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操作系统英文课件:ch3b Memory Management.ppt
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2022-06-17
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操作系统英文课件:ch3b Memory Management.ppt
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1
Memory Management
Chapter 3
Fetch Strategies
Page Replacement
-Optimal -FIFO -LRU -NRU
-Second-chance
Design Issues for paging systems
Implementation Issues
Segmentation
Summary
![](https://csdnimg.cn/release/download_crawler_static/85669796/bg2.jpg)
2
Paging Policies
Fetch Strategies
When should a page be brought into primary (main)
memory from secondary (disk) storage.
Replacement Strategies
Which page now in primary storage is to be
removed from primary storage when some other
page or segment is to be brought in and there is not
enough room.
Clean Strategies
When a page in memory is brought out?
![](https://csdnimg.cn/release/download_crawler_static/85669796/bg3.jpg)
3
Demand Fetching
Algorithm Never bring a page into primary
memory until it’s needed.
1. Page fault occurs.
2. Check if a valid virtual memory address. Kill job if
not.
3. If valid reference, check if it’s cached in memory
already (perhaps for some other process.) If so,
skip to 7.
4. Find a free page frame.
5. Map address into disk block and fetch disk block
into page frame. Suspend user process.
6. When disk read finished, add vm mapping for
page frame.
7. If necessary, restart process.
![](https://csdnimg.cn/release/download_crawler_static/85669796/bg4.jpg)
4
Prepaging
Algorithm bring a page into primary in advance.
When page-fault happens, the page needed and
adjacent pages will be brought into memory.
Advantage: improve the I/O efficiency when fetch
page.
Disadvantage: based on prediction, if the fetched
page is rare referenced, low efficiency.
Used when loading page.
![](https://csdnimg.cn/release/download_crawler_static/85669796/bg5.jpg)
5
Page Replacement
1. Find location of page on disk
2. Find a free page frame
① If free page frame use it
② Otherwise, select a page frame using the page
replacement algorithm
③ Write the selected page to the disk if necessary
and update any necessary tables
3. Read the requested page from the disk.
4. Restart the user process.
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