User's Manual

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING
Thus, so long as a particular selector value is valid (i.e., it points to a valid segment descriptor within
the bounds of the
d()scriptor table), it can be readily associated with an 8-byte descriptor. When a
selector value
is loaded into. the visible part of a segment register, the 80286 automatically loads 6
bytes of the associated descriptor into the hidden part of the register. These 6 bytes, therefore, contain
the size, base, and access type of the selected segment. Figure
6-9 illustrates this transparent process
of descriptor loading.
In effect, the hidden descriptor fields of the segment registers function
as the memory management
cache of the
80286. All the information required to address the current working set of segments~that
is, the base address, size, and access rights of the currently addressable segments-is stored in this
memory cache.
Unlike the probabilistic caches of other architectures, however, the 80286 cache is
completely deterministic: the caching of descriptors is explicitly controlled by the program.
Most memory references
do not require the translation of a full 32-bit virtual address, or long pointer.
Operands that are located within one of the currently addressable segments,
as determined by the four
segment registers, can be referenced very efficiently
by means of a short pointer, which is simply a
16-bit offset.
In fact, .most
80286 instructions reference memory locations in precisely this way, specifying only a
16-bit offset with respect
to one of the currently addressable segments. The choice of segments (CS,
DS, ES, or SS) is either implicit within the instruction itself, or explicitly specified by means of a
segment-override prefix (as described in Chapter
2).
ICPii:----- --- -----,
APPLICATION
VISIBLE
--
SEGMENT
REGISTER
-
I SELE~TOR I
1
1
.1
I
1
I
1
1
I
L __ _
r----
DESCRIPTOR
CACHE
SEGMENT
DESCRIPTOR
TYPE
BASE
LIMIT
TRANSPARENT
DESCRIPTOR
LOADING
I
I
-=--=--=1~~
I
I
I
I
_______ --1
SYSTEM
MEMORY
Figure 6-9. Descriptor Loading
6-11
DESCRIPTOR
TABLE
G30108