In terms of optimizing your computer, the most significant feature offered by DOS is the capability to load most of the operating system software into extended memory. By doing so, DOS provides more conventional memory for applications, ultimately improving the performance of your system.

Before DOS or applications software can use extended memory, you must add a command to your CONFIG.SYS file to load an Extended Memory Manager—a driver that provides a standard way for applications to address extended memory so that no two programs use the same portion of extended memory at the same time. DOS 4.0 and later versions include the Extended Memory Manager HIMEM.SYS.

HIMEM.SYS manages memory according to the rules set out in the Extended Memory Specification (XMS) Version 2.0. According to this specification, three areas of memory above the conventional 640KB barrier can be made available for programs to use:

When used on an x86 PC, HIMEM.SYS provides HMA and XMS memory to programs that know how to use it. To access upper memory, however, the PC must have at least an 80386 CPU or higher, and you must include one of the following device driver commands in CONFIG.SYS after HIMEM.SYS is loaded:

DEVICE=EMM386.EXE RAM

DEVICE=EMM386.EXE NOEMS

Refer to the next section for a full discussion of EMM386.EXE.

The syntax for using HIMEM.SYS is shown in the following:

DEVICE=d:pathHIMEM.SYS /A20CONTROL:ON|OFF /NUMHANDLES=n /EISA /HMAMIN=n/INT15=xxxx /
MACHINE:xx /SHADOWRAM:ON|OFF /CPUCLOCK:ON|OFF /QUIET

d: is the disk drive where the HIMEM.SYS resides, and path is the directory containing the device driver file. If HIMEM.SYS is contained in the DOS directory on your C drive, for example, you include the following command in CONFIG.SYS:

DEVICE=C:DOSHIMEM.SYS

In most cases, you need only this command to activate the Extended Memory Manager. In special cases, however, you might need to use one of the available switches described in the following paragraphs. The next section continues the discussion of loading DOS into upper memory.

According to XMS specifications, only one program at a time can use the high memory area. The switch /HMAMIN=n sets the minimum amount of memory that must be requested by an application before the application is permitted to use HMA. If you load DOS into HMA, you can omit this switch, as explained in the section “Loading DOS into High Memory” later in this chapter.

When the Extended Memory Manager assigns memory to a particular program, it assigns one or more extended memory block handles to the program. The /NUMHANDLES=n switch indicates the maximum number of handles available. The number n must be between 1 and 128. The default is 32 handles, usually a sufficient number. Each reserved handle requires an additional 6 bytes of memory. Unless you are running software that complains about not having enough extended memory handles, you probably do not need this option.

Most current versions of commercial software support the XMS specification for addressing extended memory. Some older versions of programs, however, use a different method of addressing extended memory, known as the Interrupt 15h (INT15h) interface. If you work with software that uses the INT15h interface and you want to load DOS into HMA, add the /INT15=xxxx switch. The number xxxx indicates the amount of extended memory you want HIMEM.SYS to assign to the INT15h interface. This number must be from 64 to 65,535 (kilobytes), and the default is 0. If you assign some of your extended memory to the INT15h interface, that memory is not available to programs that expect the XMS interface.

Internally, DOS uses a wire called the A20 memory address line to access the high memory area. What the A20 address line is and how DOS uses it are not important here, but not all brands of PCs handle the A20 line in the same way. Normally, HIMEM.SYS can detect how your computer uses its A20 line, but if it is incorrect, HIMEM.SYS displays the following error message:

Unable to control A20

If you see this message, use the /MACHINE:xx switch in the HIMEM.SYS command to specify which type of A20 handler your machine uses. For xx in the /MACHINE switch, insert the code that matches your computer. You can find the text codes in the Code column and the number codes in the Number column in Table 19.2. If you see the error message but your computer is not listed in Table 19.2, try the switch /MACHINE:1.

Before DEVICE=HIMEM.SYS in CONFIG.SYS, you might have listed a device driver that also uses the A20 line. By default, HIMEM.SYS takes control of A20 even though the line is turned on when HIMEM.SYS loads. HIMEM.SYS warns you when this condition occurs by displaying the following message:

Warning: The A20 Line was already enabled!

Determine see whether you really intend to have both drivers installed at one time. If so, you can prevent HIMEM.SYS from taking control of A20 by adding the switch /A20CONTROL:OFF (the default setting is /A20CONTROL:ON).

