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Chapter 2: Memory and Process Management, Part 1, 1.Registers and Caches,…

- - - - •Allocate the largest hole.
        •In contrast, this strategy aims to produce the largest leftover hole, which may be big enough to hold another process.
    - - •Allocate the first hole that is big enough.
        •Searching may start either at the beginning of the set of holes or where the previous first-fit search ended.
        •Stop searching as soon as first hole large enough to hold the process is found.
        •The first fit approach tends to fragment the blocks near the beginning of the list without considering blocks further down the list.
    - - •The memory manager places a process in the smallest memory in which it will fit.
        •This strategy produces the smallest leftover hole.
        •Entire list of holes is to be searched.
        •Search of entire list can be avoided by keeping the list of holes sorted by size.
    - - Worst Fit
        =Works by allocating the largest hole
        
        Weakness
        =Produce the largest leftover hole, which may be big enough to hold another process
      - First Fit
        Works by allocating the first hole that is big enough
        
        Weakness
        =Fragment the blocks near the beginning of the list
      - Best Fit
        = Works by putting a process in the smallest memory in which it will fit
        
        Weakness
        =Produces the smallest leftover hole
  - - - •Least-Recently-Used(LRU)
        
        Based on heuristic that the page that has not been used for longest will not be used in the near future
        
        List maintained where each time a page is referenced, it is moved to the head of the list. Last page in list is least recently used page, and can be replaced
      - •Least-Frequently-Used(LFU)
        
        Keeps a count of the number of times a page has been referenced.
        
        Page that has been referenced least will be replaced
      - •First-In-First-Out(FIFO)
        
        Replace the page that has been resident longest, but this could be the most heavily used page
      - •Not-Used-Recently(NUR)
      - Second Chance & Clock Page
  - - - attempt to load a piece of program or data into memory before it referenced
    - - system places the next piece of program or data in main memory when running program references it
        
        process may need to reference several pages before it can continue, but first page could be replaced before last page is fetched
- - - - A process can be swapped temporarily out of memory to a backing store and back to memory for continue execution
      - Major type of swap time is transfer time
        
        It directly proportional to the amount of memory swapped
      - Modiefied version of swapping
        
        UNIX, LINUX and Windows
      - Schematic View of Swapping
        
        Swap out
        
        Lower-prority process swapped out so higher-priority process can be swapped in (loaded and executed )
        
        Backing store
        
        -Fast disk large enough to accommodate copies of all memory images for all users and provide direct access to these memory
- - - - Common part in operating system because it provides a big benefit for users in very low cost
        
        Most computers today have something like 512 or 1 gigabytes for RAM availabe for CPU to be use but it is not enough to run all the programs that most of users expect it runs at once
        
        For example, 512 megabytes is not enough to hold all the operating system need such as email program, windows and games .
        
        With virtual memory, computer will look into RAM which area areas that have not been used recently and copy them onto the hard disk.
        
        The hard disk spaces is much cheaper than RAM chips, has an nice economic benefits
        
        This free up space in RAM to load the new application
    - - Smiliar to dynamic partitioning
        
        A program divided into several segment and each of it to a partition of varied size instead of fixed size pages.
        
        Based on the programmers' logical views of programs.
        
        Every logical address formed of a segment named and offset within that segment.
        
        In practice, segment are numbered.
        
        Program are segmented automatically by compiler or assembler.
      - Dynamic address translation
        
        Break the address into segment and displacment portions.
        
        Use the segment number to find the segment's absolute entry point address in a program segment table
        
        Add the displacement to the entry point address
    - - Operating system can store and retrieve data
        
        From secondary storage be use in main memory
        
        Break the main memory into fixed-sizes blocks called frames.
        
        Process of image into block of the same sizes called pages.
        
        The program is bought into memory for execution as pages.
        
        Paging leads to problem of Internal Fragmentation
        
        For example, when a page of 5 MB is placed in a frame of 7 MB, 2 MB of memory in the frame is wasted.
        
        This wastage of unused memory called Internal Fragmentation
      - Advantages
        
        It allows the physical address space of a process to be noncontiguous.
        
        Disadvantages
        
        Internal fragmentation occour