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┌───────────────────────────────────────────────────────────────────────────┐ │ OS/2 2.0 Technical Tips │ │ From the │ │ OS/2 Technical Support Team │ │ Boca Raton, Florida │ │ │ ├───────────────────┬─────────────────────────────────────┬─────────────────┤ │ File Name │ Description │ Number of pages │ ├───────────────────┼─────────────────────────────────────┼─────────────────┤ │ DOSSET.FAX │ OS/2 DOS Settings for VDMs │ 14 │ └───────────────────┴─────────────────────────────────────┴─────────────────┘ STANDARD DOS SETTINGS --------------------- The following is an excerpt from Chapter 11 of the IBM OS/2 Version 2.0 Technical Redbooks, Volume 2: DOS and Windows Environment (Document Number GG24-3731-00), published by the IBM Corporation. This document describes the DOS Settings in an OS/2 Virtual Dos Machine. 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 which use COM ports and which require multiple programs to access the COM port (for example, this setting prevents the COM port from being released when the first program terminates). Drawbacks: If not required by the application running in a VDM, this setting may prevent other 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 on prevents the operating system from releasing the COM port when the first program terminates. 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 that all the resources of the workstation can be monopolized. It is not uncommon for a DOS 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 virtual DOS machine is in the background reduces its demands on the system. Also see "IDLE_SENSITIVITY" in and "IDLE_SECONDS" 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: At any time. 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: At VDM creation only. 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. remove it. - Type or delete to add, change, or delete a value. Settable: At VDM creation only. 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: 16. Settable: At VDM creation only. Examples: None. 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" in topic 11.2.2.4. This may improve application performance. Default: 8. Settable: At VDM creation only. Examples: None. 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: 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 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: 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. 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 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: The default is 640KB. 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 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: 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 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: 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 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 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, 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: 20 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: - 123DOS.EXE,3,30,255 - 123.EXE,3,30,255 - LOTUS.EXE,3,30,255 DPMI_DOS_API Function: Determines whether 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 megabytes) available to DPMI applications running in the VDM. Advantages: For applications which require large amounts of DPMI memory, this setting may be used to increase the amount of available memory up to 512MB. Default: 2MB. Settable At VDM creation only. Examples: None. DPMI_NETWORK_BUFF_SIZE Function: Specifies the size, in kilobytes (KB), of the network translation buffer for DPMI programs in this session. The range is from 1 to 64 KB. Default: 8KB. 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 V2.0, 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 64KB address region known as an EMS page frame, through which programs can access expanded memory. (This allows programs to use more than 640KB of memory.) Advantages: If a user has problems when running a program that uses both a hardware device and LIM EMS expanded memory, the problem may be due to conflicting use of addresses by LIM EMS and the hardware device. If this occurs, the user should first use the EMS_HIGH_OS_MAP_REGION setting to set the extra address region used by EMS to 0. This may solve the problem. If the problem persists, the EMS_FRAME_LOCATION setting can be used to select a 64KB region that does not conflict with hardware. The user can choose where to place the frame from a list of choices or can choose to have no EMS frame for programs which do not require a frame. The user can also reduce the DOS Memory Size setting and place the frame below 640KB. Drawbacks: The best solution, when problems due to hardware conflicts occur, 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 will lead to correct choices of LIM EMS addresses. Most users will never need to change this setting. Settable: At VDM creation time only. Examples: In some cases the default choice may conflict with addresses used by hardware on the machine. 