1. Segmentation
Pradyumna

2. Definition
• Way of offering protection to different data
types and code.

3. Descriptor Tables
• Descriptor table is a array of 8k Descriptors
• 3 types
– Global Descriptor Table (GDT)
– Local Descriptor Table (LDT)
– Interrupt Descriptor Table (IDT)

4. GDT&LDT
GDT LDT
• Contains Global descriptors • Contains descriptors specific
common to all tasks to a particular task
• Contains al the descriptor • Contains segment , task
types except interrupt & gate & call gate descriptors.
trap descriptors

5. IDT
• The Interrupt Descriptor Table (IDT) is a data
structure used to implement an interrupt
vector table.
• The IDT is used by the processor to determine the
correct response to interrupts & exceptions.
• Use of the IDT is triggered by three types of
events: hardware interrupts, software
interrupts, and processor exceptions, which
together are referred to as "interrupts".
• The IDT consists of 256 interrupt vectors

6. Descriptors
• These carry all the info about a segment and its
access rights
• These have 20 bit segment limit and 32-bit
segment address.
• 5 types of descriptors
– Code or data segment Descriptors
– System Descriptors
– Local Descriptors
– TSS (Task State segment) Descriptors
– GATE Descriptors

7. Segment descriptor
• Segment descriptors are a part of the
segmentation unit, used for translating
a logical address to linear address. Segment
descriptors describe the memory
segment referred in the logical address

8. TSS
• The Task State Segment is a structure which
holds information about a task.
• It is used by the OS kernel for task management.
• Specifically, the following information is stored in
the TSS:
– Processor register state
– I/O Port permissions
– Inner level stack pointers
– Previous TSS link

9. Gate Descriptors
• Mechanism, for changing the privilege level of
the CPU when it executes a
predefined function callusing a CALL FAR
instruction.

10. • BASE  Base Address of the • A Accessed Bit
segment • G  Granularity Bit-1=Segment
• LIMIT the length of the Length is page
segment granular,0=Segment length is byte
granular
• P Present BIT-1=Present,0=Not
Present • D  Default Operation Size-1=32
bit segment,0=16-segment
• DPL  Descriptor Privilege Level
0-3 • 0  Bit must be zero for
compatibility with future
• S  Segment Descriptor - processors
0=System Descriptor ,1=Code or
Data Segment Descriptor • AVL  Available field for user or
OS
• TYPE  type of segment

11. Advantages
• memory protection added to segment table
like paging
• sharing of memory similar to paging (but per
area rather than per page)

12. Drawbacks
• Allocation algorithms as for memory partitions
• External fragmentation, back to compaction
problem.