Segmentation
Download as PPTX, PDF0 likes427 views
Segmentation provides memory protection by separating memory into different segments for code and data. There are three main descriptor tables - the Global Descriptor Table (GDT) containing global descriptors, the Local Descriptor Table (LDT) containing task-specific descriptors, and the Interrupt Descriptor Table (IDT) containing interrupt vectors. Descriptors contain information about segments including limits, addresses, and access rights. The Task State Segment (TSS) stores task state information for task switching. Gate descriptors allow changing privilege levels during function calls. Segmentation adds memory protection while allowing memory sharing between tasks.
Segmentation
- 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.