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" in topic 11.2.4.1. 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 avoiding conflicts with addresses used by devices and programs. Default: The value set is the size of the region in kilobytes. The default is 32KB. 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 kilobytes. The default is 384KB. Settable: At VDM creation only. Examples: None. EMS_MEMORY_LIMIT Function: This setting controls the amount of EMS memory available to a VDM. Advantages: The user 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 whether they use EMS on the box or in their manuals. Default: The value set is the size of the region in kilobytes. The default size is 2MB. 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 restarted. HW_NOSOUND Function: Enables or disables sound started by a DOS program. Advantage: Any sound from a program is heard unless sounds is disabled. An "x" in the check box indicates that the sound is to be heard. Drawbacks: No error sound will be 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 the user wishes 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 may patch BIOS. Default: OFF. Settable: At VDM creation only. Examples: This setting is useful if debugging the kernel. The change would allow normal breakpoints to be set in ROM and allow stepping over calls and loops. Warning: If an application writes to a memory address used by the ROM while this setting is enabled, it may 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 will not run (or will 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 may not function properly. If the setting is enabled first, the VDM will not detect changes to the divisor correctly, and the simulated interrupt frequency will be incorrect. Also, multiple applications using this setting may interfere with one another. Default: Off. Most applications will operate normally with timer virtualization. Settable: At any time. It is useful to alter this setting dynamically and watch for changes in application performance. Examples: The ROMs on some machines 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. Also see "IDLE_SENSITIVITY" in topic 11.2.6.2 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 may pause, for instance, 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 virtual machine boot session, (See Chapter 12, "Virtual Machine Boot") the DOS shell may fail to complete displaying the directory of the C: drive 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 will be 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 instance, a program may wait for a response by repeatedly checking to see if the user has hit a key. In a multitasking environment such as OS/2 Version 2.0, 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%, then 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 may 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" in topic 11.2.6.1. 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. Also see "DOS_BACKGROUND_EXECUTION" in topic 11.2.2.1. KBD_ALTHOME_BYPASS Function: When enabled, prevents the Alt+Home key sequence from switching the VDM between full screen and windowed mode. Advantages: Enabling this setting allows normal behavior for applications which themselves make use of the Alt+Home key sequence. Drawbacks: When enabled, the user must use the Ctrl+Esc sequence to switch to Presentation Manager from a full screen VDM, then use the context menu of the class to switch the VDM to windowed mode. Default: Off (Alt+Home will cause a switch between full screen and windowed mode). Settable: At any time. KBD_BUFFER_EXTEND Function: Increases a VDM's keyboard type-ahead buffer size. Advantages: Provides greater keystroke buffering, consistent with the level available in VIO windows. Note that Ctrl-Break will flush the entire buffer, just as it does with the standard buffer. Drawbacks: Applications which bypass the ROM BIOS input buffer and/or INT 16h may not benefit from this feature. There is also a small amount of additional memory overhead for every VDM. Default: On. Most applications will benefit, and those that do not should not be adversely affected. Settable: At any time. This facilitates easy experimentation by the user in the (rare) event that a problem does arise. Examples: None. KBD_CTRL_BYPASS Function: When enabled, inhibits one or more control key sequences, allowing an application in the VDM to use these sequences for its own purposes. Advantages: Enabling this setting allows normal behavior for applications which make use of control key sequences normally used by OS/2 Version 2.0. Drawbacks: Enabling this setting may prevent certain operations from being performed with OS/2 Version 2.0 and the Workplace Shell. Default: NONE (All control key sequences behave in the normal manner). Settable: At any time. Examples: None. KBD_RATE_LOCK Function: Prevents a DOS application in a VDM from changing the system keyboard repeat rate. Advantages: