Intel 80287, 80286 manual ~,m~

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING,

SEGMENT ADDRESS TRANSLATION REGISTERS
16-BIT	48-BIT HIDDEN DESCRIPTOR CACHE

						I,~_,m''''~'"
						DATA SEGMENT REGISTER
						EXTRA SEGMENT REGISTER
						STACK SEGMENT REGISTER
	63	48 47	4039		16 15	0
		ACCESS		SEGMENT BASE	SEGMENT
		RIGHTS		ADDRESS	SIZE
SYSTEM ADDRESS REGISTERS
GDTR	II------------t------,IGLOBAL DESCRIPTOR TABLE REGISTER
		40 BIT EXPLICIT REGISTER
IDTR _					INTERRUPT DESCRIPTOR TABLE REGISTER
	39			16 15	o
		BASE		LIMIT
	16-BIT VISIBLE			40-BIT HIDDEN DESCRIPTOR CACHE		ILOCAL DESCRIPTOR TABLE REGISTER
	SELECTOR	(AUTOMATICALLY LOADED FROM LDTR WITHIN GDT)
	L5-5--------4-0~3-9------------------1-6~1~5----------~0
				BASE	LIMIT

G30108

Figure 6-8. Memory Management Registers

The operations that load these registers-or, more exactly, those that load the visible portion of these

registers-arc normal program instructions. These instructions may be divided into two categories:

I, Direct segment-register load instructions_ These instructions (such as LDS, LES, MOV, POP,

etc.) can explicitly reference the SS, DS, or ES segment registers as the destination operand_

2.Implied segment-register load instructions. These instructions (such as intersegment CALL and JMP) implicitly reference the CS code segment register; as a result of these operations, the contents of CS are altered.

Using these instructions, a program loads ine visiul~ pal i (If the s6giTierll register 'v',:ithu16-bit ~ele~tGr

(i.e., the high-order word of a virtual address pointer). Whenever this is done, the processor automati- cally uses the selector to reference the appropriate descriptor and loads the 48-bit hidden descriptor cache for that segment register.

The correspondence between selectors and descriptors has already been described. Remember that the selector's TI bit indicates one of the two descriptor tables, either the LDT or the GDT. Within the indicated table, a particular entry is chosen by the selector's 13-bit INDEX field. This index, scaled by a factor of 8, represents the relative displacement of the chosen table entry (a descriptor).

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