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Mrs. Shilpa S. Paygude. Flexible 32-Bit Microprocessor 8, 16, 32-Bit Data Types 8 General Purpose 32-Bit Registers Very Large Address Space 4 Gigabyte.

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Presentation on theme: "Mrs. Shilpa S. Paygude. Flexible 32-Bit Microprocessor 8, 16, 32-Bit Data Types 8 General Purpose 32-Bit Registers Very Large Address Space 4 Gigabyte."— Presentation transcript:

1 Mrs. Shilpa S. Paygude


3 Flexible 32-Bit Microprocessor 8, 16, 32-Bit Data Types 8 General Purpose 32-Bit Registers Very Large Address Space 4 Gigabyte Physical 64 Terabyte Virtual 4 Gigabyte Maximum Segment Size

4 Integrated Memory Management Unit Virtual Memory Support Optional On-Chip Paging 4 Levels of Protection Virtual 8086 Mode Allows Running of 8086 Software in a Protected and Paged System Optimized for System Performance Pipelined Instruction Execution On-Chip Address Translation Caches 20, 25 and 33 MHz Clock

5 No Memory Management Unit Application program size limited to available memory (RAM) Single tasking environment Segment size maximum 64KB

6 Multitasking Protection –Restricted Access to Data, Code & Stack Secure Access to I/O devices With minimum available memory, execution of application of any size

7 Real Address Mode Protected Mode Virtual 86 Mode

8 Multitasking Protection Virtual Memory- Demand Paging

9 Microprocessor can execute one instruction at a time Microprocessor works very fast ( in microseconds ) compared to human response time


11 Segmentation Unit : Deals with segments of a program. Paging Unit : Divides programs in fixed size (4KB) blocks.

12 Defines various types of segments with different attributes and provides segment level protection. Cannot be Disabled. It converts Logical Address to Linear Address.

13 Divides a program in fixed sized pages(4KB). Swaps pages between RAM & secondary device as required. Provides page level protection. It converts Linear address to Physical address. Optional -can be enabled or disabled.

14 Virtual / logical address to Linear Address CS : IP Base address + IP Linear address Linear Address to Physical Address Linear Address Base address from page table + offset

15 Segmentation is one method of memory management. Segmentation provides the basis for protection. Segments are used to encapsulate regions of memory which have common attributes. Segment selectors can be considered the logical name of a program module or data structure

16 Segments are variable sized blocks of linear addresses which have certain attributes associated with them There are two main types of ( non system) segments: code and data Segments are of variable size - as small as 1 byte or as large as 4 gigabytes Base address (32bit) + Offset (32bit) Linear Address Base address defined in the descriptor by system

17 A segment is described by a structure – Segment Descriptor Each segment (Code/data/stack) has a unique descriptor in memory All descriptors are maintained in a table – Descriptor Table


19 GDT (Global Descriptor Table), IDT (Interrupt Descriptor Table), LDT (Local Descriptor Table), TSS (Task State Segment).


21 Segment Descriptor n Segment Descriptor 1 Segment Descriptor 0 NULL Descriptor Base AddressLimit - 1 + Linear address (32bit) Segment base address (32 bit) Offset (32bit) GDT GDTR Selector Segment register




25 The Global Descriptor Table (GDT) contains descriptors which are possibly available to all of the tasks in a system. The GDT can contain any type of segment descriptor except for descriptors which are used for servicing interrupts (i.e. interrupt and trap descriptors). Every Intel386 DX system contains a GDT

26 Generally the GDT contains code and data segments used by the operating systems and task state segments, and descriptors for the LDTs in a system. GDT contains descriptors for segments which are common to all tasks

27 LDTs contain descriptors which are associated with a given task. Generally, operating systems are designed so that each task has a separate LDT. The LDT may contain only code, data, stack, task gate, and call gate descriptors. There can be 0 or many LDTs.

28 The visible portion of the LDT register contains only a 16-bit selector. This selector refers to a Local Descriptor Table descriptor in the GDT. LDTs provide a mechanism for isolating a given task's code and data segments from the rest of the operating system





33 Privilege Level-One of the four hierarchical privilege levels. Level 0 is the most privileged level and level 3 is the least privileged. More privileged levels are numerically smaller than less privileged levels.

34 System tables are manipulated by the operating system. Therefore, the load descriptor table instructions are privileged instructions. The instructions that can be executed at CPL=0 are privileged instructions.


36 Paging is another type of memory management useful for virtual memory multitasking operating systems Freed space of an outgoing task always same as that needed for an incoming task A page most likely corresponds to only a portion of a module or data structure

37 The page directory The page tables The page itself (page frame)

38 A uniform size of 4KB for all of the elements simplifies memory allocation and reallocation schemes, since there is no problem with memory fragmentation. Protection can further be increased at page level.

39 Directory Entry Page Table Entry 4KB page PDBR Index to Directory Index to TableOffset in Page 31 22 21 12 11 0 Page Table Directory Page Table Page CR 3 Linear Address Base address


41 CR3 is the Page Directory Physical Base Address Register. It contains the physical starting address of the Page Directory. Page Directory is always page aligned. CR2 is the Page Fault Linear Address register. It holds the 32-bit linear address which caused the last page fault detected

42 The Page Directory is 4K bytes long and allows up to 1024 Page Directory Entries. Each Page Directory Entry contains the address of the next level of tables, the Page Tables and information about the page table. Page Directory Entry points to Page Table


44 Each Page Table is 4K bytes and holds up to 1024 Page Table Entries. Page Table Entries contain the starting address of the page frame and statistical information about the page Page tables can be shared between tasks and swapped to disks.


46 The paging mechanism distinguishes between two levels of protection: User which corresponds to level 3 of the segmentation based protection, and supervisor which encompasses all of the other protection levels (0, 1, 2). Programs executing at Level 0, 1 or 2 bypass the page protection, although segmentation based protection is still enforced by the hardware.

47 The U/S and R/W bits are used to provide User/Supervisor and Read/Write protection for individual pages or for all pages covered by a Page Table Directory Entry The U/S and R/W bits in the first level Page Directory Table apply to all pages described by the page table pointed to by that directory entry.

48 The U/S and R/W bits in the second level Page Table Entry apply only to the page described by that entry. The U/S and R/W bits for a given page are obtained by taking the most restrictive of the U/S and R/W from the Page Directory Table Entries and the Page Table Entries

49 Performance would degrade substantially if the processor was required to access two levels of tables for every memory reference. Intel386 DX keeps a cache of the most recently accessed pages, this cache is called the Translation Lookaside Buffer (TLB).

50 The TLB is a four-way set associative 32- entry page table cache. It automatically keeps the most commonly used Page Table Entries in the processor. The 32-entry TLB coupled with a 4K page size, results in coverage of 128K bytes of memory addresses.


52 The paging unit hardware receives a 32-bit linear address from the segmentation unit. The upper 20 linear address bits are compared with all 32 entries in the TLB to determine if there is a match. If there is a match (i.e. a TLB hit), then the 32-bit physical address is calculated and will be placed on the address bus.

53 Intel386 DX will read the appropriate Page Directory Entry. If P = 1 on the Page Directory Entry indicating that the page table is in memory, then the Intel386 DX will read the appropriate Page Table Entry



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