Segment Register—Selects Descriptor Table and Entry
When a programmer wishes to gain access to an area of memory, the respective segment register (the CS, SS, or one the data segment registers—DS, ES, FS, or GS) must be loaded with a 16-bit value that identifies the area of memory. In Real Mode, the value loaded into the segment register represents the upper 16 bits of the 20 bit start address of the segment in memory. In Protected Mode, the value loaded into a segment register is referred to as the segment selector, illustrated in the upper part (i.e., the segment register's visible part) of Figure 7-3 on page 114:
The Requestor Privilege Level (RPL) field is described in “Code Segments” on page 133 and “Data and Stack Segments” on page 157 ...
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