Essay on Computer Memory : Virtual Memory

Essay on Computer Memory : Virtual Memory

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Abstract:
UNIX has a technique called Virtual Memory to accommodate for the shortage of memory space. Virtual memory increases the available memory the computer has by enlarging the "address space," or places in memory where data can be stored. This paper describes concepts like swapping, demand paging, page fault, page replacement algorithm, thrashing, some virtual memory allocation algorithms and the Memory Management Unit (MMU).
Introduction:
Each program running on the machine is called a process and each process has its own address space that the process uses to store its data and code segments. Sometimes there is not enough memory to hold the entirety of large programs or when too many programs are running together on the machine. As a get around, UNIX uses a technique called Virtual Memory. Virtual memory is the use of space on the hard disk to simulate additional RAM [1]. In order to free up space in memory, the UNIX operating system transfers data that is not immediately needed from memory to the HDD; when that data is needed again, it is copied back into memory. That is, when all of the RAM is busy performing some task, a computer with virtual memory enabled will ‘swap’ data to the HDD and back to memory as needed, thus, in effect, increasing the total system memory. The operating system and hardware map the virtual address to the physical address when information needs to be stored or retrieved by a program
The physical main memory behaves like a set of windows or caches on a much larger "virtual" memory space, most of which at any given time is actually stored on disk in a special zone called the swap area. Virtual memory permits software to run in a memory space (i.e., a logical memory) whose size is greater than...


... middle of paper ...


...cal memory for other uses thus allowing many more programs to run simultaneously.
• Virtual memory allows non-contiguous physical memory blocks to be used as contiguous virtual memory.
Conclusion:
When a virtual address needs to be translated into a physical address, the TLB is searched first. If a match is found (a TLB hit), the physical address is returned and memory access can continue. However, if there is no match (called a TLB miss), the handler will typically look up the address mapping in the page table to see whether a mapping exists (a page walk). If one exists, it is written back to the TLB (this must be done, as the hardware accesses memory through the TLB in a virtual memory system), and the faulting instruction is restarted (this may happen in parallel as well). This subsequent translation will find a TLB hit, and the memory access will continue.





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