Segment Selector and Segment Descriptor + Difference

Difference between Segment Selector and Segment Descriptor?

Segment Selector

• The selector is located in the segment register.
• Describe the length and access rights of the segment of memory.
• It selects one of 8192 descriptor from one of two tables of descriptors.
• Indirectly, the register still selects a memory segment, but not directly as in real mode.

Segment Descriptor

• Two types of Descriptor table selected by the segment register.
• Global descriptor contain segment definitions that apply to all programs.
• A global descriptor might be called a system descriptor
• Location descriptor are usually unique to an application.
• Location descriptor an application descriptor.
• Global and local descriptor tables are a maximum of 64k Bytes in length.

Main Difference of Segment Selector and Descriptor.
Role of selector is to select on descriptor from the table of descriptors and the role of descriptor is to define the actual base address.

Explanation of Segment Selector and Descriptor.

• Each Descriptor is 8 bytes in length.
• The base address of the descriptor indicates the starting location of the memory segment.
• The G, or granularity bit allows a segment length of 4K to 4G bytes in steps of 4k bytes.
• 32-bit offset address allows segment lengths of 4G bytes.
• 16-bit offset address allows segment lengths of 64K bytes.