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Memory and Process Management, image, Four Level of Memory Hierarchy,…

- - - - Fixed partitions have fixed size and usually cannot be easily expanded or shrunk
        
        Advantage
        
        Easy to implement
        
        Multiprogramming
        
        Simplicity
        
        Multiple programs can be loaded into memory and executed concurrently
        
        Fixed-Partition Allocation
        
        Fixed-Partition Memory Management divides the available space into
        fixed-length partitions each of which holds one program
        
        Abstract View Of Memory Allocation
- - - - Dynamic partitions can change size by themselves when
        they need it.
      - Allow the partitions to be variable in size at load time.
      - Allocate to the process the exact amount of memory it
        requires.
      - Process
        
        loaded into consecutive areas until the
        memory is filled / more likely, the remaining space is
        too small to accommodate another process.
      - transient
        area
        
        treated as a pool of unstructured free space
        
        Because a program gets only the space it needs,
        relatively little is wasted.
    - - Jobs are given only as much memory as they request.
      - Still, contiguous blocks are needed
    - - Suffer external fragmentation.
- - - - Resident routine
        
        Permanent in memory.
        
        A computer does not have enough memory
        to hold all the programs you use.
        
        Also known as swapping
        
        A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution
        
        Major part(swap time)
        
        transfer time; total transfer time is directly proportional to the amount of memory swapped
        
        Modified version (founded on many systems)
        
        Example
        
        UNIX
        
        Linux
        
        Windows
        
        Certain programs
        
        can be marked as being memory resident, which means that the operating system is not permitted to swap them out to a storage device; they will always remain in memory
        
        The programs and data
        
        used most frequently are the ones that
        should be memory resident
        
        Example
        
        calendars and
        calculators
        
        that you want to be able to access immediately.
        
        Another term
        
        RAM resident.
        
        DOS systems
        
        memory-resident programs are called pop-up
        utilities or TSRs (terminate and stay resident).
      - Term overlay
        
        commonly used instead, and refer to a program
        module that is brought to memory
      - Meaning
        
        a module that, once loaded into main memory, is
        expected to remain in memory for a short time.
      - Program modules
        
        written to allow different modules to share
        the same memory region and the main program itself
        
        responsible of exchanging modules between disk and memory as
        necessary
      - The operating system ( Resident & Transient Routines )
        
        occupies low
        memory.
        
        The remining memory
        
        Transient area
      - Transient routine
      - Comparison
- - - - • When to bring data into primary memory
    - - Where in primary memory should data be placed
    - - Which page should be replaced, if primary memory is full.
  - - - Memory allocation is the process of assigning blocks of memory on request.
      - Typically the allocator receives memory from the operating system in a small number of large blocks that it must divide up to satisfy the requests for smaller blocks.
      - It must also make any returned blocks available for reuse.
      - There are many common ways to perform this, with different strengths and weaknesses.
  - - -  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.
    - - Suppose a process request 12KB of memory and the memory manager currently has a list of unallocated blocks of 6KB, 14KB, 19KB,11 KB, and 13KB blocks. The best fit strategy will allocate the 13 KB block to the process
    - - Avoid negative reminder
      - Scan and compare
      - Allow extra space
  - - - Allocate the largest hole.
      - In contrast, this strategy aims to produce the largest leftover hole, which may be big enough to hold another process.
    - - If the list of unallocated blocks consists of 6KB, 14KB, 19KB, 11 KB, and 13KB blocks and a process request a 12KB block, Then the worst fit will split the 19KB block into a 12KB block to allocate to the process, leaving a 7KB block for future use.
      - Scan and compare
        
        Avoid negative remainder
        
        Allow MORE extra space
  - - - 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.
    - -  If the list of unallocated blocks consists of 6KB, 14KB, 19KB, 11 KB, and 13KB blocks and a process request a 12KB block, First Fit will allocate the 14KB block to the process.
      - 1.Avoid negative remainder
        
        Allow extra space
  - - -  When a process generates a page fault, the memory manager must locate referenced page in secondary storage, load it into page frame in main memory and update corresponding page table entry.
      -  If no frame is free, we find one that is not currently being used and free it.
      -  We can free a frame by writing its contents to swap space and changing the page table (and all other tables) to indicate that the page is no longer in memory
    - - Least-Recently-Used (LRU)
      - Least-Frequently-Used (LFU)
      -  First-In-First-Out (FIFO)
      - Not-Used-Recently (NUR)
  - - - Worst Fit
        
        First Fit
        
        Best Fit
- - - - Memory devices at a lower level are
        
        faster to access
        
        smaller in size,
        
        more expensive per byte
        
        using a smaller unit of transfer
        
        having a higher bandwidth
      - Disk Drives and Tape Units
      - Registers and Caches