ID:V1 Virtualization: What is it?
Quarterdeck Technical Note #201 Filename: VIRTUALI.TEC
by Michael Bolton and Eric Wieling CompuServe: VIRTIZ.TEC
Last revised: 1/22/92 Category: DV
Subject: This note discusses what screen virtualization is, why it is needed,
and how to use it. This information is important if you wish to run
graphical programs in background under DESQview 386.
Q. What is virtualizing?
Q. What does "virtualize text and graphics" mean?
Q. What does "writes directly to screen" mean?
Q. How can I tell if my program writes directly to screen?
Q. I've set "Runs in background" to Y. Why isn't my program running in
Q. What is "virtualizing"?
Programs can handle display in three ways in DESQview parlance. These are
explained in more detail below, but briefly,
1) the "well-behaved" application uses DOS or your system's BIOS services to
display its information;
2) the "DESQview-aware" program writes directly to the screen when not running
under DESQview, but writes to a special area of memory (under DESQview's
control) so that DESQview may properly manage display of multiple windows.
3) the "misbehaved" application writes directly to the screen hardware with no
consideration for DESQview.
Misbehaved applications present a special problem for most multitasking
environments, in that direct screen-writing can "bleed through" into
foreground windows. However, on a 386 processor, DESQview 386 (which is
simply DESQview and QEMM running on the same computer) can "virtualize"
misbehaved applications; that is, it can fool these applications into
believing that they have exclusive control of the screen. Thus, such
applications may be run in background and/or in a small window without
interfering with the display of other applications.
For the purposes of this discussion, the 80386SX, the 80386 (also known as the
80386DX), the i486, and the i486SX are equivalent; all provide the memory-
management features of the 386; henceforth, the term "386" refers to any
processor in this family.
Unless you are working on a 386, AND using both DESQview and QEMM-386 (which
together comprise DESQview 386), virtualizing means little to you; anything
less than a 386 is not capable of virtualizing the screen, and only DESQview
386 provides the memory management services that allow this trick to be
performed. This is another of the many compelling reasons to move to a 386
processor if you have not already done so.
From DESQview's perspective, there are two basic types of programs, as far as
display is concerned. The first type of program, the "well-behaved" program
in DESQview parlance, uses DOS or the system's Basic Input/Output Services
(BIOS) to put text on the screen. The BIOS gets instructions from the
program, and then puts values into screen memory addresses; these values are
then translated by the display adapter to show up as the characters that you
see on your monitor. "Well-behaved" applications, then, use resources built
into the system to display information. To run such programs in a small
window or in background, DESQview (without QEMM's help, and on any processor)
intercepts DOS and BIOS calls and places the results into a "virtual screen."
This is an area of memory (or "buffer") the same size as screen memory, which,
to the application, looks and feels exactly like the real screen. The
application therefore behaves as it would if it had the computer all to
itself. The "well-behaved" program believes that display work has been done,
and continues about its business without complaints.
Hercules, CGA, EGA, or VGA graphical applications (this includes all programs
with graphical user interfaces) normally do not use the BIOS for display.
Because of the greater complexity involved in displaying graphics, such
programs do not have the option of writing directly to a DESQview memory
buffer, nor are there BIOS services efficient enough to allow such programs to
run at a satisfactory speed. These applications are therefore, by definition,
NOT well-behaved and are called (predictably) "misbehaved" in DESQview
parlance. This type of program avoids DOS and the BIOS, and puts text or
graphics on the screen itself by writing values directly into screen memory
For text-based applications, writing directly to the screen hardware is
considerably faster than using BIOS calls, so many text applications write
directly to in the interests of speed. Graphics-oriented BIOS functions are
limited in power and flexibility, and in any case are unacceptably slow, so
graphical applications write directly to the screen.
The DESQview-aware program checks for the presence of DESQview (or TopView, an
IBM environment with which DESQview is compatible) and asks for the address of
the memory buffer that DESQview will use to store the program's video
information. The program then writes directly to DESQview's memory buffer
instead of to the video hardware. This method produces the same well-governed
results as writing through DOS or the BIOS, and is considerably faster. This
type of application is called "DESQview-aware."
