Intel 80286, 80287 manual Segments and Segment Descriptors, LOT Descriptor

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING

						r
			,J	I
			C			ONE
,	,					SEGMENT
						OF THE
			RESERVED-ZERO			TASKS
						LOCAL
			1. BASE 23-16		1-	(private)
						ADDRESS
	RESERVED-ZERO					SPACE
	IBASE"_'6	1-	BASE 15-0
			LIMIT '5-0
	BASE,S_O
	LIMIT '5-0
	LDT
	DESCRIPTOR
	IN THE
	GDT
	IN MEMORY
~		~	~		~
			DESCRIPTOR			SEGMENT
			TABLES			IN
			IN RAM			RAM

f-,

Figure 6-5. LOT Descriptor

6.5 SEGMENTS AND SEGMENT DESCRIPTORS

,

SEGMENT

LIMIT

SEGMENT

BASE

h

G3010B

Segments are the basic units of 80286 memory management. In contrast to schemes based on fixed- size pages, segmentation allows for a very efficient implementation of software: variable-length segments can be tailored to the exact requirements of an application_ Segmentation, moreover, is consistent with the way a programmer naturally deals with his virtual address space: programmers are encouraged to divide code and data into clearly defined modules and structures which are manipulated as consistent entities. This reduces (minimizes) the potential for virtual memory thrashing_ Segmentation also elimi- nates the restrictions on data structures that span a page (e.g., a word that crosses page boundaries)_

Each segment within an 80286 system is defined by an associated segment descriptor, which may appear in one or more descriptor tables. Its inclusion within a descriptor table represents the presence of its associated segment within the virtual address space defined by that table. Conversely, its ommis- sion from a descriptor table means that the segment is absent from the corresponding address space.

6-7