**** NOTE: THIS IS AN EXTREMELY LARGE DOCUMENT ****
DESCRIPTION
This document contains the standard DOS settings in an OS/2 Virtual DOS
Machine (VDM).
Depending on hardware configuration, and software level, some of these
settings might not exist.
RESOLUTION
AUDIO_ADAPTER_SHARING
Function: Allows access to audio hardware for the DOS session.
Two applications cannot use an audio adapter even if
one is not required to run the program. This will
allow you to minimize conflicts by defining audio
specifications for each DOS session.
Select Optional to indicate that a program in this
DOS session should use an audio adapter if one is
available.
Select Required to indicate that a program in this
DOS session must have access to an audio adapter.
Select None to indicate that a program in this DOS
session does not require an audio adapter.
Default: None.
Settable: At any time.
COM_DIRECT_ACCESS
Function: When set on, VCOM.SYS allows direct access to the COM
ports.
Advantages: Programs that need direct access, such as
AS/400 Asynch Router, FastLynx, FSDUAT, and MS Word
will now work.
Drawbacks: Buffers in COM.SYS cannot be used. Characters might
be lost, and some applications might suffer from the
lack of buffering.
Default: Off.
Settable: At VDM creation only.
COM_HOLD
Function: When set on, provides exclusive access to COM
ports for the specified VDM, preventing other
processes from using the port and preventing the
operating system from releasing the port until the
VDM terminates.
Advantages: For certain applications that use COM ports and
require multiple programs to access the COM port
(for example, this setting prevents the COM port
from being released when the first program ends).
Drawbacks: If not required by the application running in a
VDM, this setting might prevent their applications
from accessing COM ports.
Default: Off.
Settable: At VDM creation only.
Examples: Certain bulletin board applications use one
program to dial the BBS and another to exchange
information; setting COM_HOLD to on prevents the
operating system from releasing the COM port when
the first program ends.
COM_RECEIVE_BUFFER_FLUSH
Function: Allows control of the received data buffers when the
DOS session is switched to the foreground, or when
the DOS program enables the received data interrupt.
Select Receive Data Interrupt Enable to indicate
that, for this DOS session, the operating system
is to discard data in the received data buffer when
the DOS program enables the received data interrupt.
Select Switch to Foreground to indicate, for this
DOS session, the operating system is to discard data
in the received data buffer when the DOS program is
switched to the foreground.
Select ALL to indicate that communications data is
to be discarded when a DOS program enables the
received data interrupt or the program is switched
to the foreground.
Select NONE to indicate, for this DOS session, the
operating system is to keep data in the received
data buffer.
Default: None.
Settable: At VDM creation only.
COM_SELECT
Function: When set, allows a program to select and use one
communication port.
Advantages: You can limit your program to just the COM port
that it requires; some programs, such as like
LapLink Pro, try to take over every available COM
port. Communications that are not selected are
hidden from the program.
Drawbacks: Some programs need to have access to the COM ports
to work, even if they are not using them.
Default: All (Full access to the COM ports.)
Settable: At VDM creation only.
DOS_AUTOEXEC
Function: To specify a different batch file other than the
default AUTOEXEC.BAT.
Advantages: Different batch files could be specified for
different DOS programs.
Default: AUTOEXEC.BAT in root directory.
Example: A new batch file that contains some SET statements
relative to an individual program, can be kept
separately.
Settable: At VDM creation only.
DOS_BACKGROUND_EXECUTION
Function: When set off, suspends execution of the program
when it is in the background.
Advantages: Many DOS applications are written on the assumption
that they are single-tasking and all the resources
of the work station can be monopolized. It is not
uncommon for a program to continually poll for
keyboard input (examples are WordPerfect 5.1 and
Lotus 1-2-3 R2.2). In a multitasking environment,
this can impact system performance, especially when
more than one such program is running. Turning the
DOS application off when its VDM is in the
background reduces its demands on the system. See
also IDLE_SENSITIVITY and IDLE_SECONDS.
Drawbacks: Communications programs fail if background execution
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(FCBs) that can be opened by applications running in
the VDM. Note that this setting affects only those
modules that use file-sharing.
Advantages: Reducing this setting might improve DOS application
performance in a resource-constrained networking
environment. When the maximum number of FCBs is
opened by an application, the least recently used
FCB is closed to allow additional files to be
opened; see DOS_FCBS_KEEP below.
Drawbacks: Reducing this setting to an excessively low number
might inhibit the performance of applications that
use large numbers of files. Check application
documentation for recommended FCB settings.
Default: 16.
Settable: At VDM creation only.
Examples: None.
DOS_FCBS_KEEP
Function: Specifies the number of file control blocks (FCBs)
protected against automatic closure.
