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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