All PCs store some of their basic operating instructions in read-only memory (ROM); however, computers usually cannot access instructions from ROM as fast as they can access instructions from RAM. If you have plenty of memory and an 80386 (or better) computer, you can usually increase the performance of your computer by asking the computer to copy the instructions from ROM into an upper memory block (which is RAM). This technique is known as shadow RAM.

Shadow RAM uses some of your upper memory blocks. If you prefer to increase the amount of upper memory available for device drivers and memory-resident programs, use the /SHADOWRAM:OFF switch. As HIMEM.SYS loads, the following message appears:

Shadow RAM disabled.

In some cases, DOS cannot turn off shadow RAM. Instead, DOS displays a message telling you that shadow RAM is in use and cannot be disabled. (Check your hardware documentation for other methods of disabling shadow RAM.)

The /CPUCLOCK:ON switch ensures that HIMEM.SYS does not slow your computer's clock speed, the speed at which your computer processes instructions. (Any change in clock speed does not affect your computer's real-time clock, which keeps time of day.) On the front panel of many PCs is an LED or other indicator that indicates the current clock speed. To prevent the clock speed from slowing, add the /CPUCLOCK:ON switch to the DEVICE=HIMEM.SYS command in CONFIG.SYS.

If you are using a machine that uses an Extended Industry Standard Architecture (EISA) bus with more than 16MB of memory, HIMEM.SYS does not normally allocate all the extended memory. Use the /EISA switch to tell HIMEM.SYS to automatically allocate all available extended memory.

When HIMEM.SYS loads, it usually prints status messages. If you want HIMEM.SYS to load without the usual status messages, add the /QUIET switch.

DOS enables you to load most of the operating system into an area of extended memory known as the high memory area (HMA), the first 64KB of extended memory (except the first 16 bytes, which overlap the upper memory area). After the device driver HIMEM.SYS is loaded into the computer's memory, the command DOS=HIGH in CONFIG.SYS loads DOS into high memory.

The next time you boot the computer, DOS uses about 14KB of space in conventional memory and loads the remainder of the operating system into the HMA. If, however, you don't use this command in CONFIG.SYS or don't have extended memory installed in your computer, DOS occupies more than 62KB of memory. By loading the operating system into high memory, DOS 6.2 can free about 47KB of conventional memory.

Because expanded memory was introduced before extended memory, many PC applications were written to take advantage of expanded memory, not extended memory. Today, however, extended memory is far less expensive and much more common. The DOS device driver EMM386.EXE enables applications to use extended memory as though it were expanded memory, freeing you from the need for the special memory boards and device drivers that expanded memory requires. Thus, EMM386.EXE emulates expanded memory by using extended memory.

Some applications today can use either expanded memory or extended memory by accessing whichever is available. If you have a choice, use extended memory, which is faster.

In addition to its role as an expanded memory emulator, EMM386.EXE also is a UMB provider, working with HIMEM.SYS to provide upper memory blocks (UMBs) into which you can load device drivers and memory-resident programs. See the next section, “Loading Device Drivers and TSRs into Upper Memory,” for further discussion of providing UMBs.

The syntax of the command for EMM386.EXE, used as a device driver, is shown in the following:

DEVICE=EMM386.EXE ON|OFF|AUTO memory W=ON|OFF
    Mx|FRAME=address /Pmmmm Pn=address X=mmmm-nnn
    I=mmmm-nnn B=address L=minxms A=altregs H=handles
    D=nnn RAM NOEMS MIN=n /VERBOSE ROM=mmmm-nnnn NOVCPI
     WIN=mmmm-nnnn NOMOVEXBDA NOHIGHSCAN ALTBOOT NOHI

In most cases, one of the following commands is sufficient:

DEVICE=EMM386.EXE RAM

DEVICE=EMM386.EXE NOEMS

The first command loads the expanded memory emulator and allocates 256KB of extended memory to be used as expanded memory. The RAM switch enables upper memory. The second command also enables upper memory but tells EMM386 not to allocate extended memory for use as expanded memory.

Sometimes you might need the remaining switches for the EMM386.EXE device driver to customize your computer for use with particularly demanding software or hardware. You can specify ON, OFF, or AUTO in the EMM386.EXE device driver command to indicate whether your computer starts in the EMM386.EXE active, inactive, or automatic mode, respectively. By default, the device driver is active, and EMM386.EXE makes extended memory available. However, some applications might not run properly when EMM386.EXE is active because EMM386.EXE places the computer in a mode known as Virtual 8086 mode. When you use EMM386.EXE as a device driver, the driver loads in memory and remains active (ON) unless you specify otherwise by using the OFF switch.

