt    	mem48
lidt    	mem48

Operation

MEM48 -> GDTR MEM48 -> IDTR

Description

The GDTR and IDTR are loaded with a linear base address and limit value from a six-byte operand in memory by the lgdt/lidt instructions. For a 16-bit operand:

• Load the register with a 16-bit limit and a 24-bit base.

• The six-byte data operand high-order eight bits are not used.

For a 32-bit operand:

• Load the register with a 16-bit limit and a 32-bit base.

• The six-byte data operand high-order eight bits are used as the high-order base address bits.

All 48-bits of the six-byte data operand are always stored into by the sgdt/sidt instructions. For a 16-bit and a 32-bit operand, the upper eight-bits are written with the high-order eight address bits. lgdt or lidt, when used with a 16-bit operand to load the register stored by sgdt or sidt, stores the upper eight-bits as zeros.

lgdt and lidt are not used in application programs; they are used in operation system. lgdt and lidt are the only instructions that load a linear address directly in 80386 Protected Mode.

Example

Load the Global/Interrupt Descriptor Table Register from memory address 0x55555555:

lgdt 0x55555555
lidt 0x55555555