User's Manual

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING
+7
+5
+3
+1
o 7
INTEL RESERVED'
MUST BE 0
pIDPLI~1
TYPE
I
BASE
23
'
16
BASE 15.0
LlMIT
15
·
0
15
8 7
ACCESS RIGHTS BYTES:
P - PRESENT
OPl = DESCRIPTOR PRIVILEGE LEVEL
5 :;::. SEGMENT DESCRIPTOR
TYPE
= TYPE OF SPECIAL DESCRIPTOR
(Includes control and system segments)
o = INVALID DESCRIPTOR
1 = AVAILABLE TASK STATE SEGMENT
2 = LOT DESCRIPTOR
3 = BUSY TASK STATE SEGMENT
4-7 = CONTROL DESCRIPTOR (see Chapter 7)
8 = INVALID DESCRIPTOR (reserved by Intel)
9-F = RESERVED BY INTEL
'",UST BE SET TO 0 FOR
COMPATIBILITY WITH IAPX
386
+6
+4
+2
Figure 6-4. System Segment Descriptor or Gate Descriptor (S = 0)
6.4 VIRTUAL-TO-PHYSICAL ADDRESS TRANSLATION
G3010B
The translation of a full 32-bit virtual address pointer into a real 24-bit physical address is shown by
figure 6-6.
When the segment's base address is determined as a result of the mapping process, the
offset value
is added to the result to obtain the physical address.
The actual mapping is performed on the selector component of the virtual address. The 16-bit segment
selector
is mapped to a 24-bit segment base address via a segment descriptor maintained in one of the
descriptor tables.
The TI bit in the segment selector (see figure 6-1) determines which of two descriptor tables, either
the GOT or the current LOT, is to be chosen for memory mapping. In either case, using the GOTR or
LOTR register, the processor can readily determine the physical base address of the memory-resident
table.
.
1 he INDEX fieici in the segment seiecwr speClIles a parliculaI ue,CIil"UI clIlly wi,llill the Cll0SeH
table. The processor simply multiplies this index value by 8 (the length of a descriptor), and adds the
result to
the base address of the descriptor table in order to access the appropriate segment descriptor
in the table.
Finally, the segment descriptor contains the physical base address of the
target segment, as well as size
(limit) and access information.
The processor sums the 24-bit segment base and the specified 16-bit
offset to generate the resulting 24-bit physical address.
6-6