IBM PC 300GL Types 6275, Information Manual

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memory above the first 1 MB is not written to or read from directly. It is broken into 16 KB
pieces called pages, or 64 KB pieces called frames, and is moved in and out of the reserved
area as needed. When a page or frame is moved into the reserved area, it can be read from
and written to as any other address in conventional memory.
Other Types of Memory Management
Operating systems such as Microsoft Windows 95, Windows 98, and Windows NT have
very sophisticated memory-management systems. These operating systems use a technique
called disk swapping or disk paging. If your computer does not have enough memory to meet
the needs of your active programs, these operating systems transfer the least-used
information from memory to the hard disk to make more memory available. When the
information on the hard disk is needed, it is exchanged with other information in memory.
Microsoft Windows 95, Windows 98, and Windows NT also use memory in another way.
These operating systems assign blocks of memory as virtual DOS machines (VDM). Each
VDM runs independently of the others, providing the same function as a separate computer
running DOS. With very few exceptions, these operating systems are fully compatible with
application programs written for DOS.
If you are using any of these operating systems, adding memory can increase the overall
performance.
Physical and Virtual Memory
Physical memory consists of all the addressable memory locations in the computer. Physical
memory is used to store such items as the operating system, video data, and instructions
and data the computer uses to run programs.
Virtual memory is memory that appears to be allocated to application programs. The
operating system uses a portion of the hard disk as virtual memory, swapping data and
instructions between the hard disk and physical memory.
Virtual memory makes multitasking possible. In a multitasking session, the memory
requirements of all the programs that might be running in the system at the same time can
far exceed the amount of physical memory that is available. The operating system allocates
virtual memory to meet the total memory requirements of each program and then manages
the available physical memory to meet the actual requirements. Thus, the amount of virtual
memory that is allocated can be much greater than the amount of physical memory installed
in the computer.
6 Microprocessors and Memory