Lecture Outline

Aims and Objectives
Memory Management Techniques (1) - Paging

Sharing

• Paged Systems

• Segmentation Systems

• Combined Paging / Segmentation System

Concepts

• A page is a fixed-size block

• Virtual address, v = (p,d) [page number, displacement]

• Physical Memory is organised into Page Frames, which are equal in size to the Virtual page.

• If a page is not in PM, then it can be swapped in from secondary storage and placed in any free page frame.

Dynamic Address Translation for Paged Systems

• Item's virtual memory address referenced by v = (p, d).

• Look up p in page map table to determine that p is on page frame p'

• Multiply p'by the page size to obtain the address of the page frame

• Add d to P' to obtain the real address of the item

• But what happens if page is not in PM? - 3 different techniques

Page Address Translation by Direct Mapping

• If required page not already in PM, then Missing Page Fault issued

• Page map table contains entry for every page in process' virtual address space

• Process execution time effectively doubled, because every time virtual address referenced, 2 PM reads are required - 1 to look up physical address, other to read contents of phy. address!

Page Address Translation by Associative Mapping

• Costs of Associative memory are prohibitive, so usually this method of dynamic address translation is combined with Direct Mapping

Page Address Translation with combined Associative / Direct Mapping

• The Associative Storage will contain a partial associative page map of the most-recently referenced pages only (Translation Lookaside Buffer)

• Virtual Page entry searched for in TLB. If Found, continue as per associative store.

• Otherwise, access full page map table in PM.

• Update TLB with this page's details, replacing least-recently referenced page.

• Only 8 or 16 cells needed to achieve 90% performance of full asociative page maps.

• Page Frames can be shared by pointing to the same page frame from each processes' page map table.

Advantages and disadvantages

+ easy to tell how many pages can be held in memory simultaneously

- Internal Fragmentation

• Segment is a chunk of virtual memory. Segments can be variably sized, to accommodate entire process.

• Virtual Address is v = (s, d) [segment no., displacement]

• Segments transferred betwen PM and Secondary Storage as complete units

• Address Translation is the same as for Paged Systems. If a segment is not in PM a Missing Segment Fault is generated.

• Incoming segments are placed in any large enough free area of PM (Best Fit / First Fit).

Sharing in a Segmented System

• Different processes can reference the same segment through their segment table

Advantages and Disadvantages

+ No internal Fragmentation

- External Fragmentation

- Entire Segment needs to be in PM

• Combines Paging and Segmentation systems to benefit from advantages of both systems.

• Segments will contain a variable number of pages.

• Not all pages need be in Primary Memory (PM) at the same time.

• Virtual address is referenced by v = (s, p, d) [segment number, page number within segment, displacement within page]

Dynamic Address Translation in Paging/Segmentation Systems (using combined associative / direct mapping)

Sharing in a Paging/ Segmentation System

• Entries in different segment map tables for different processes can point to the same page map table.

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