Please enable JavaScript.

Coggle requires JavaScript to display documents.

MEMORY AND PROCESS MANAGEMENT :, ABDULLAH, HAZIQ, SHAHRUL - Coggle Diagram

- - - - Unlike registers, main memory is accessed via a transaction on the memory bus
      - Main memory and the registers built into the processor itself
      - Registers that are built into the CPU are generally accessible within one cycle of the CPU clock
        
        If the data are not in memory, they must be moved there before the CPU can
        operate on them.
    - - This situation is unable to be endured because of frequency of the memory accesses
      - A memory buffer used to accommodate a speed differential, called a
        cache.
      - Memory access may take many cycles of the CPU clock to complete
- - - - -The memory manager places a process in the
        smallest memory in which it will fit.
        -This strategy produces the smallest leftover hole.
        -Search of entire list can be avoided by keeping the
        list of holes sorted by size.
      - For Example:
        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.
      - How it works?
        Put a process in the smallest memory in which it will fit.
      - Weakness
        Produces the smallest leftover hole
    - - Allocate the largest hole.
      - In contrast, this strategy aims to produce the largest leftover hole, which may be big enough to hold another process.
      - For example:
        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.
      - How it works?
        Allocate the largest hole
      - Weakness
        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.
      - For example:
        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.
      - How it works?
        Allocate the first hole that is big enough
      - Weakness
        Fragment The Blocks Near The Beginning Of The List
- - - - Normally, a computer does not have enough memory
        to hold all the programs you use.
      - Permanent in memory.
      - When you want to run a program, therefore, the operating system is obliged to “ free some memory by copying data or programs from main memory to a disk.”
        
        This process is known as swapping^^.
      - The programs and data used most frequently are the ones that should be memory resident. This includes central portions of the operating system and special programs, such as calendars and calculators, that you want to be able to access immediately.
      - Certain programs, however, 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
      - Another term for memory resident is RAM resident.
      - In DOS systems, memory-resident programs are called pop-up
        utilities or TSRs (terminate and stay resident).
    - - A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution.
      - Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped.
      - Modified versions of swapping are found on many
        systems, i.e., UNIX, Linux, and Windows.
  - - - -The Operating System ( Resident & Transient Routines ) occupies low
        memory.
      - --The remaining memory is the transient area.
- - - - Fixed partitions have fixed size and usually cannot be easily expanded or shrunk.
    - - Easy to implement.
      - Multiprogramming
      - Simplicity.
      - Multiple programs can be loaded into memory and
        executed concurrently
  - - - In contrast to fixed partition, is the dynamic partition
      - 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.
      - Processes are loaded into consecutive areas until the memory is filled, or, more likely, the remaining space is too small to accommodate another process.
      - Under dynamic memory management, the transient
        area is treated as a pool of unstructured free space.
      - When the system decides to load a particular program, a region of memory just sufficient to hold the program is allocated from the pool.
      - Because a program gets only the space it needs,
        relatively little is wasted.
    - - Advantage
        
        Jobs are given only as much memory as they request. Still, contiguous blocks are needed. Internal fragmentation problem is solved
      - Disadvantage
        
        Suffer external fragmentation.
- - - - When RAM runs low, virtual memory moves data from RAM to a space called a paging file.
        
        Moving data to and from the paging file frees up RAM to complete its work.
        
        Virtual memory combines your computer’s RAM with temporary space on your hard disk.
        
        The more RAM your computer has, the faster your programs will generally run.
        
        Virtual memory is used by the operating system in order to enhance the storage capacity of the working memory, without requiring the installation of additional Random Access Memory modules
        
        If a lack of RAM is slowing your computer, you might be tempted to increase virtual memory to compensate.
        
        Technique that allows the execution of processes that are not completely in memory.
        
        However, your computer can read data from RAM much more quickly than from a hard disk, so adding RAM is a better solution.
    - - Virtual memory is a common part of most operating systems on desktop computers.
        
        It has become so common because it provides a big benefit for users at a very low cost.
        
        Most computers today have something like 512 or 1 Gigabytes of RAM available for the CPU to use.
        
        Unfortunately, that amount of RAM is not enough to run all of the programs that most users expect to run at once.
        
        For example, if you load the operating system, an e-mail program, a several tabs Web browser, word processor, Adobe Photoshop and 3D game into RAM simultaneously, 512 megabytes is not enough to hold it all.
        
        If there were no such thing as virtual memory, then once you filled up the available RAM your computer would have to say, "Sorry, you can not load any more applications.
        
        Please close another application to load a new one." With virtual memory, what the computer can do is look at RAM for areas that have not been used recently and copy them onto the hard disk
        
        This frees up space in RAM to load the new application.
      - Because this copying happens automatically, you don't even know it is happening, and it makes your computer feel like is has unlimited RAM space even though it only has 512 megabytes installed
        
        The hard disk space is so much cheaper than RAM chips, it also has a nice economic benefit.
        
        The read/write speed of a hard drive is much slower than RAM, and the technology of a hard drive is not geared toward accessing small pieces of data at a time.
        
        If your system has to rely too heavily on virtual memory, you will notice a significant performance drop.
        
        The key is to have enough RAM to handle everything you tend to work on simultaneously then, the only time you "feel" the slowness of virtual memory is when there's a slight pause when you're changing tasks. When that's the case, virtual memory is perfect.
        
        When that's the case, virtual memory is perfect.
        
        When it is not the case, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.
        
        The area of the hard disk that stores the RAM image is called a page file. It holds pages of RAM on the hard disk, and the operating system moves data back and forth between the page file and RAM.
  - - - Another memory management approach, segmentation, is similar to dynamic partitioning.
        
        With segmentation, a program is divided into several segments, each similar to a partition of varied size
        
        Segmentation, is usually visible, which is actually based on the programmers’ logical view of programs.
        
        With segmentation, programs are divided into variable size segments, instead of fixed size pages.
        
        Every logical address is formed of a segment name and an offset within that segment.
        
        In practice, segments are numbered.
        
        Programs are segmented automatically by compiler or assembler.
      - Dynamic address translation.
        
        To dynamically translate a segment address to an absolute address:
        
        2) Use the segment number to find the segment’s absolute entry point address in a program segment table
        
        1) Break the address into segment and displacement portions
        
        3) Add the displacement to the entry point address.
    - - (i)Create the Physical Memory
        
        (i)Find the Physical Address when the generated logical address is <0,400> and <3,650>
  - - - From secondary storage for use in main memory.
      - Break the main memory into fixed-sized blocks called frames.
      - The ways in which the operating system can store and retrieve data
      - And break the program or process image into blocks of the same size called pages.
      - The program is bought into memory for execution as pages.
      - The program takes the pages from main memory during execution.
      - When the page required is not in main memory, the operating system fetches the required page from hard disk into main memory and moves the other page from main memory back to hard disk.
      - This process is referred as Paging.
      - When a process is to be executed, its pages are loaded into any available frames, as illustrated in Figure 8
    - - When a page of smaller size than the size of frame is placed in a larger size frame, it leads to unused memory space within a frame.
      - 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 is called Internal Fragmentation.
      - Paging leads to problem of Internal Fragmentation.
    - - It allows the physical address space of a process to be noncontiguous.
    - - Internal fragmentation occur.
  - - - With segmentation and paging, addresses are divided into a segment number, a page number within that segment, and a displacement within that page.
      - After the instruction control unit expands the relative address, dynamic address translation begins.
      - First, the program’s segment table is searched for the segment number, which yields the address of the segment’s page table.
      - The page table is then searched for the page’s base address, which is added to the displacement to get an absolute address.