The Unabridged Pentium 4 IA32 Processor Genealogy

Segment Descriptor Describes a Memory Area in Detail

In a multitasking environment, the OS programmer must be able to specify the following characteristics of each segment:

• The task that it belongs to.

• Its start address anywhere in the 4GB memory address range.

• Its length (anywhere from one byte to 4GB in length).

• How it may be accessed: read-only, execute-only, read/writable.

• The minimum privilege level a program must have to access the segment.

• Whether it's a code or data segment, or a special segment that is only used by the OS.

• Whether the segment of information is currently present in memory or resides on a mass storage device.

Figure 7-1 on page 111 illustrates the manner in which the processor interprets the contents of a segment register while ...