Memory Segment - Segment Selector
When writing application code, programmers generally create segment selectors with assembler directives and symbols. The assembler and other tools then create the actual segment selector values associated with these directives and symbols. If writing system code, programmers may need to create segment selectors directly.
The value of a segment selector is hold in a segment register.
To access a particular segment in memory, the segment selector for that segment must be present in the appropriate segment register.
When using a segmented memory model, some instructions assume that pointers in certain registers are relative to specific segments.