Segmentation

Segmentation

• 1.
  Segmentation Pradyumna
• 2.
  Definition • Way ofoffering protection to different data types and code.
• 3.
  Descriptor Tables • Descriptortable 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 InterruptDescriptor 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 carryall 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 • Segmentdescriptors 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 TaskState 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 protectionadded to segment table like paging • sharing of memory similar to paging (but per area rather than per page)
• 12.
  Drawbacks • Allocation algorithmsas for memory partitions • External fragmentation, back to compaction problem.