User's Manual

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
40-BIT EXPLICIT
REGISTER
GDTR II------------t------,I GLOBAL DESCRIPTOR TABLE REGISTER
IDTR _ INTERRUPT DESCRIPTOR TABLE REGISTER
39
16 15 o
BASE LIMIT
16-BIT VISIBLE 40-BIT
HIDDEN DESCRIPTOR CACHE
SELECTOR
(AUTOMATICALLY LOADED FROM LDTR WITHIN GDT)
I LOCAL DESCRIPTOR TABLE REGISTER
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',:ith u 16-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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