User's Manual

PROTECTION
An important distinction exists between tasks and programs. Programs (e.g., instructions in code
segments) are static and consist of a fixed set of code and data segments each with an associated
privilege level. The privilege assigned to a program determines what the program may
do when executed
by a task. Privilege
is assigned to a program w~en the system is built or when the program is loaded.
Tasks are dynamic; they execute one or more programs. Task privilege changes with time according
to
the privilege level of the program being executed. Each task has a unique set of attributes that define
it, e.g., address space, register values, stack, data, etc. A task may execute a program if that program
appears in the task's address space. The rules of protection control determine when a program may be
executed by a task, and once executed, determine what the program may
do.
7.2 MEMORY MANAGEMENT AND PROTECTION
The protection hardware of the 80286 is related to the memory management hardware. Since protec-
tion attributes are assigned to segments, they are stored along with the memory management infor-
mation in the segment descriptor. The protection information is specified when the segment is created.
In addition
to privilege levels, the descriptor defines the segment type (e.g., Code segment, Data segment,
~tc.). Descriptors may be created either by program development tools or by a loader in a dynamically
loaded reprogram mabie environment.
The protection control Information consists
of a segment type, its privilege level, and size. These are
fields
in the access byte of the segment descriptor (see figure 7-2). This information is saved on-chip
in the programmer invisible section of the segment register for fast access during execution. These
entries are changed only when a segment register
is loaded. The protection data is used at two times:
upon loading a segment register and upon each reference to the selected segment.
The hardware performs several checks while loading a segment register.
These checks enforce the
protection rules before any memory reference
is generated. The hardware verifies that the selected
segment
is valid (is identified by a descriptor, is in memory, and is accessible from the privilege level
in which the program is executing) and that the type is consistent with the target segment register. For
example,
you cannot load a read-only segment descriptor into SS because the stack must always be
writable. '
PROGRAM VISIBLE
SEGMENT SELECTORS'
::~, '
ssr-----i
I I
15
SEGMENT REGISTERS
(loaded by program)
o
r- - --------P"R<iQR'i"MiNviSIBLE-------- --...,
I 'ACCESS I
I
RIGHTS SEGMENT BASE ADDRESS SEGMENT SIZE I
i I I I I !
: I . I I I :
I 47 40 39 16 15 0 I
I SEGMENT DESCRIPTOR CACHE REGISTERS I
L ________ (!T~M:TI~':.Y .:o~E~ B': c:u~ _______ J
G30108
Figure 7-2. Descriptor Cache Registers
7-4