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.

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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.


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

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__> segpresent__>phymem.

3) exception load__>segmemreturn__> interrupted prog.

4) 1st & 2nd step satisfied MMU will add 16 bit offset.

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