sgdt

SGDT stands for Store Global Descriptor Table Register. It is an x86 assembly instruction that copies the current contents of the Global Descriptor Table Register (GDTR) into a memory location specified by a memory operand. The GDTR holds the base address and limit of the Global Descriptor Table (GDT), which defines the memory segmentation used by protected mode and some compatibility modes.

The memory operand receives the GDTR data. In 32-bit operation, SGDT writes 6 bytes: a 16-bit limit followed by a 32-bit base address. In 64-bit operation, the base becomes 64 bits, and the total size written is 10 bytes (2 bytes for the limit plus 8 bytes for the base). The exact layout is a compact representation of the GDT descriptor: limit first, then base.

SGDT is used mainly in low-level system software such as operating systems, hypervisors, and boot code, where saving or inspecting the descriptor table state is necessary for initialization, debugging, or context switching. It is the counterpart to LGDT, which loads the GDTR from memory, and to SIDT, which stores the Interrupt Descriptor Table Register (IDTR). Together, these instructions allow software to read and manipulate the processor’s descriptor tables and related structural data.

See also SGDT’s siblings: LGDT and SIDT, and related concepts like the Global Descriptor Table (GDT) and Interrupt Descriptor Table (IDT).