The OFF switch starts the computer with EMM386.EXE loaded in memory but inactive. The XMS memory allocated as EMS memory is unavailable for any purpose. You can activate the driver by entering the following command at the DOS command prompt:

EMM386 ON

The OFF switch is not compatible, however, with the RAM or NOEMS switches, discussed later in this section.

Use AUTO if you want EMM386.EXE to activate only when an application requests EMS memory. This setting provides maximum compatibility with software that might not work properly in Virtual 8086 mode. Like the OFF switch, AUTO is not compatible with the RAM or NOEMS switches.

EMM386 OFF

The memory parameter enables you to specify the amount of XMS memory you want EMM386.EXE to allocate as EMS memory. Type the number of kilobytes in the range from 16 to 32,768. EMM386.EXE rounds any number you type down to the nearest multiple of 16. All unallocated memory remains available as XMS memory. The default EMS memory allocated is 256KB.

As a general rule, allocate only as much EMS memory as is required by your applications. Any memory allocated as EMS memory is no longer available as XMS memory. Most of the newer DOS applications, especially games, require the maximum amount of XMS you can make available. If none of your applications need expanded memory, use the NOEMS option so that no memory is allocated as expanded memory.

Use the L=minxms switch, in which minxms is the number of kilobytes, to indicate the minimum XMS memory that EMM386.EXE is to allocate. This parameter overrides the memory parameter.

If you installed a Weitek math coprocessor chip—a special computer chip that improves the performance of computation-intensive software, such as computer-aided design (CAD) software—use the W=ON switch. By default, the device driver does not support this type of coprocessor. You also can turn on or off support for the Weitek coprocessor by entering one of the following commands at the DOS prompt:

EMM386 W=ON

EMM386 W=OFF

In some circumstances, you might want to use upper memory for device drivers and memory-resident programs, but you don't need EMS memory. In this case, use the NOEMS switch with the EMM386.EXE driver to free the maximum amount of upper memory and to provide no EMS memory. You might intend to run Windows 3.1 on your computer, for example. Because Windows can use all your XMS memory, this software doesn't need EMS memory.

The /VERBOSE switch causes EMM386 to print status and error messages when it loads. Normally, EMM386 suppresses these messages. You can abbreviate /VERBOSE as /V.

EMM386 scans the upper memory area (UMA) for all available free memory. Unusual architectures might confuse EMM386, causing trouble when your computer boots or while you use the computer. The NOHIGHSCAN parameter limits EMM386's scanning of the UMA. If you experience problems when you use EMM386, adding the NOHIGHSCAN parameter might solve the problem.

EMM386 loads part of itself into the upper memory area. If one of your programs needs extra upper memory blocks, and you're willing to give up some of your conventional memory, the NOHI parameter causes EMM386 to load all of itself into conventional memory, and does not use any UMA.

The remaining switches available for use with EMM386.EXE are highly technical and beyond the scope of this book. Refer to your DOS User's Guide or the Technical Reference manual for more information.

In addition to enabling you to run DOS in high memory, DOS provides the capability of loading memory-resident programs and device drivers into upper memory blocks, freeing more conventional memory for other applications. DOS can access this area of memory in 80386 and 80486 PCs (including SXs) that have 1MB or more of memory.

To load device drivers or memory-resident programs, also called terminate-and-stay-resident (TSR) programs, into upper memory, the following conditions must be met:

If you also want to use the command DOS=HIGH to load DOS into high memory, you can combine the two commands as shown in the following:

DOS=HIGH, UMB

You can load two types of programs into upper memory: device drivers and memory- resident programs (TSRs). You already know that device drivers normally are loaded using the DEVICE command. When you want to load a device driver into upper memory, however, you use the DEVICEHIGH command. The syntax for this configuration command is as follows:

DEVICEHIGH=filename.ext /switches

To load into upper memory the screen driver ANSI.SYS, for example, you need the following command in CONFIG.SYS:

DEVICEHIGH=C:DOSANSI.SYS

When you boot the computer, DOS attempts to load ANSI.SYS into the upper memory area.

