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ID:VM DESQview/X: Virtual Memory
Quarterdeck Technical Note # 130
By Susan Easmon
Last Revision: 29 January 1993
WHAT IS VIRTUAL MEMORY?
Virtual memory allows a computer to work as if it has more RAM than is
physically present. This is done through memory addressing that involves
storing some parts of the program on disk and other parts in memory. When
parts of the program that are not in physical memory are needed they are
loaded from disk. The operating system (OS) determines where the information
is located and where it must go, in order to minimize any performance loss.
DESQview/X VIRTUAL MEMORY SYSTEMS
The display servers use a 16-bit VMM (Virtual Memory Manager) configuration
file which can be found in the \DVX\SERVER directory. DESQview/X display
servers have the ability to use your system's free disk space for virtual
memory.
The 256-colour server uses XC16.VMC as its configuration file and the
16-colour server uses XB16.VMC as its configuration file.
The defaults for both .VMC files are:
minmem = 600 - The minimum amount of RAM managed by the VMM.
maxmem = 1000 - The maximum amount of RAM managed by the VMM.
swapmin = 128 - The size of the swapfile at startup.
swapinc = 128 - The increment by which a swapfile grows (in K).
swapname=tmp\server.swp - The swapfile name and location.
reservelow = 16 - Reserve 16K of low DOS memory for use by the VMM.
freelow - Free all low memory if program does Spawn().
envlow - Allocate environment space from low memory.
The figures on the MINMEM and MAXMEM field come from your pool of expanded
memory. The display server uses RAM (physical memory) up to the MAXMEM limit
before it uses virtual memory, i.e., before it starts to swap to disk. You can
tell that virtual memory is being used when system performance degrades and
your computer accesses the hard disk without one of your programs explicitly
reading or writing to the hard disk (i.e. the light starts to flash
continuously). Please note that virtual memory is at least several thousand
times slower than physical memory. If virtual memory becomes active, and you
have sufficient expanded memory available on your system, you may wish to
increase the MAXMEM parameter for the appropriate display server and restart
DVX to see if this will improve the performance of your computer. It is
important to leave the default setting on the MAXMEM field for the 256-colour
server. This figure may be increased if need be. We do not recommend
reducing the figure to any value less than 1000K.
It is highly unlikely that the 16-colour server would benefit from more than
1000K of server memory. The 256-colour server (especially at higher
resolutions) would benefit from more server memory in most cases; as mentioned
earlier on, (depending on how much RAM you have available), you may try
increasing MAXMEM to 2048K; if performance does improve, you may try
increasing it again to 3072K to see if you get even better results. 3
megabytes should be adequate. It is important to remember that you need to
have the RAM available before increasing the figure in the MAXMEM field.
THE 32-BIT VIRTUAL MEMORY FEATURE
The second virtual memory system is a 32-bit VM intended for use with large
applications built using the 32-bit DOS Extender. This VM is configured by
setting options in the \DVX\DOS4GX.CFG file. Quarterdeck ships no applications
in the current release that use this VM.
************************************************************************
* Trademarks are property of their respective owners. *
*This technical note may be copied and distributed freely as long as it*
*is distributed in its entirety and it is not distributed for profit. *
* Copyright (C) 1993 by Quarterdeck Office Systems *
************************ E N D O F F I L E *************************