VDM - Virtual DOS Machine
VDM - Virtual DOS Machine
A DOS session created by OS/2 and Windows 95/98/NT/2000 in order to run a DOS or Windows 3.1 application. Depending on the operating system version, an application may run in its own Virtual DOS Machine or all applications run in the same one. It may or may not use the Virtual 8086 mode of the x86 chip.
Virtual 8086 Mode
An operational state in an Intel CPU that allows it to perform as multiple 8086/8088 CPUs, which was the architecture of the first x86 chip. Starting with the 386 in 1985, the mode was created to multitask multiple DOS "Real Mode" applications side by side with 32-bit Windows applications. Virtual 8086 Mode divides the computer into multiple address spaces and maintains virtual registers for each virtual machine.
Note: Virtual 8086 Mode is a "virtual machine" mode, not a "virtual memory" mode, which extends main memory to disk.
Virtual Machine
(1) One system image in a computer that supports multiple system images. Each system image contains the operating system and its associated applications, and each image may have the same operating system or a different operating system. Computers are built with hardware circuits that support virtual machine capability. Years ago, IBM's VM mainframe operating system was the first to provide this capability, and it did it entirely in software. Later, hardware circuits were added to provide virtual partitioning of the computer
Virtual Memory
Simulating more memory than actually exists, allowing the computer to run larger programs or more programs concurrently. It breaks up the program into small segments, called "pages," and brings as many pages from disk into memory that fit into a reserved area for that program. When additional pages are required, it makes room for them by swapping them to disk. It keeps track of pages that have been modified so that they can be retrieved when needed again.
If a program's logic points back and forth to opposite ends of the program, excessive disk accesses, or "thrashing," can slow down execution.
Virtual memory can be implemented in software only, but efficient operation requires virtual memory hardware. Virtual memory claims are sometimes made for specific applications that bring additional parts of the program in as needed; however, true virtual memory is a hardware and operating system implementation that works with all applications
Windows Swap File
A disk file used by Windows for its virtual memory. A virtual memory system temporarily stores segments of the application on disk when there is not enough memory to hold all the programs called for.
WINDOWS 95/98
Windows 95/98 creates only a temporary swap file (WIN386.SWP) that is dynamically sized and abandoned. It can also reside on a compressed drive as long as it is under the control of the DRVSPACE.VXD Protected Mode driver. To adjust the Windows 95/98 swap file, double click on the System Control Panel and select the Performance tab, then Virtual Memory.
WINDOWS NT 4
NT 4 creates a temporary swap file, or paging file (PAGEFILE.SYS), that is generally equal to the size of memory plus 12MB. It also allows for one additional swap file to be created on each logical partition on the hard disk. To adjust the size of the file, double click on the System Control Panel and select the Performance tab, then Virtual Memory, then Change.
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