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Ezra Rogers
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Operating System Notes Vtu Pdf 92 [EXCLUSIVE]


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Operating System Notes Vtu Pdf 92



Now, recalling where we last left off, there are two types of Locks available Shared S(a) and Exclusive X(a). Implementing this lock system without any restrictions gives us the Simple Lock-based protocol (or Binary Locking), but it has its own disadvantages, they do not guarantee Serializability. Schedules may follow the preceding rules but a non-serializable schedule may result.


The main purpose of a computer system is to execute programs. During the execution of these programs together with the data they access must be stored in main memory. Memory consists of a large array of bytes. Each Byte has its own address. CPU fetches instructions from memory according to the value of the program counter.


Note: It is the responsibility of the users to design their programs to support Dynamiclinking. Operating systems may help the programmer by providing library routines toimplement dynamic loading.


Dynamic LinkingDynamically linked libraries are system libraries that are linked to user programs when theprograms are running. In static linking system libraries are treated like any other object module and they are combined by the loader into the binary program image. In Dynamic linking, the linking is postponed until execution time. This feature is usually used with system libraries, such as language subroutine libraries. Without dynamic linking, each program on a system must include a copy of its language library in the executable image. This will waste both disk space and main memory.


Version information is included in both the program and the library. More than one version of a library may be loaded into memory and each program uses its version information to decide which copy of the library to use. Versions with minor changes retain the same version number, whereas versions with major changes increment the number. Thus, only programs that are compiled with the new library version are affected by any incompatible changes incorporated in it. Other programs linked before the new library was installed will continue using the older library. This system is also known as shared libraries.Note: Dynamic linking and shared libraries require help from the operating system.


Swapping on Mobile systemsMobile systems such as iOS and Android do not support swapping. Mobile devices generally use flash memory rather than more spacious Hard disks as their persistent storage. Mobile operating-system designers avoid swapping because of the less space constraint. Flash memory can tolerate only the limited number of writes before it becomes unreliable and the poor throughput between main memory and flash memory in these devices.


Variable partition scheme (MVT)In the variable-partition scheme, the operating system keeps a table indicating which parts ofmemory are available and which are occupied. Initially, all memory is available for user processes and it is considered one large block of available memory called as Hole. Eventually the memory contains a set of holes of various sizes. As processes enter the system, they are put into an Input Queue. The operating system takes into account the memory requirements of each process and the amount of available memory space in determining which processes are allocated memory.


When a process is allocated space, it is loaded into memory and it can then compete for CPU time. When a process terminates, it releases its memory. The operating system may use this free fill with another process from the input queue.


Then operating system can wait until a large block is available for the process or it can skipthe process and moves down to the input queue to see whether the smaller memoryrequirements of some other process can be met.


The memory blocks available comprise a set of holes of various sizes scattered throughout main memory. When a process arrives and needs memory, the system searches the set for a hole that is large enough for this process. If the hole is too large, it is split into two parts. One part is allocated to the arriving process and the other part is returned to the set of holes. When a process terminates, it releases its block of memory, which is then placed back in the set of holes. If the new hole is adjacent to other holes, these adjacent holes are merged to form one larger hole. At this point, the system may need to check whether there are processes waiting for memory and whether this newly freed and recombined memory could satisfy the demands of any of these waiting processes.


STRUCTURE OF THE PAGE TABLEPage tables can be structured in 3 ways: 1. Hierarchical Paging 2. Hashed Page Tables 3. Inverted Page TablesHierarchical PagingMost modern computer systems support a large logical address space (2 32 to 2 64 Bytes) thatleads to excessively larger page tables.


Example of Two level paging: VAX Mini Computer The VAX was the most popular minicomputer from 1977 through 2000. The VAX architecture supported a variation of Two-Level paging. The VAX is a 32- bit machine with a page size of 512 bytes. The logical address space of a process is divided into 4-equal sections, each of which consists of 230 bytes. Each section represents a different part of the logical address space of a process. The first 2 high-order bits of the logical address designate the appropriate section. The next 21 bits represent the logical page number of that section and the final 9 bits represent an offset in the desired page. By partitioning the page table in this manner, the operating system can leave partitions unused until a process needs them. Entire sections of virtual address space are frequently unused and multilevel page tables have no entries for these spaces, greatly decreasing the amount of memory needed to store virtual memory data structures.


A hierarchical memory system has the following specification, 20 MB main storage with access time of 300 ns, 256 bytes cache with access time of 50 ns, word size 4 bytes, page size 8 words. What will be the hit ratio if the page address trace of a program has the pattern 0, 1, 2, 3, 0, 1, 2, 4 following LRU page replacement technique? 350c69d7ab


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