lec10_mem

lec10_mem - Review CSE120 Principles of Operating Systems...

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1 CSE120 Principles of Operating Systems Prof Yuanyuan (YY) Zhou Lecture 10: paging Nov 2nd, 2010 Review Fixed and variable size partition Simple to implement (base and limit registers) Internal vs. external fragmentation Best, worst, fast fit Bitmap vs. link-list implementation of free memory regions Paging Page size is a power of 2 Virtual page number physical page frame How to calculate VPN from virtual address? How to get physical address? Segmentation Segmentation table Hybrid: paging+segmentation 11/1/2010 CSE 120 – Lecture 10 – Paging 2 11/1/2010 CSE 120 – Lecture 10 – Paging 3 Today’s Lecture Today we’ll cover more paging mechanisms: Optimizations Managing page tables (space) Efficient translations (TLBs) (time) Demand paged virtual memory (space) Recap address translation 11/1/2010 CSE 120 – Lecture 10 – Paging 4 Page Mapping Hardware Contents(P,D) Contents(F,D) P D F D P-> F 0 1 0 1 1 0 1 Page Table Virtual Memory Physical Memory Virtual Address (P,D) Physical Address (F,D) P F D D P
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2 11/1/2010 CSE 120 – Lecture 10 – Paging 5 Page Mapping Hardware Contents(4006) Contents(5006) 004 006 005 006 4-> 5 0 1 0 1 1 0 1 Page Table Virtual Memory Physical Memory Virtual Address (004006) Physical Address (F,D) 004 005 006 006 4 Page size 0x1000 Number of Possible Virtual Pages 0x1000 Number of Page Frames 8 11/1/2010 CSE 120 – Lecture 10 – Paging 6 Paging Issues Page size is 2 n usually 512, 1k, 2k, 4k, or 8k E.g. 32 bit VM address may have 2 20 (1MB) pages with 4k (2 12 ) bytes per page Page table: 2 20 page entries take 2 22 bytes (4MB) 11/1/2010 CSE 120 – Lecture 10 – Paging 7 Managing Page Tables The page table for a 32-bit address space w/ 4K pages to be 4MB This is far too much overhead for each process How can we reduce this overhead? Observation: Only need to map the portion of the address space actually being used (tiny fraction of entire addr space) How do we only map what is being used? Can dynamically extend page table… Does not work if addr space is sparce (internal fragmentation), why ? So now what? Discussion How to reduce page table size if the virtual pages are sparse? 11/1/2010 CSE 120 – Lecture 10 – Paging 8
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3 11/1/2010 CSE 120 – Lecture 10 – Paging 9 Two-Level Page Tables Two-level page tables Virtual addresses (VAs) have three parts: Master page number, secondary page number, and offset Master page table maps VAs to secondary page table
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lec10_mem - Review CSE120 Principles of Operating Systems...

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