Os Memory Management Questions Medium
The LRU-4 (Least Recently Used with 4th Chance) page replacement algorithm is a variation of the LRU (Least Recently Used) algorithm that aims to improve its performance by providing additional chances for a page to be referenced before being replaced.
The working of the LRU-4 algorithm can be described as follows:
1. Each page in the memory is assigned a reference bit, which is initially set to 0. This reference bit is used to track the recent usage of pages.
2. When a page is referenced, its reference bit is set to 1, indicating that it has been recently used.
3. When a page needs to be replaced, the algorithm starts by examining the reference bits of all pages in the memory.
4. If a page has a reference bit of 0, it means that it has not been recently used and can be replaced immediately. The selected page is then replaced with the new page.
5. If all pages have a reference bit of 1, indicating that they have all been recently used, the algorithm proceeds to the next step.
6. The algorithm then checks the second chance bit of each page. This bit is initially set to 0 for all pages.
7. If a page has a second chance bit of 0, it means that it has not been given a second chance yet. In this case, the second chance bit is set to 1, and the algorithm moves on to the next page.
8. If a page has a second chance bit of 1, it means that it has already been given a second chance. In this case, the algorithm proceeds to the next step.
9. The algorithm then checks the third chance bit of each page. This bit is initially set to 0 for all pages.
10. If a page has a third chance bit of 0, it means that it has not been given a third chance yet. In this case, the third chance bit is set to 1, and the algorithm moves on to the next page.
11. If a page has a third chance bit of 1, it means that it has already been given a third chance. In this case, the algorithm proceeds to the next step.
12. The algorithm then checks the fourth chance bit of each page. This bit is initially set to 0 for all pages.
13. If a page has a fourth chance bit of 0, it means that it has not been given a fourth chance yet. In this case, the fourth chance bit is set to 1, and the algorithm moves on to the next page.
14. If a page has a fourth chance bit of 1, it means that it has already been given a fourth chance. In this case, the page is selected for replacement, and its fourth chance bit is reset to 0.
15. The selected page is then replaced with the new page, and its reference bit, second chance bit, third chance bit, and fourth chance bit are all set to 0.
By providing additional chances for pages to be referenced before being replaced, the LRU-4 algorithm aims to reduce unnecessary page replacements and improve the overall efficiency of memory management.