Insulates machine from applications that modify the repeat rate in an uncontrolled or undesirable way. Drawbacks: Prevents the application's repeat rate from taking effect even when the application is the focus session. Default: Off. Most applications do not modify the repeat rate, and those that do are generally in accordance with the user's wishes. Settable: At any time. Examples: None. MEM_EXCLUDE_REGIONS Function: This setting is used to specify address ranges which should be protected from use by EMS/XMS and direct access by applications. This setting is intended for experienced users who understand the hardware. Advantages: This setting restricts the use of EMS/XMS on certain ranges in the region between RMSIZE and 1MB. It also protects these ranges from being touched by user applications by portraying ROM there. Drawbacks: Some hardware adapters stop functioning if their addresses are touched in random fashion. If these ranges are defined excessively, they will adversely impact the function and performance of EMS and XMS services. Default: By default, this setting is void. Each address is specified in hex and if there is no range specified, the length taken is a page (4KB). Settable: At VDM creation only. Examples: None. MEM_INCLUDE_REGIONS Function: Specify regions which should be made available to EMS/XMS. This setting is used to specify some address ranges between RMSIZE and 1MB for use by EMS and XMS. Advantages: If there is a hardware adapter in this range which the user knows is not going to be used by a particular VDM session, then the address range used by this adapter should be made available to EMS and XMS. This will improve the performance of EMS and XMS services. Only advanced users who know the addresses used by a card should use this setting. Default: By default, this setting is void. Settable: At VDM creation only. Examples: See discussion in "Expanded Memory (EMS) and Upper Memory (UMB)" in topic 6.2. MOUSE_EXCLUSIVE_ACCESS Function: This setting allows VDMs to run applications which maintain their own mouse pointers. Some DOS applications manage their own mouse positions and movements; in many cases, the application's values for mouse sensitivity and/or double speed threshold are different from those of Presentation Manager. As a result, a Presentation Manager mouse pointer may be outside the VDM window while the application pointer is somewhere in the window not receiving any mouse events. This means having two asynchronous mouse pointers on the screen. Advantages: The user forces the physical mouse driver to send its events directly to the virtual mouse driver without going through Presentation Manager. Only one mouse pointer appears when the particular VDM window has the focus. Default: OFF. Settable: At any time. However, this only marks the VDM window and does not actually activate the setting. In order to activate it, the user must press a mouse button within the VDM window. The Presentation Manager pointer disappears, leaving only the application pointer. In order to regain the Presentation Manager pointer, the user must press any of the hot-keys (Alt, Ctrl+Esc, Shift+Esc). Examples: WordPerfect 5.1 has its own block-shaped mouse pointer, which will appear together with the system mouse pointer when the window has the focus. Turning MOUSE_EXCLUSIVE_ACCESS on allows the user to remove the system mouse pointer when in WordPerfect. PRINT_TIMEOUT Function: Use this setting to adjust the amount of time, in seconds, that the OS/2 V2.0 print subsystem waits before forcing a print job to the printer. In DOS, information sent by a program for printing goes directly to a printer. However, the OS/2 V2.0 print subsystem assembles print information in a spool file. After a specified period of time, during which the spool file does not grow larger, OS/2 V2.0 print subsystem sends the information to the printer as a single print job. Advantage: There is no need to exit the DOS program before the print job is released by the OS/2 V2.0 print subsystem. This is useful for applications which do not explicitly close their print jobs. Default: 15 seconds, configurable from 0 to 3600 seconds (0 seconds is no timeout). Settable: At any time. Examples: A timeout of 1 or 2 seconds is sufficient for small print jobs, such as copying the contents of the screen. However, when printing large files, formatting documents, or running calculations, the value must be set high enough to allow all print results to reach the spooler before the time limit expires. If not, results go in two or more spool files instead of one, and the resulting output may be instead of one, and the resulting output may be unsatisfactory. VIDEO_FASTPASTE Function: Speeds up input from other sources than the keyboard. Advantages: Improves the speed of paste operations from the clipboard to a DOS application. Drawbacks: Does not work with all applications (in particular, some applications which monitor keyboard interrupts directly may experience errors). Default: Off. Settable: At any time. This