GDT - Global Descriptor Table

Enhanced Definition

The Global Descriptor Table (GDT) is a data structure used in **x86 architecture** (Intel/AMD processors) to define memory segments, their base addresses, sizes, and access rights. It is a fundamental component of the protected mode memory management scheme on x86 systems. **Crucially, the GDT is not a concept or component found within IBM mainframe (z/Architecture) systems or the z/OS operating system.**

Key Characteristics

- x86 Specific: The GDT is entirely specific to the Intel x86 family of processors and their protected mode operation. It is integral to how operating systems like Windows, Linux, and older OS/2 manage memory segmentation and protection on these platforms.
- Not Applicable to z/Architecture: IBM z/Architecture (the underlying hardware architecture for z/OS) employs a fundamentally different memory management and protection model that does not utilize segment descriptors or a GDT.
- Hardware-Managed: On x86, the GDT is managed by the operating system and pointed to by a special CPU register (GDTR). The CPU hardware uses the GDT to perform address translation and enforce memory access rules.
- Segmentation-Based: The GDT facilitates a segmentation-based memory model, where memory is divided into logical segments, each described by an entry in the GDT.

Use Cases

- No z/OS Use Cases: As the GDT is an x86-specific construct, there are no use cases for it within the IBM mainframe environment (z/OS, z/VM, z/VSE, etc.). Mainframe operating systems do not interact with or manage a GDT.
- x86 Context (for clarity): In x86 systems, the GDT is used by the operating system kernel to define segments for code, data, stack, and task state, enabling memory protection and multi-tasking.