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Top 10 List of Week 05

1. Logical and Physical Memory Separation
   Virtual memory involves the separation of logical memory as perceived by users from physical memory. This separation allows an extremely large virtual memory to be provided for programmers when only a smaller physical memory is available.

2. Demand Paging
   In Operating Systems, demand paging is a method of virtual memory management. In a system that uses demand paging, the operating system copies a disk page into physical memory only if an attempt is made to access it and that page is not already in memory.

3. Page Fault
   A page fault is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit into the virtual address space of a process.

4. Demand Paging Performance
   The performance of demand paging is often measured in terms of the effective access time. Effective access time is the amount of time it takes to access memory, if the cost of page faults are amortized over all memory accesses. In some sense it is an average or expected access time.

5. COW (Copy On Write)
   COW (Copy On Write), sometimes referred to as implicit sharing or shadowing, is a resource-management technique used in computer programming to efficiently implement a “duplicate” or “copy” operation on modifiable resources.

6. Page Replacement Algorithm
   In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out, sometimes called swap out, or write to disk, when a page of memory needs to be allocated.

7. Page-Buffering Algorithms
   The operating system maintains a pool of free frames. When a page fault occurs, a page isselected for replacement and written into the pool of free frames. The faulty page is swapped outof disk and the page table is modified.

8. Allocation of Frames
   An important aspect of operating systems, virtual memory is implemented using demand paging. Demand paging necessitates the development of a page-replacement algorithm and a frame allocation algorithm. Frame allocation algorithms are used if you have multiple processes; it helps decide how many frames to allocate to each process.

9. Thrashing
   In computer science, thrashing occurs when a computer’s virtual memory resources are overused, leading to a constant state of paging and page faults, inhibiting most application-level processing. This causes the performance of the computer to degrade or collapse.

10. This Week’s Checklist
    The Checklists of this week and previous weeks are all related to running “chktoken some word” and downloading and decrypting tar.bz2.asc files and then putting the result to txt files. I don’t really know what’s that about or why, but it keeps me wondering what that is preparing us for.