facilitates easy experimentation by the user. Examples: Pasting into the DOS command prompt, or any application using DOS Console I/O functions, will generally work. However, the Microsoft Editor (M) and its successor, Programmer's Workbench (PWB), can fail when using fast pasting because they rebuffer keystrokes in an internal buffer, which can overflow. VIDEO_MODE_RESTRICTION Function: Extends the 640KB DOS address space by limiting video mode support. Advantages: For text-based or CGA graphics based applications, the video memory normally reserved just above 640KB for high-resolution graphics modes can be remapped to conventional memory, providing an additional 64KB (or 96KB, depending on graphics mode) for DOS applications, TSRs, and other programs. This is valuable for applications that do not take advantage of EMS or XMS memory extenders. Drawbacks: It is not possible to completely hide the fact that the video adapter is high-resolution graphics-capable; some applications may attempt to enable those modes and use the memory above 640KB as video memory, inadvertently corrupting application data. Care must therefore be taken when using this feature. Default: NONE. The complete list of settings is: - None - CGA modes only (adds 96KB) - MONO modes only (adds 64KB). Settable: At VDM creation only. Examples: None. VIDEO_ONDEMAND_MEMORY Function: Reduces swap space requirements for fullscreen VDMs. Advantages: Allows a full-screen VDM to run without pre-allocating a virtual video buffer for the worst-case video modes (high-resolution graphics modes). Using this setting does not prevent execution of graphics applications; it simply means that allocation of the buffer is delayed until it is needed. This can save a substantial amount of memory/swap space, which might be important under certain low-memory conditions. It also enables you to start a program quickly. Drawbacks: If allocation of a virtual video buffer for a full-screen VDM fails at the time the application changes video modes, the session must be frozen and switched back to the shell. Unless the user is able to free memory from another session, he may be unable to get the DOS application running again. This is a concern if the application contains unsaved data. Default: Off. Settable: At any time. This allows the user to save memory the next time the session is switched to full-screen. Examples: None. VIDEO_RETRACE_EMULATION Function: Simulates the video retrace status port to provide faster access. Advantages: DOS applications that poll the video retrace status port often write to the screen only during the retrace interval, even though it is safe (on EGA and VGA adapters) to draw at any time without causing interference (also known as "snow"). This feature causes most applications to write to the screen more often, and compensates for the performance drag imposed by monitoring the port in the first place. Drawbacks: Some applications may poll the port in such a way that overall performance is worse; this is sometimes true of applications that draw only during vertical (not horizontal) retrace. Unfortunately, while turning off trace emulation will restore performance, there is a risk that screen-switching will not be as reliable. Default: On. Reliable screen-switching has higher priority over the minority of applications that will experience some drag in performance. Settable: At any time. This allows the user to experiment with different settings in the event of a performance problem. Examples: None. VIDEO_ROM_EMULATION Function: Emulates selected INT 10h ROM Video functions. Advantages: Provides faster output for selected video functions than ROM services typically provide. This also has a dramatic effect on the performance of those functions in a window. Drawbacks: Some ROMs may offer enhanced services that are not included in the emulation. Applications which rely upon these services may not execute correctly. Default: On. Because the INT 10h ROM Video services are well-documented, incompatibilities are unlikely and the performance benefits of using the emulation are quite significant. Settable: At any time. This allows the user to experiment in the event of a compatibility problem. Examples: None. VIDEO_SWITCH_NOTIFICATION Function: Notifies a DOS application of a switch to/from full-screen mode. Advantages: Allows applications that monitor this notification to redraw their screens as needed. This may be necessary for some video adapters that provide modes (and applications that use those modes) which are not fully supported by the OS/2 video those modes) which are not fully supported by the OS/2 video driver or which are slightly incompatible. It is also valuable in situations where an OS/2 video driver has not allocated a virtual video buffer (see "VIDEO_8514_XGA_IOTRAP" in topic 11.2.11.8). Use this setting if you use the VIDEO_ONDEMAND_MEMORY DOS setting, because concurrent buffer allocation and screen switching can make a screen go black. Drawbacks: When used indiscriminately, this