DOS Environment
The following settings affect the behavior of the DOS emulation environment
within a virtual DOS machine.
DOS_BACKGROUND_EXECUTION
Function:
When set off, suspends execution
of the program when it is in the background.
Advantages:
Drawbacks:
Communications
programs will fail if background execution is turned off, as will DDE for
Windows applications.
Try changing the values of IDLE_SECONDS and IDLE_SENSITIVITY before
turning DOS_BACKGROUND_EXECUTION off.
Default:
On (Background execution is enabled).
Settable:
Examples:
If more than two DOS programs are running and
tuning with IDLE_SENSITIVITY and IDLE_SECONDS does not provide
sufficient improvement, turn DOS_BACKGROUND_EXECUTION off for the
least used application.
DOS_BREAK
Function:
Enables or disables Ctrl+Break
for the specified VDM. Also check for the BREAK statement in the
CONFIG.SYS. Set BREAK=ON in the CONFIG.SYS to make Ctrl+Break and
Ctrl+C working in addition to setting DOS_BREAK on.
Advantages:
Enables a DOS application running in the VDM
to be interrupted using the Ctrl+Break or Ctrl+C key sequences.
Drawbacks:
This setting is useful only if an application
must be quickly interrupted; the user may easily terminate a VDM by closing
it from the Window List.
Default:
Off
(Ctrl+Break is disabled).
Settable:
Examples:
If
the user wishes to have the option to interrupt a DOS batch file running
in a virtual DOS machine, this setting should be turned on.
DOS_DEVICE
Function:
This setting can be used to add
or modify information about DOS device drivers for the specified VDM, in
addition to the information specified in CONFIG.SYS.
Default:
When this setting is selected, a list is displayed
which contains information about each DOS device driver specified in CONFIG.SYS.
The information consists of the path and file name of each DOS device driver
and its current parameters, if applicable. For example:
c:\os2\mdos\ansi.sys
The user may:
Type the name
of a DOS device driver to add it. Typing should begin on a new line.
Delete all the information about a device driver
to remove it.
Type or delete to add, change, or delete a
value.
Settable:
Examples:
A
program to support hardware such as a scanner may include a device driver
that is needed only for itself. The device driver should be loaded with
the DOS_DEVICE setting instead of in the CONFIG.SYS.
DOS_FCBS
Function:
Specifies the maximum number of
file control blocks (FCBs) which may be opened by applications running in
the VDM. Note that this setting affects only those modules which use file-sharing.
Advantages:
Reducing
this setting may 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 may inhibit the performance of applications which use large numbers
of files. Check application documentation for recommended FCB settings.
Default:
Settable:
Examples:
DOS_FCBS_KEEP
Function:
Specifies the number of FCBs that
will be 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
above. This may improve application performance.
Default:
Settable:
Examples:
DOS_FILES
Function:
Specifies the maximum number of
file handles which may be opened in a VDM.
Advantages:
Setting this value higher than the default
may improve performance for applications which 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:
Settable:
Examples:
DBASE
IV requires a DOS_FILES setting of at least 40.
DOS_HIGH
Function:
Determines whether DOS is loaded
outside the 640KB low memory address space.
Advantages:
Loading DOS into high memory allows more available
memory for application code and data within the 640KB address space.
Drawbacks:
Applications which require access to DOS internal
control structures require DOS to be loaded into low memory, and therefore
cannot use this setting.
Default:
Off
(DOS is loaded into low memory).
Settable:
Examples:
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.
Default:
Settable:
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 which is available to DOS applications.
Advantages:
The virtual video device driver uses this setting
on certain video adapters to set even more than 640KB.
Drawbacks:
This setting is of little use to most users
as there is no point specifying less than 640KB.
Default:
Settable:
Examples:
DOS_SHELL
Function:
To specify the DOS command processor,
or to add parameters to affect the command processor. This setting points
by default to COMMAND.COM. If a user has a different command processor,
it should be specified here.
Advantages:
The
user may specify a command processor other than the default COMMAND.COM,
if required by a specialized application, or may alter the environment space
available for the VDM.
Default:
C:\OS2\MDOS\COMMAND.COM
C:\OS2\MDOS /P
Settable:
Examples:
C:\OS2\MDOS\COMMAND.COM
/E:1024 /P
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
may not be as good as the VDM kernel, which is optimized for the OS/2 V2.0
environment.
Default:
The
DOS Emulation kernel is loaded.
Settable:
Examples:
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 on allows use of the DEVICEHIGH= and LOADHIGH statements, to load
device drivers and TSR programs into Upper Memory Blocks, thereby preserving
space in low memory for use by applications.
Drawbacks:
Certain applications which make use of UMBs
need to access and manage the UMBs directly; such applications will not
run when DOS_UMB is set on, because DOS owns the UMBs.
Default:
Off (UMBs are owned by certain types of TSR
programs and DOS device drivers if necessary).
Settable
Examples:
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, in order to support applications which check for a DOS version
number.
Advantages:
Allows
some programs that will not start unless they detect a prerequisite DOS
version to run in DOS Emulation
Default:
Settable:
Before
application initiation.
Examples:
Lotus
1-2-3 R3+ will run 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: