Segment Descriptor

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Segment Descriptor
Segment Descriptor
• Segments are areas of memory defined by a
programmer and can be a code, data or stack segment.
• In 80386 segments need not be all the same size and
aligned. And segments need not be exactly 64 KB long,
but we can define them to be any length from 1 byte to
4 GB.
• In 80386 memory segmentation, it is not possible to
use a 16-bit segment register to represent all the
information related to segment. When multiple
privilege levels and intertask protection are required a
special structure called a segment descriptor is used.
Segment Descriptor
• The description of a segment includes its base
address, length, type, privilege level and some
security information.
• The lower base address is specified in bits 16
to 39 and upper 8 bits are specified in bits 56
to 63.
• The lower 16 bits of segments limit are
specified in 0-15 and the remaining 4 bits are
specified in 48-51.
Types of segment descriptors
Non System Segment descriptors
• Defines data, code, stack segments.
• Used by both system and application
programs.
• S=1 in access right byte.
Descriptors
Bit Position
Name
7
Present (P)
6-5
4
3
2
1
3
2
1
0
Function
P=1 Segment is mapped into physical memory
P=0 No mapping to physical memory exists, base and limit
are not used
Descriptor Privilege
Segment privilege attribute used in privilege tests
Level (DPL)
Segment Descriptor S=1 Code or Data segment descriptor
Type (S)
S=0 System segment Descriptor or Gate Descriptor
Executable(E)
E=0 Data segment descriptor type is :
Expansion
ED=0 Expand Up segment
Direction(ED)
ED=1 Expand Down segment
Writeable(W)
W= 0 Data segment may not be written into
W= 1 Data segment may be written into
Executable(E)
E=1 Code segment descriptor type is:
Conforming(C)
C=1 Code segment may only be executed
When CPL DPL and CPL remains
unchanged
Readable(R)
R=0 Code segment may not be read
R=1 Code segment may be read
Accessed(A)
A=0 Segment has not been accessed
A=1 Segment selector has been loaded into segment
register or used by selector test instruction
Non System Segment descriptors
• P bit indicates whether segment is present in
memory or not. P = 0 -> Segment is not
present and P = 1-> Segment is present.
• DPL: defines privilege level of the segment.
Used to protect segment from low privilege
caller.
Non System Segment descriptors
• S: Used to distinguish between non system segment
and system segment descriptors.
• S = 1-> Non system segment descriptor.
• E: Executable; Used to distinguish between data and
code segments. E=0 -> Data segment including stack.
E = 1 -> Code segment.
• ED/C: Expand direction/conforming; When E=0, then
this bit functions as ED, ED indicates whether the
segment is data or stack. ED = 0 -> Data segment(
access segment randomly). Offset address  limit. ED
= 1 -> Stack segment LIFO.
Non System Segment descriptors
• When E =1, then this bit functions as C (
conforming) bit. Used to distinguish between
conforming and nonconforming code segments.
• C= 0 -> Non conforming code segment.
• C = 1 -> Conforming code segment.
• R/W Read/Write. When E= 0( data segment),
then this bit functions as W bit. This bit indicates
whether data segment is writable or not.
• W = 0 -> data segment is not writable. W= 1 ->
data segment is writable.
Non System Segment descriptors
• When E =1 (code segment) then this bit
functions as R bit. This bit indicates whether
code segment is readable or not. R = 0 ->
code segment is not readable and R = 1 ->
code segment is readable.
• A: Accessed; This bit indicates whether the
segment is accessed or not. A= 1 -> Segment
accessed. This bit is reset by OS periodically.
A= 0 -> Segment not accessed
System Segment descriptor Type
• All system descriptors are present in GDT while
some system descriptors are present in LDTs.
• Normally system segment descriptor are used by
OS.
• The value of S in right access byte is 0.
• Their functions are fixed and specified by Intel.
• The type of system descriptor is indicated by type
field.
• The system segment descriptors have no Accessed
bit, instead the type field (3 bits) is now extended
to 4 bits.
• The system segment descriptors contain the
information about tables (LDT), tasks(TSS) and gates
(call gate, interrupt gate, task gate, trap gate) of the
OS.
System descriptors
Name
Value
Description
Type
0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
0
1
Reserved
Available 16 bit TSS
LDT
Busy 16 bit TSS
Call Gate
Task Gate
16 bit Interrupt gate
16 bit Trap gate
Reserved
Available 32 bit TSS
Reserved
Busy 32 bit TSS
32 bit Call Gate
Reserved
32 bit Interrupt Gate
32 bit Trap Gate
Descriptor contents are not valid
Descriptor contents are valid
DPL
0-3
Descriptor Privilege level 0,1,2, or 3
BASE
32-bit number
Base address of special system data segment in memory
LIMIT
20-bit number
Offset of last byte in segment from the base
P
LDT descriptor (s=0, Type 2)
LDT descriptor (s=0, Type 2)
• The LDT descriptors are present in the GDT.
They contain the information about the LDT.
• LDT contains the segment descriptors that are
unique to a particular task.
• The DPL field of the descriptor is ignored as
this descriptor can only be accessed with a
privilege level of 0.
• Here type field =2 i.e. it specifies a LDT
descriptor.
TSS (Task State Segment) Descriptors
(s=0, Type 1, 3, 9 and B)
• Whenever a computer is performing more than
one tasks at a time it may also switch between
these tasks.
• The task may be a single program or a group of
program.
• When one task switches to another task , it
stores all the necessary information required to
restart the task where it was left. This
information is called as the “ state of the task”
• For storing the state of the task the 80386
processor uses a special segment called the “Task
State Segment (TSS)”
TSS (Task State Segment) Descriptors
(s=0, Type 1, 3, 9 and B)
• The task segment is addressed with the help
of TSS descriptor. It contains information
about the location, size and privilege level of a
TSS.
• A TSS descriptor appears only in GDT and not
in IDT or LDT.
• The TSS consists the linkage field for the nest
task that permits the nesting of the tasks.
TSS (Task State Segment) Descriptors
(s=0, Type 1, 3, 9 and B)
• B bit indicates whether task is busy or not.
B=0 : Task is not busy
B=1 : Task is busy
Gate descriptors (S=0, Type 4,5,6,7,C,F)
• Whenever 4,5,6,7 is specified in type field it
specifies a call gate, task gate, interrupt gate
and trap gate respectively.
• All fields are same as specified earlier except
the word count, selector and offect.
• The word count field specifies the number of
parameters that are to be copied from caller’s
stack to the called procedure’s stack.
Gate descriptors (S=0, Type 4,5,6,7,C,F)
• Call gate are used to modified privilege levels.
• Trap and interrupt gates are used in interrupt
and exception handling.
• The task gates are used in multitasking system
Protected Virtual Address Mode
Physical address calculation in the protected virtual address mode
Protected Virtual Address Mode
1) PLs > Pldesc->access to segment allowed.
2)MMU checks__> Pdesc determine__> seg
present__> phy mem.
3) exception load__>seg mem return__>
interrupted prog.
4) 1st & 2nd step satisfied MMU will add 16 bit
offset.

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