feature may cause unnecessary and time-consuming screen redrawing. For standard MONO/CGA/EGA/VGA video modes, the OS/2 video driver should be able to restore application screens without assistance. Default: Off. For standard hardware and standard video modes, this feature is not necessary. Settable: At any time. This allows the user to experiment in the event of a compatibility problem. Examples: Windows 2.x and 3.x understand this notification and will redraw themselves accordingly. For WIN-OS/2 sessions, set this setting on. VIDEO_WINDOW_REFRESH Function: Adjusts the window update frequency for a given VDM. Advantages: For applications (particularly graphics) that write frequently to video memory, this value can be increased to reduce time spent updating the window and provide more processor time for the application. Note: This has no effect on updates based on other events such as keyboard input or synchronous scrolling operations or any video events other than refresh. Drawbacks: A large refresh period can make an application unusable (or at least, very hard to use). Default: 0.1 seconds. This has been found to yield the best overall performance. Settable: At any time, in increments of 0.1 seconds. This allows for experimentation. The range is from 0.1 to 60.0 seconds. Examples: This setting affects normal TTY-style output. Compare a DIR or TYPE operation before and after altering this setting. VIDEO_8514_XGA_IOTRAP Function: When set OFF, unrestricted access to 8514/A display adapter hardware. Note that this setting is only available for systems with 8514/A display adapters installed. Advantages: Achieves higher performance for 8514/A applications and eliminates the overhead of the 1MB 8514/A virtual video buffer normally allocated for each VDM when set OFF. Drawbacks: Screen-switching away from the application will result in immediate freezing of the application, and the system may not be able to reliably switch back; that is, the screen image may not be correct. This may be overcome by setting VIDEO_SWITCH_NOTIFICATION on, which notifies applications to redraw their own screen images. Note however, that not all applications will take advantage of the notification. Note: An application with this setting enabled may not be run in windowed mode, or copied to the clipboard, because there is no complete information about its state. Default: Off. Settable: No; may be set at VDM creation only. Examples: When executing Windows 3.0 with the 8514/A display driver, certain operations such as painting dithered backgrounds will run significantly faster. XMS_HANDLES Function: Specifies the number of XMS extended memory block (EMB) handles. A handle is used with each XMS EMB. This number is required because XMS pre-allocates all the handle space to be compatible with XMS specifications. This setting should be used only if an application uses a large number of handles. Advantages: This setting restricts the number of block handles, thereby reducing memory consumption. Drawbacks: Specifying a large number of handles will increase memory consumption and adversely impact system performance. Default: The default value of this setting is 32. Settable: At VDM creation only. Examples: None. XMS_MEMORY_LIMIT Function: Specifies the per VDM XMS memory limit. This setting should be used under the same guidelines as described above in "XMS_HANDLES" in topic 11.2.12.1. The global limit is the overall maximum XMS memory consumption, and the per-VDM limit is the maximum allowed for each VDM. See also "Initialization" in topic 6.3.1.2 for defining global and per-VDM limit in the CONFIG.SYS. Drawbacks: Specifying a large number may adversely affect system performance. Default: The default value is 2MB per-VDM. Settable: At VDM creation only. Examples: 4096; this specifies a limit for each VDM. XMS_MINIMUM_HMA Function: Specifies the minimum HMA memory request allowed. This setting allows the user to fine tune the XMS. HMA is slightly less than 64KB in size. Only one request can be fulfilled from this area at a time. Advantages: If a TSR takes a very small allocation, then it will waste this area for other applications. In such cases a limit can be specified. Default: The default value is zero, which means all the requests will be allowed. Settable: At VDM creation only. Examples: 2048; this sets a limit of 2KB. WIN_RUNMODE Function: OS/2 V2.0 can use two modes to run Windows programs: - Real - Standard Real mode is the mode that Windows 2.0 programs run in. Windows 3.0 programs usually run in standard mode. For a detailed discussion, see Chapter 8, "Windows Applications." Use this setting to specify WIN-OS/2 mode for your session. Default: AUTO (the system selects standard mode as long as it has the OS/2 V2.0 Virtual Device Drivers required to support a standard mode WIN-OS/2 session in the OS/2 V2.0 operating system). AUTO enables the system to automatically choose between real and standard. Examples: None. -------------------------------------------------------------------------------