Advantages: If this setting is specified as "n", the first "n"
files are protected against automatic closure, as
described in DOS_FCBS. This might improve application
performance.
Default: 8.
Settable: At VDM creation only.
Examples: None.
DOS_FILES
Function: Specifies the maximum number of file handles that
can be opened in a VDM.
Advantages: Setting this value higher than the default might
improve performance for applications that use a
large number of files. Check application
documentation for recommended settings.
Drawbacks: Setting the number of file handles higher than
necessary reduces the available memory.
Default: 20.
Settable At any time.
Examples: DBASE IV requires a DOS_FILES setting of at least
40.
DOS_HIGH
Function: Determines whether DOS is loaded outside the 640 KB
low-memory address space.
Advantages: Loading DOS into high memory allows more available
memory for application code and data within the
640 KB address space.
Drawbacks: Applications that require access to DOS internal
control structures require DOS to be loaded into
low memory, and so cannot use this setting.
Default: Off (DOS is loaded into low memory).
Settable: At VDM creation only.
Examples: None.
DOS_LASTDRIVE
Function: Specifies the highest available logical drive
letter for the specified VDM. This setting is
similar to the LASTDRIVE statement in a DOS
CONFIG.SYS file.
Default: Z.
Settable: At VDM creation only.
Examples: Each additional drive letter uses about 100 bytes.
Setting the LAST_DRIVE to a lower letter, such as
"J" or "K," provides more conventional memory for
an application.
DOS_RMSIZE
Function: Specifies the DOS memory size. This is the amount of
memory available to DOS applications.
Advantages: The virtual video device driver uses this setting on
certain video adapters to set even more than 640 KB.
Drawbacks: This setting is of little use to most users, as there
is no point specifying less than 640 KB.
Default: The default is 640 KB.
Settable: At VDM creation only.
Examples: None.
DOS_SHELL
Function: To specify the DOS command processor, or to add
parameters to affect the command processor. This
points, by default, to COMMAND.COM. If you have a
different command processor, it should be specified
here.
Advantages: You can specify a command processor other than the
default COMMAND.COM, if required by a specialized
application, or you can alter the environment space
available for the VDM.
Default: C:\OS2\MDOS\COMMAND.COM C:\OS2\MDOS /P
Settable: At VDM creation only.
DOS_STARTUP_DRIVE
Function: Specifies the location of the DOS kernel to be
loaded into the VDM.
Advantages: Allows specific versions of DOS to be loaded into
a VDM using the VMB facility, allowing the
execution of version-dependent DOS applications.
Drawbacks: Performance might not be as good as the VDM kernel
that is optimized for the OS/2 environment.
Default: The DOS Emulation kernel is loaded.
Settable: At VDM creation only.
Examples: See Chapter 12, "Virtual Machine Boot."
DOS_UMB
Function: Specifies whether DOS owns Upper Memory Blocks
(UMBs), and manages the loading of device drivers
and TSR programs.
Advantages: Setting DOS_UMB to on allows use of the DEVICEHIGH
and LOADHIGH statements to load device drivers and
TSR programs into UMBs, thereby preserving space in
low memory for use by applications.
Drawbacks: Certain applications that make use of UMBs need to
access and manage the UMBs directly; such
applications do not run when DOS_UMB is set to on,
because DOS owns the UMBs.
Default: Off (UMBs are owned by certain types of TSR programs
and DOS device drivers if necessary).
Settable: At VDM creation only.
Examples: None.
DOS_VERSION
Function: Allows the operating system to report a "fake"
DOS version number in response to a request from
a program in the VDM, to support applications
that check for a DOS version number.
Advantages: Allows some programs that do not start unless they
detect a prerequisite DOS version to run in DOS
Emulation.
Default: 20.
Settable: Before application initiation.
Examples: Lotus 1-2-3 R3+ runs in DOS emulation if it is
"fooled" into thinking that it is running under DOS
3.3 by putting the following lines into the
DOS_Version list box:
123DOS.EXE,3,30,255
123.EXE,3,30,255
LOTUS.EXE,3,30,255
DPMI_DOS_API
Function: Determines if DOS API translation is enabled for
the specified VDM.
Default: AUTO (API translation is enabled if required).
Settable: At VDM creation only.
Examples: None.
DPMI_MEMORY_LIMIT
Function: Specifies the maximum amount of protected mode
memory (in MB) available to DPMI applications
running in the VDM.
Advantages: For applications that require large amounts of DPMI
memory, this setting can be used to increase the
amount of available memory up to 512 MB.
Default: 2 MB.
Settable: At VDM creation only.
Examples: None.
