Memory Management
- 1. Memory management Presented by M . Lavanya M.sc(CS & IT) Nadar Saraswathi college of arts & science. Vadapudupatti ,Theni.
- 2. MEMORY MANAGEMENT • Main Memory refers to a physical memory that is the internal memory to the computer. • The word main is used to distinguish it from external mass storage devices such as disk drives. Main memory is also known as RAM. • The computer is able to change only data that is in main memory. • Therefore, every program we execute and every file we access must be copied from a storage device into main memory.
- 3. MEMORY MANAGEMENT REQUIREMENTS. Five requirements: • Relocation • Protection • Sharing • Logical organization • Physical organization
- 4. • Relocation – programmer cannot know where the program will be placed in memory when it is executed – a process may be relocated in main memory due to swapping – swapping enables the OS to have a larger pool of ready-to- execute processes – memory references in code (for both instructions and data) must be translated to actual physical memory address
- 5. • Protection – processes should not be able to reference memory locations in another process without permission – impossible to check addresses at compile time in programs since the program could be relocated – address references must be checked at run time by hardware
- 6. • Sharing – must allow several processes to access a common portion of main memory without compromising protection • cooperating processes may need to share access to the same data structure • better to allow each process to access the same copy of the program rather than have their own separate copy
- 7. • Logical Organization – users write programs in modules with different characteristics • instruction modules are execute-only • data modules are either read-only or read/write • some modules are private others are public – To effectively deal with user programs, the OS and hardware should support a basic form of module to provide the required protection and sharing.
- 8. • Physical Organization – secondary memory is the long term store for programs and data while main memory holds program and data currently in use – moving information between these two levels of memory is a major concern of memory management (OS) • it is highly inefficient to leave this responsibility to the application programmer
- 9. FIXED PARTITIONING • Split the physical memory into partitions into each a process may be assigned Two difficulties: • The programmer may have to rewrite a program in order to fit into even the largest available partition. Overlaying may be used. • Internal fragmentation typically results--there's memory that is unused within each partition.
- 10. PLACEMENT ALGORITHM • With equal sized partitions there's not much to consider. Swapping out for more throughput may require some policies • For unequal sized partitions, the programs may be queued to use the "best" partition. This may not give the best throughput as some partitions may go unused.
- 11. RELOCATION • Because of swapping and compaction, a process may occupy different main memory locations during its lifetime • Hence physical memory references by a process cannot be fixed • This problem is solved by distinguishing between logical address and physical address
- 12. VIRTUAL MEMORY • Virtual Memory is a space where large programs can store themselves in form of pages while their execution and only the required pages or portions of processes are loaded into the main memory. This technique is useful as large virtual memory is provided for user programs when a very small physical memory is there. Benefits of Virtual Memory : • Large programs can be written, as virtual space available is huge compared to physical memory. • Less I/O required, leads to faster and easy swapping of processes. • More physical memory available, as programs are stored on virtual memory, so they occupy very less space on actual physical memory.
- 13. PAGING • Main memory is partition into equal fixed-sized chunks (of relatively small size) • Trick: each process is also divided into chunks of the same size called pages • The process pages can thus be assigned to the available chunks in main memory called frames (or page frames) • Consequence: a process does not need to occupy a contiguous portion of memory
- 15. PAGE TABLE • The OS now needs to maintain (in main memory) a page table for each process • Each entry of a page table consist of the frame number where the corresponding page is physically located • The page table is indexed by the page number to obtain the frame number • A free frame list, available for pages, is maintained
- 16. ADDRESS TRANSLATION IN PAGING SYSTEM Address translation by paging is similar to segmentation. A virtual address is comprised of a page number and an offset. Page tables are used to translate the page number into a page frame number; that is, the physical location of the page in main memory.