Intel 80286 User Manual

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING
VIRTUAL ADDRESS
I I I
TARGET
SELECTOR OFFSET
SEGMENT
TI
0- PHYSICAL
DATUM
DESCRIPTOR
ADDRESS
I
TABLE
I
SEGMENT
SEGMENT
BASE
DESCRIPTOR
----
INDEX
G3010B
Figure 6-6. Virtual-to-Physical Address Translation
As shown previously in figure 6-3, an 8-byte segment descriptor encodes the following information
about a particular segment:
Size. This 16-bit field, comprising bytes
0 and 1 of a segment descriptor, specifies an unsigned
integer as the size,
in bytes (from 1 byte to 64K bytes), of the segment.
Unlike segments
in the 8086 (or the 80286 in Real Address Mode)-which are never explicitly
limited to
less than a full 64K bytes-Protected Mode segments are always assigned a specific
size value. In conjunction with the protection features described in Chapter
7, this assigned size
allows the enforcement of a very desirable and natural rule: inadvertent accesses to locations beyond
a segment's actual boundaries are prohibited.
Base. This 24-bit field, comprising bytes 2 through 4 of a segment descriptor, specifies the physi-
cal base address of the segment; it thus defines the actual location of the segment within the
16-
megabyte real memory space. The base may be any byte address within the 16-megabyte real
memory space.
Access. This 8-bit field comprises byte 5 of a segment descriptor. This access byte specifies a
variety of additional information about a segment, particularly
in regard to the protection features
of the 80286. Fer example, cede seg!TI.ents are distinguished from certain special
access restrictions (such as Execute-Only or Read-Only) may be defined for segments of each
type. Access byte values of
OOH or 80H will denote "invalid."
Figure
6-7 shows the access byte format for both code and data segment descriptors. Detailed discus-
sion of the protection related fields within an access byte (Conforming, Execute-Only, Descriptor Privi-
lege Level, Expand Down, and Write-Permitted), and their use
in implementing protection policies, is
deferred to Chapter 7. The two fields Accessed and Present are used for virtual memory
implementations.
6-8
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Intel 80286 Specifications

General

Processor Type	Microprocessor
Clock Speed	6 MHz to 25 MHz
Transistor Count	134, 000
Addressable Memory	16 MB
Instruction Set	x86
Architecture	x86
Data Bus Width	16-bit
Address Bus Width	24-bit
Operating Modes	Real mode, Protected mode
Process Technology	1.5 µm
Voltage	5V
Introduced	1982
Package	68-pin PGA