Segmentation and the Global Descriptor Table (GDT)
Learn how x86 segmentation works alongside paging: segment selectors, descriptors, the GDT, and how flat memory models set the stage for protected mode.
Segmentation and the Global Descriptor Table (GDT) is a free Assembly Language & x86 Low-Level Systems Programming lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Assembly Language & x86 Low-Level Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What Is Segmentation?
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.
Real Mode Segments
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.
Protected Mode Selectors
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.
The Segment Descriptor
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 Global Descriptor Table
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.
Loading the GDT
You point the CPU at your GDT with the lgdt instruction, which reads a 6-byte pseudo-descriptor (limit + base).
gdt_descriptor:
dw gdt_end - gdt_start - 1 ; limit
dd gdt_start ; base
lgdt [gdt_descriptor] ; load GDTRDefining a Flat Code Segment
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.
gdt_code:
dw 0xFFFF ; limit low
dw 0x0000 ; base low
db 0x00 ; base mid
db 10011010b ; present, ring 0, code, readable
db 11001111b ; granularity 4K, 32-bit, limit high
db 0x00 ; base highActivating Protected Mode
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.
mov eax, cr0
or eax, 1 ; set PE bit
mov cr0, eax
jmp 0x08:pmode ; far jump, 0x08 = code selectorSelectors in Practice
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).
Segmentation vs Paging Today
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.
Why It Still Matters
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.
Quick Check
Test your segmentation knowledge.
Recap
You learned x86 segmentation:
- A selector indexes a descriptor in the GDT
- Descriptors hold base, limit, type, and privilege level
lgdtloads the table; setting CR0.PE plus a far jump enters protected mode- The flat model neutralizes segmentation so paging does the work
Frequently asked questions
Is the “Segmentation and the Global Descriptor Table (GDT)” lesson free?
Yes — the full text of “Segmentation and the Global Descriptor Table (GDT)” is free to read here on the web, and the Assembly Language & x86 Low-Level Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Assembly Language & x86 Low-Level Systems Programming course, upgrade to CoddyKit PRO.
What will I learn in “Segmentation and the Global Descriptor Table (GDT)”?
Learn how x86 segmentation works alongside paging: segment selectors, descriptors, the GDT, and how flat memory models set the stage for protected mode. You practise Assembly Language & x86 Low-Level Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Assembly Language & x86 Low-Level Systems Programming?
No prior experience is required. Assembly Language & x86 Low-Level Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Segmentation and the Global Descriptor Table (GDT)” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Assembly Language & x86 Low-Level Systems Programming lesson?
Yes. Every Assembly Language & x86 Low-Level Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Paging and Memory Management Unit (MMU)
- Protection Rings and Privileges
- Hypervisors and Virtualization Basics
- Segmentation and the Global Descriptor Table (GDT)