DPMI_NETWORK_BUFF_SIZE
Function: Specifies the size, in KB, of the network translation
buffer for DPMI programs in this session. The range
is from 1 to 64 KB.
Default: 8 KB.
Settable: At VDM creation only.
Examples: This setting allows you to configure the size of the
translation buffer for Windows programs that transfer
data over a network. If a network-specific Windows
program does not run correctly under OS/2 2.x,
increase this setting; then restart the session.
EMS_FRAME_LOCATION
Function: This DOS setting allows you to change the location of
the LIM EMS region. LIM EMS uses a 64 KB address
region known as an EMS page frame, through which,
programs can access expanded memory. (This allows
programs to use more than 640 KB of memory.)
Advantages: If you have problems running a program that uses
both a hardware device and LIM EMS expanded memory,
the problem might be due to conflicting use of
addresses by LIM EMS and the hardware device. If
this occurs, first use the EMS_HIGH_OS_MAP_REGION
setting to set the extra address region used by EMS
to 0. This might solve the problem. If not, the
EMS_FRAME_LOCATION setting can be used to select
the 64 KB region that does not conflict with
hardware.
You can choose where to place the frame from a list
of choices, or can choose to have no EMS frame for
programs that do not require one. You also can
reduce the DOS Memory Size setting and place the
frame below 640 KB.
Drawbacks: The best solution, when problems are due to hardware
conflicts, is to use the MEM_EXCLUDE_REGIONS and
MEM_INCLUDE_REGIONS settings to specify the
addresses that the hardware uses, rather than using
this setting.
Default: The default AUTO setting leads to correct choices of
LIM EMS addresses. You might never need to change
this setting.
Settable: At VDM creation time only.
Examples: In some cases the default choice might conflict with
addresses used by hardware on the system. This can
happen only for devices that are not supported by a
virtual device driver.
EMS_HIGH_OS_MAP_REGION
Function: In addition to the EMS page frame, some programs can
use additional addresses to access expanded memory.
This setting gives advanced users the capability to
adjust the size of the additional EMS region. See
also EMS_FRAME_LOCATION.
Advantages: An advanced user can use the MEM_EXCLUDE_REGIONS
and MEM_INCLUDE_REGIONS settings to specify the
addresses used by devices that do not have virtual
device drivers, and can then set the size of the
EMS_HIGH_OS_MAP_REGION appropriately for their
program. This helps to avoid conflicts with
addresses used by devices and programs.
Default: The value set is the size of the region in KB.
The default is 32 KB.
Settable: At VDM creation only.
Examples: None.
EMS_LOW_OS_MAP_REGION
Function: Some programs can use remappable conventional
memory. Others do not use this feature. This
setting allows advanced users to set the size of the
remappable conventional memory available in a VDM.
Default: The value set is the size of the region in KB.
The default is 384 KB.
Settable: At VDM creation only.
Examples: None.
EMS_MEMORY_LIMIT
Function: This setting controls the amount of EMS memory
available to a VDM.
Advantages: You can set this to a higher value for running
programs that require a large amount of EMS memory.
Other programs do not use EMS at all. The size can
be set to 0 in such cases, to disable EMS support
for that VDM. Programs generally state if they use
EMS on the box or in their documentation.
Default: The value set is the size of the region in KB.
The default size is 2 MB.
Settable: At VDM creation time only.
Examples: If a spreadsheet runs out of memory, the amount of EMS
memory can be increased, and the VDM can be restarted.
HW_NOSOUND
Function: Enables or disables sound started by a DOS program.
Advantage: Any sound from a program is heard unless sound is
disabled. An "x" in the check box indicates that the
sound should be heard.
Drawbacks: No error sound is heard if HW_NOSOUND is turned on.
Default: Off.
Settable: At any time, including while a program is running
in a VDM.
Examples: Output from a music program may be disabled when you
want to hear another music program, or switch to
another session.
HW_ROM_TO_RAM
Function: Enabling HW_ROM_TO_RAM causes the operating system to
copy read-only memory (ROM) and run the copy in 32-bit
random access memory (RAM). With this setting enabled,
BIOS operations run faster and system utilities can
patch BIOS.
Default: Off.
Settable: At VDM creation only.
Examples: This setting is useful if debugging the kernel. The
change allows normal breakpoints to be set in ROM and
allows stepping over calls and loops.
Attention: If an application writes to a memory address used by
the ROM while this setting is enabled, it might cause
unpredictable results for that application and for
every application run thereafter in the VDM.
HW_TIMER
Function: When enabled, allows an application to have direct
access to the 8253 timer ports, and prevents the
operating system from trapping, or intercepting, the
timer request and emulating a timer.
