Thursday, December 13, 2007

Assignment #3

#4a.What is the cause of thrashing?
@Thrashing is caused by under allocation of the minimum number of pages required by a process, forcing it to continuously page fault.

b. How does the system detect thrashing?
The system can detect thrashing by evaluating the level of CPU utilization as compared to the level of multiprogramming.

c.Once it detects thrashing, what can the system do to eliminate this problem?
It can be eliminated by reducing the level of multiprogramming.

a.Multiprogrammng. Why is it to use?-The concept of multiprogramming is that the operating system keeps several jobs in memory simultaneously. The operating system selects a job from the job pool and starts executing a job, when that job needs to wait for any i/o operations the CPU is switched to another job. So the main idea here is that the CPU is never idle.

b.Internal Fragmentation.How does it occur?-
Internal fragmentation is the space wasted inside of allocated memory blocks because of restriction on the allowed sizes of allocated blocks. Allocated memory may be slightly larger than requested memory; this size difference is memory internal to a partition, but not being used

c.External Fragmentation. How does it occur?
External Fragmentation happens when a dynamic memory allocation algorithm allocates some memory and a small piece is left over that cannot be effectively used. If too much external fragmentation occurs, the amount of usable memory is drastically reduced. Total memory space exists to satisfy a request, but it is not contiguous.

Sunday, December 2, 2007

Assingment No. 2

Windows Operating System;

The Windows NT Virtual Memory ManagerIn Windows NT, responsibility for managing the relationship between the virtual organization of memory (as seen by applications) with the physical organization of memory (as seen by the operating system) falls on a component of the Windows NT executive called the virtual memory (VM) manager .Memory Management GoalsWindows NT is a portable, secure, multithreaded, multiprocessing operating system. As a result, its virtual memory manager must:• Be compatible with multiple processor types • Protect the NT Executive from applications • Protect applications from each other • Provide mechanisms for programs to efficiently share physical memory (RAM). • Be efficientAn Application's View of MemoryIn Windows NT, applications access memory using a 32-bit linear addressing scheme. This scheme is sometimes referred to as flat memory model because applications view memory as one linear (or flat) array of memory locations. Applications address memory using simple 32-bit offsets from address zero (0). Since a 32-bit offset can specify 232 memory addresses, each application can access up to 4 Gb of (virtual) memory. The range of addresses an application can access is called the application's address space (Figure 7).The 32-bit flat memory model makes Windows NT portable because it is compatible with the memory addressing of processors such as the MIPS R4000 and DEC Alpha. It also simplifies porting of applications originally written for flat memory model environments such as Unix and the Apple Macintosh.The flat memory model used in Windows NT contrasts sharply with the segmented model used in Windows for MS-DOS. In Windows for MS-DOS, memory is broken into many differently sized segments, each with a maximum length of 64K. This has been a major difficulty for developers of Windows applications for a very long time because changing segments is somewhat difficult and slow. This difficulty has led to many 64K limits in a lot of software, including the Windows 3.1 resource heap. The 32-bit flat memory model does away with all of these constraints.


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