Category : Alternate Operating Systems - Quarterdeck DesqView, CP/M, etc
Archive   : QWHITE13.ZIP
Filename : VIRTMEM.TEC

 
Output of file : VIRTMEM.TEC contained in archive : QWHITE13.ZIP

ID:VM DESQview/X: Virtual Memory
Quarterdeck Technical Note #130 Filename: VIRTMEM.TEC
by Susan Easmon CompuServe: VRTMEM.TEC
Last revised: 2/23/93 Category: DVX

Subject: Explains the concept of "virtual memory" and how it is used with
DESQview/X.


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.


HOW DOES DVX USE VIRTUAL MEMORY FOR ITS DISPLAY SERVERS?

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 starts using 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.

Since the 256-colour server tends to use quite a bit more RAM than the
16-colour server, it will more likely benefit from more server memory. In
any case, regardless of which server is being used, it will become obvious
that virtual memory is being used when system performance degrades ie. the
hard disk light starts to flash continuously. As mentioned earlier on,
depending on how much RAM you have available, you may try increasing MAXMEM
to 2000; if performance does improve, you may try increasing it again to 3000
to see if you get even better results. 3000 should be adequate in all but the
most unusual circumstances.

It is IMPORTANT to remember that you should actually have the physical RAM
available before increasing the figure in the MAXMEM field and you also need
to make sure that you do have some expanded memory left over if you do run
other DOS applications, in other words do NOT allocate all the expanded
memory to the server, because it will use it all IF and WHEN the need arises
and that will basically leave you with no memory for running DOS
applications.


THE 32-BIT VIRTUAL MEMORY FEATURE

The second virtual memory system in DESQview/X 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.

The default for the 32-bit VM is as follows:


[VMM]

on = n - Enables VMM. The default is off (n).
minmem = 64 - The maximum amount of RAM managed by the VMM.
maxmem = 8192 - The maximum amount of RAM managed by the VMM.
virtualsize = 8192 - Maximum virtual memory
swapmin = 2048 - The size of swapfile at startup
swapinc = 1024 - The increment by which a swapfile grows

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* 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 *
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