Intel 80286, 80287 manual PiDPll~1

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING

			o 7
+7		INTEL RESERVED'
			MUST BE 0
i!:!
..	piDPll~1
			A
m +5		TYPE	I I	BASE23· 1e	-4
~a: +3
			BASE,s·o		+2
..
			LIMIT 15.0
t1
	15		B 7

ACCESS RIGHTS BYTES:

P• PRESENT

DPL • DESCRIPTOR PRIVILEGE LEVEL

S = SEGMENT DESCRIPTOR

TYPE - SEGhfENT TYPE AND ACCESS INFORMATION

(I.e Figure 6·7)

A= ACCESSED

.MUST BE SET TO 0 FOR COMPATIBILITY WITH IApX 3B6

G30108

Figure 6·3. Code or Data Segment Descriptor (S = 1)

are segment descriptors for all of the segments that comprise a system's global address space. Similarly, within a task's LDT, there must be a descriptor for each of the segments that are to be included in that task's local address space.

Each local descriptor table is itself a special system segment, recognizable as such by the 80286 archi- tecture and described by a specific type of segment descriptor (see figure 6-4). Because there is only a single GDT segment, it is not defined by a segment descriptor. Its base and size information is maintained in a dedicated register, GDTR, as described below (section 6.6.2).

Similarly, there is another dedicated register within the 80286, LDTR, that records the base and size of the current LDT segment (i.e., the LDT associated with the currently executing task). The LDTR register state, however, is volatile: its contents are automatically altered whenever a task switch is made from one task to another. An alternate specification independent of changeable register contents must therefore exist for each LDT in the system. This independent specification is accomplished by means of special system segment descriptors known as descriptor table descriptors or LDT descriptors.

Figure 6-4 shows the format of a descriptor' table descriptor. (Note that it is distinguished from an ordinary segment descriptor by the contents of certain bits in the access byte.) This special type of descriptor is used to specify the physical base address and size of a local descriptor table that defines the virtual address space and address mapping for an individual user or task (figure 6-5).

Each LDT segment in a system must lie within that system's global address space. Thus, all of the descriptor table descriptors must be included among the entries in the global descriptor table (the GDT) of a system. In fact, these special descriptors may appear only in the GDT. Reference to an LDT descriptor within an LDT will cause a protection violation. Even though they are in the global address space available to all tasks, the descriptor table descriptors are protected from corruption within the GDT since they are special system segments and can only be accessed for loading into the LDTR register.

6-5