Virtual Memory Manager. The Virtual Memory Manager has two main duties: to create and manage address maps for processes and to control physical memory allocation. NT 4.0 implements a 32-bit (4GB) address space; however, applications can directly access only the first 2GB. This portion of the address space is the user-mode half of the address map, and it changes to reflect the currently executing program's address map (e.g., Netscape, Notepad, Word). The 2GB to 4GB portion of the address space is for the kernel-mode portions of NT, and it doesn't change. NT 4.0 Service Pack 3 (SP3) and NT Server, Enterprise Edition 4.0, let administrators move the boundary in the address space so that user-mode applications can access 3GB of the map and kernel-mode components can use only 1GB.
The Virtual Memory Manager implements demand-paged virtual memory, which means it manages memory in individual segments, or pages. In x86 systems, a page is 4096 bytes; in Alpha systems, a page is 8192 bytes. The total memory applications require can exceed the computer's physical memory space. The Virtual Memory Manager stores the data that exceeds a computer's physical memory on the hard disk in page files. The Virtual Memory Manager transfers data to physical memory from a paging file when an application requests the data.
The Virtual Memory Manager has advanced capabilities that implement file memory mapping, memory sharing, and copy-on-write page protection. NT uses file memory mapping to load executable images and DLLs efficiently. In memory mapping, the Virtual Memory Manager learns through the operating system that a portion of a process' address map is connected to a particular file. When the process touches these portions of its address map (e.g., when it tries to execute code), the Virtual Memory Manager automatically loads the data into physical memory. NT uses memory sharing to enhance physical memory use and to communicate between processes. For example, multiple instances of a program share a memory-mapped file image, and this sharing increases memory efficiency.
Copy-on-write is an optimization related to memory sharing in which several programs share common data that each program can modify individually. When one program writes to a copy-on-write page that it shares with another program, the program that makes the modification gets its own version of the copy-on-write page to scribble on. The other program then becomes the original page's sole owner. NT uses copy-on-write optimization when several applications share the writable portions of system DLLs.
The Virtual Memory Manager divides physical memory among several executing programs. It uses a function called working-set tuning to allocate additional memory to programs that require it and ensure that other executing programs have enough memory to keep running.
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