Segment Descriptor

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 Function7

6-5

4

Present (P)

Descriptor PrivilegeLevel (DPL)Segment DescriptorType (S)

P=1 Segment is mapped into physical memoryP=0 No mapping to physical memory exists, base and limit are not used Segment privilege attribute used in privilege tests

S=1 Code or Data segment descriptorS=0 System segment Descriptor or Gate Descriptor

32

1

Executable(E)ExpansionDirection(ED)Writeable(W)

E=0 Data segment descriptor type is :ED=0 Expand Up segmentED=1 Expand Down segmentW= 0 Data segment may not be written intoW= 1 Data segment may be written into

32

1

Executable(E)Conforming(C)

Readable(R)

E=1 Code segment descriptor type is:C=1 Code segment may only be executed When CPL DPL and CPL remains unchangedR=0 Code segment may not be readR=1 Code segment may be read

0 Accessed(A) A=0 Segment has not been accessedA=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 descriptorsName Value Description

Type 0 1 2 3 4 5 6 7 8 9 A B C D E F

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

P 0 1

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

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)”


• 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.


• 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.

Segment Descriptor

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