Advantages: Certain timing-critical applications do not run (or
run much slower) if accesses to timer ports are
trapped and virtualized. In addition, the values they
read do not accurately reflect the amount of time
passed, because they do not take trapping overhead
into account. Enabling this setting allows certain
timing-dependent code to run more effectively.
Drawbacks: Applications that change the divisor before this
setting is enabled, and then read the timer ports
after the setting has been enabled, might not
function properly. If the setting is enabled first,
the VDM does not detect changes to the divisor
correctly, and the simulated interrupt frequency
will be incorrect. Also, multiple applications using
this setting might interfere with one another.
Default: Off. Most applications operate normally with timer
virtualization.
Settable: At any time. It is useful to change this setting
dynamically and watch for changes in application
performance.
Examples: The ROM on some systems implement very brief delays
by polling the timer ports. These delays become
unacceptably long unless direct timer port access
is allowed.
IDLE_SECONDS
Function: When programs appear to be doing nothing but waiting
for input, the operating system gives them less time
to run. This is done to give preference to programs
that are doing useful work.
Some programs periodically appear to be waiting for
input, but then change their behavior and continue
after a time. This setting disables the
IDLE_SENSITIVITY function for a period of time after
useful work has been detected. See also
IDLE_SENSITIVITY below for more details on idle
detection.
Advantages: If a program appears to run slowly when there is an
option for the user to provide input, this value
should be increased.
Drawbacks: Setting the value too high gives the DOS program more
resources than it needs.
Default: This value is in seconds. The default is no idle time
allowed.
Settable: The setting can be changed while the program is
running to tune it to the proper value.
Examples: A game might pause, for example, to wait for the user
to make a choice, but then continues if the user does
not react.
When DOS 5 is run in a VMB session, the DOS shell
might fail to complete displaying the directory of
drive C if IDLE_SENSITIVITY is set too low.
IDLE_SECONDS should then be raised.
IDLE_SENSITIVITY
Function: The idle sensitivity level sets a threshold for
judging when applications are considered idle. The
value is the percentage of the maximum possible
polling rate the application can perform. If an
application polls at a rate higher than this value,
it is considered "idle."
DOS programs often "poll" for input when they are
waiting for a user response. For example, a program
might wait for a response by repeatedly checking to
see if the user has pressed a key. In a multitasking
environment such as OS/2 2.x, this wastes time
when other programs could be running instead. The
operating system detects idle programs by looking
for a high rate of polling for input. When programs
are judged to be waiting for input, they are given
less time to run.
For example, if idle sensitivity is set to 75%, an
application repeatedly checking to see if input is
available would have to do this checking at more than
75% of the maximum possible rate before it would be
judged idle. Idle detection is a "best guess" of what
the program is doing. It could be that the program is
polling at a very high rate, but is still doing useful
work in between checking. It might be that the
application checks at a fairly slow rate but still is
doing nothing but waiting. The idle sensitivity
threshold allows adjustment of the threshold for a
particular application. Also see IDLE_SECONDS.
Advantages: If an application receives input while running, and
seems to run slower than expected, the idle
sensitivity should be set to a higher value. This
lets the application poll at a higher rate without
being judged idle. Setting the level to 100 turns
idle detection off altogether. The application will
be allowed to poll for input as often as it likes.
If an application is waiting for input and other
applications do not appear to be running, the idle
sensitivity should be adjusted downward. This lowers
the threshold for judging the application idle.
Default: The default is 75%.
Settable: The setting can be changed while the program is
running to tune it to the proper value.
Examples: Overall system performance can usually be improved
when there are multiple DOS applications running,
if IDLE_SENSITIVITY is turned down. See also
DOS_BACKGROUND_EXECUTION.
*INT_DURING_IO
Function: When set to on, this creates a second thread for the
application to use for interrupt handling when the
primary thread is busy with I/O operations.
Advantage: Very useful in multimedia applications, MSCDEX
applications, and many games.
Drawbacks: Creates extra overhead on the system for
processing and memory requirements, and can cause
degradation of performance for other applications.
Default: Off.
Settable: At VDM creation only.
Example: Unless your application is interrupt-sensitive,
leave this setting Off.
KBD_ALTHOME_BYPASS
Function: When enabled, prevents the Alt+Home key sequence
from switching the VDM between full screen and
window mode.
Advantages: Enabling this setting allows normal behavior for
applications, which themselves make use of the
Alt+Home key sequence.
Drawbacks: When enabled, you must use the Ctrl+Esc sequence
to switch to Presentation Manager from a
full-screen VDM, and then use the context menu of
the class to switch the VDM to window mode.
Default: Off (Alt+Home causes a switch between full-screen
and window mode).
Settable: At any time.
KBD_BUFFER_EXTEND
Function: Increases a VDM's keyboard type-ahead buffer size.
