การแบ่งเซกเมนต์และตารางตัวบอกส่วนกลาง (GDT)

Segmentation divides memory into variable-sized regions called segments, each with a base address, limit, and access rights. It predates paging and is the first translation step in x86 address generation.

In 16-bit real mode, a physical address is computed as segment * 16 + offset. This gave access to 1 MB using 16-bit registers but offered no protection at all.

In protected mode a segment register no longer holds a base directly. It holds a selector — an index into a descriptor table plus a privilege level and table indicator bit.

Each descriptor is 8 bytes and encodes:

• Base (32-bit start address)
• Limit (size of the segment)
• Type (code, data, system)
• DPL (descriptor privilege level 0-3)
• Flags (granularity, default size)

The GDT is an array of these descriptors in memory. The CPU finds it through the GDTR register, which holds the table base and limit. Entry 0 is always the null descriptor.

You point the CPU at your GDT with the lgdt instruction, which reads a 6-byte pseudo-descriptor (limit + base).

Modern OSes use a flat model: base 0 and a 4 GB limit, so segmentation becomes a no-op and paging does the real work. Here is a typical code descriptor.

You enter protected mode by setting bit 0 (PE) of control register CR0, then performing a far jump to flush the prefetch queue and load CS with a protected-mode selector.

A selector like 0x08 breaks down as: index 1 (byte offset 8 / 8), table indicator 0 (GDT), requested privilege level 0. After the jump, data segment registers are loaded with the data selector (often 0x10).

In long (64-bit) mode segmentation is largely disabled: base and limit are ignored for most segments. Protection and translation are handled by paging. Segmentation survives mainly for FS/GS base pointers used for thread-local storage.

Even though the flat model neutralizes segmentation, every protected-mode and long-mode OS must set up a valid GDT to boot. Understanding selectors and descriptors is essential for kernel bring-up and exception handling.

You learned x86 segmentation:

• A selector indexes a descriptor in the GDT
• Descriptors hold base, limit, type, and privilege level
• lgdt loads the table; setting CR0.PE plus a far jump enters protected mode
• The flat model neutralizes segmentation so paging does the work