Writing directly to screen memory addresses eliminates the middleman, but the
catch is that direct-to-screen display information cannot be intercepted and
redirected as easily as BIOS calls. Under anything other than DESQview-386,
misbehaved programs may interfere with the display of others. Misbehaved
programs interact directly with the display hardware, and will write outside
the borders of their DESQview windows; if allowed to run in background or in a
small window, these programs will "leak" or "bleed" into the display windowsnd
of other applications. The alternative is to suspend the operation of theay
misbehaved program while it is in background, which is DESQview's defaultf
method of managing such things. This is quite satisfactory for those who want
to use DESQview as a task-switching environment, but not for those who desire
Happily, the 386 processor, QEMM-386, and DESQview, all working together, can
trap the direct screen writes of misbehaved text or graphics programs and
redirect the output of these applications to DESQview's virtual screen
buffers. As far as the application knows, it is writing to the screen. This
process is called "virtualizing." The advantage of virtualizing is that,
under DESQview 386, misbehaved text programs and even programs using Hercules,
CGA, EGA, or VGA graphics can continue run in background or in a small window,
without causing display conflicts.
Q. What is the difference between "virtualizing text" and "virtualizing text
First, memory usage: a memory buffer for a virtualized text screen only takes
up as much memory (in bytes) as there are places to display a character on the
screen. That works out to 80 (columns) X 25 (rows) X 2 bytes per character
(one byte for the character, and the other for its display attribute -- color,
highlighting, or underlining), for a total of 4000 bytes. Creating a virtual
screen to hold text-based data costs relatively little memory. DESQview uses
expanded memory to virtualize video output, and expanded memory can only be
allocated in 16K chunks (called "pages"). Consequently 16K (rather than 4000
bytes) of expanded memory is needed to virtualize a text program.
Virtualizing a graphics program requires as much as 272K of available expanded
memory, depending on the graphics mode that the program is using. Check your
DESQview manual for entries indexed under "graphics pages" for more detailed
Second, there is a difference in the use of the processor's time. A
virtualized text program is approximately as fast as a nonvirtualized text
program, but virtualizing graphics takes a heavier toll on the processor.
(Incidentally, when you are running a virtualized program full-screen and in
foreground, DESQview temporarily suspends virtualization and lets the program
write to the real video memory region; in this circumstance, there is little
extra processor overhead.)
Third, it is worth noting that under DESQview 386, even protected mode
programs, also known as DOS-extended programs, can be multitasked. This is
thanks to a memory-management specification co-authored by Quarterdeck called
the Virtual Control Program Interface (VCPI). It is possible in most
circumstances to virtualize the text output of a program that runs in
protected mode, because most protected mode programs actually switch into real
mode to write text to the screen. However, programs that write graphics
directly to the screen while in protected mode cannot be virtualized, so
DESQview 386 will by default halt the operation of such programs when they are
not using the full screen. A newer specification for multitasking, the DOS
Protected Mode Interface, will allow virtualization of graphical protected-
mode programs; future versions of DESQview and QEMM are expected to support
this specification. If it is vital that a protected mode graphics program
continue to run in background, it will not be halted if Runs in Background (on
the second page of the Change a Program Menu) is set to Y. The program will
continue to write its graphics to the screen, which may interfere with the
display of foreground programs, but the ability to run the program in
background may be worth the inconvenience.
Q. How can I tell if my program is writing directly to screen?
To tell if an application is writing directly to the video hardware inside
DESQview, make the following changes in the application's Change a Program
1) Set "Writes text directly to screen" to N;
2) Set "Virtualize text/graphics" to N;
3) On the Advanced Options screen, blank out the following four fields
in the "Window Position" section: Starting Height, Starting Width,
Starting Row, and Starting Column. Put blanks in these fields, not
After these changes have been made, open the program. DESQview will place a
small window border on the screen; if the program comes up and stays within
the small window border, it does not write directly to the screen. If the
program demolishes the window border, takes the full screen, or writes
anywhere outside the window, it is writing directly to the hardware. Such a
program should be virtualized if it is to be run in background or in a small
Q. Why doesn't my program STAY in background? It bleeds through on to the
screen when it's background; even though I've set "Runs in background" to Y,
the information from this program keeps showing up in the foreground window.
If it is an application that writes text or graphics directly to screen, one
reason might be that it is not set up to virtualize. You can amend this by:
1) selecting Change a Program from the DESQview Open Window Menu,
2) selecting the program that you want to change, and
3) on page one of the Change a Program Menu, setting "Virtualize text
and graphics" to Y (for both text and graphics) or T (for text only).
Q. When should I virtualize both text and graphics?
If an application does work which you want to continue while the program is in
background or in a small window, and it is a program that does this work while
displaying graphics, set "Virtualize text and graphics" to Y.
Q. When should I virtualize text only?
1) If you have an application which writes text directly to the screen, but
never uses graphics, set Virtualize to T. Misbehaved text-based
applications are good candidates for this treatment.
2) If you have an application which writes both text and graphics directly to
the screen, but which does not need to run in background while graphics are
being displayed, set Virtualize to T. A good example of this type of
program is a word processor or a spreadsheet which has a graphical element
to it (a charting or print preview module, for example) that you never need
to run in background or in a small window. Even if you're not virtualizing
this type of program, you can still place your chart or print preview in a
small window -- the program simply halts, while retaining the information on
screen. You can still use DESQview's screen management to see all the parts
of the screen you like, but the program's operation is suspended while it's
in the small window. Recalculating and printing, since they usually take
place while the screen is in text mode, can typically continue to run in
background in such programs, since the text mode of the program is being
3) If you have an application which writes text in real mode, but writes
graphics to the screen while in protected mode, again set "Virtualize text
and graphics" to T. Do this because the program can still be virtualized
while it's in text mode, even though it can't be virtualized while it's
Q. I've done all these things and my program STILL won't run in background.
There are a few possibilities.
* You may have set "Virtualize Text and Graphics" on Page 1 of the program's
Change a Program Menu to N.
Solution: set Virtualize Text and Graphics to Y
* You may simply be out of memory; when DESQview virtualizes an application's
graphics, it can require up to 272K of free expanded memory.
Solution: make sure that you have enough memory available for virtualizing.
If you have several applications open, close down one or more. Make sure
that you have not allocated a large number of graphics pages unnecessarily;
if the number of graphics pages is greater than one, try decreasing it. If
you have a disk cache that "lends" memory to applications, try disabling
this feature and reducing the size of the cache. Finally, you might also
want to consider upgrading your system by adding more memory.
* DESQview 386 cannot virtualize if you have set the Expanded Memory Page
Frame to 0 with QEMM's FRAME=NONE or FRAMELENGTH=0 parameters.
Solution: make sure neither of the above parameters is on the QEMM line in
* Although DESQview can save and restore all standard IBM VGA video modes,
there are a few non-standard VGA modes, and Super VGA modes, that it can't
virtualize. This is because Super VGA implementations vary from card to
Solution: try using a lower resolution for the program in question. Text
mode, sometimes known as "CGA mode" for some programs, is a good place to
start. "CGA mode" is something of a misnomer, since you'll actually still get
your normal resolution on an EGA or VGA -- the program just writes to the
screen without being in graphics mode. If your program uses a very high
resolution graphics mode, (e.g. 1024 x 768), or a very high number of
colours (e.g. 256 colour mode), try using lower resolutions or lower numbers
of colours until you are successful.
* To virtualize graphics, you may need more graphics pages.
Solution: on the Advanced Options screen of the Change a Program Menu, try
setting Graphics Pages to a higher number, such as 2 or 4. Keep in mind
that each graphics page can take up to 128K.
* You may not have enough Real Alternate Maps to virtualize a large number of
Solution: QEMM must provide DESQview with a Real Alternate Map for each
virtualized window. Since QEMM's default number of Real Alternate Maps is
8, if you are using more than 8 windows, you'll be unable to virtualize
window 9. Close down an application or two, or specify MAPS=n on the QEMM
line in CONFIG.SYS, where n is a number larger than 8. Numbers larger than
20 are unlikely to be helpful. Each alternate map uses 4k of expanded
* Programs that start as part of a DESQview startup script, and are put into
background by the script, may not virtualize if they change video modes;
when the video mode changes, they will be suspended.
Solution: for this type of program, make sure that "Runs in Background" is
set to Y, and is not left blank.
* A program may be grabbing a hardware interrupt, and may write directly to
screen from inside its hardware interrupt handler. This most often results
in a trail of oddly-coloured blocks, left behind on the screen when the
mouse is moved (mouse droppings).
Solution: Consult Quarterdeck Technical Note #238, "'Mouse Droppings' in
DESQview" (MOUSDROP.TEC) for an explanation and a solution. The file
should be available from the same source as this one; it is available on the
Quarterdeck BBS at (310) 314-3227, Compuserve (!GO QUARTERDECK), or large
local BBS systems. It is also available through our QFAX fax
retrieval service at (310) 314-3214; call from the handset on your fax
Finally, it should be noted that DESQview 386 is presently the only DOS-based
program in existence that will multitask and virtualize misbehaved DOS
applications which use EGA and VGA graphics. Virtualization is one of the
principal ways by which DESQview can multitask these applications;
understanding the above ideas about virtualization can help you to take the
greatest advantage of DESQview's power.
*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) 1990-2 by Quarterdeck Office Systems *
************************ E N D O F F I L E *************************