To load a memory-resident program into upper memory, precede the program's startup command with LOADHIGH. (You can use the abbreviation LH in place of LOADHIGH.) The syntax for the command for LOADHIGH is as follows:

LOADHIGH programname /switches

As an example, assume that you want to load DOSKEY (discussed in Chapter 18, “Mastering DOSKEY and Macros”) into upper memory each time you start your computer. To do so, you add the following command to your AUTOEXEC.BAT file:

LOADHIGH DOSKEY

The next time you reboot the computer, DOS attempts to load DOSKEY into upper memory. DOS does not load a program into upper memory if the program requests that DOS allocate more memory during initialization than is available in the largest available upper memory block. If DOS is not successful when it tries to load device drivers or TSRs into the upper memory area, it loads the program into conventional memory instead.

So that you can make the most efficient use of DOS's memory management utilities, DOS also enables you to display the amount of free and used memory at any point during a DOS session. Use the MEM command for this purpose.

The syntax for the MEM command is as follows:

MEM /CLASSIFY /DEBUG /FREE /MODULE modulename /PAGE

Most of these options (/DEBUG, /FREE, and /MODULE) are primarily of interest to programmers. For any option, you can specify just the first letter, such as /C in place of /CLASSIFY, for example.

With no switches, MEM gives a basic report of how the memory on your machine is being used. The following provides a sample of a basic MEM report from a typical computer with 8MB of memory:

Memory Type        Total  =   Used  +   Free
-------------      ------     -----    ------
Conventional         640K       79K      562K
Upper                 71K       33K       38K
Reserved             384K      384K        0K
Extended (XMS)*    7,097K    2,537K    4,560K
-------------      ------    ------    ------
Total memory       8,192K    3,033K    5,159K

Total under 1 MB     711K      112K      599K

Total Expanded (EMS)                7,488K (7,667,712 bytes)
Free Expanded (EMS)*                4,800K (4,915,200 bytes)

* EMM386 is using XMS memory to simulate EMS memory as needed.
  Free EMS memory may change as free XMS memory changes.

Largest executable program size       561K (574,752 bytes)
Largest free upper memory block        22K  (22,016 bytes)
MS-DOS is resident in the high memory area.

Even this most basic report includes a wealth of information:

With the /CLASSIFY switch, MEM lists all DOS programs and device drivers currently loaded. Along with the information shown previously, MEM lists this additional information if you use /CLASSIFY:

Modules using memory below 1 MB:

  Name           Total       =   Conventional   +   Upper Memory
  --------   ---------------    ---------------     --------------
  MSDOS       18,029   (18K)     18,029   (18K)          0    (0K)
  HIMEM        1,168    (1K)      1,168    (1K)          0    (0K)
  EMM386       3,120    (3K)      3,120    (3K)          0    (0K)
  DBLSPACE    37,664   (37K)     37,664   (37K)          0    (0K)
  COMMAND      3,184    (3K)      3,184    (3K)          0    (0K)
  MOUSE       17,088   (17K)     17,088   (17K)          0    (0K)
  SETVER         816    (1K)          0    (0K)        816    (1K)
  ANSI         4,240    (4K)          0    (0K)      4,240    (4K)
  SMARTDRV    29,024   (28K)          0    (0K)     29,024   (28K)
  Free       613,472  (599K)    574,976  (562K)     38,496   (38K)

This sample report shows the programs and device drivers currently loaded and displays the amount of conventional and upper memory used by each:

After you identify a driver or memory-resident program that appears to be the right size to fit in the available UMB, edit CONFIG.SYS or AUTOEXEC.BAT to add DEVICEHIGH or LOADHIGH to the appropriate command. Reboot your computer and issue the MEM /C command again to see whether the driver or program loaded.

Arriving at the optimal combination of device drivers and memory-resident programs loaded into upper memory might require some experimentation. DOS loads programs in the largest available UMB first, so try loading the largest drivers and programs first by placing their startup commands earliest in CONFIG.SYS or AUTOEXEC.BAT. To alleviate some of the trial-and-error necessary in arriving at the optimal memory arrangement, DOS now provides the MemMaker utility, which is described in the following section.

Understanding and configuring a PC's memory is one of the most challenging activities most users face. Because most users usually configure their computer's memory once, they never really get a chance to build any experience fiddling with the configuration. After several years of watching users stumble at configuring their PCs, Microsoft finally offered help. DOS includes MemMaker, a utility that analyzes your PC and makes the appropriate changes to your CONFIG.SYS and AUTOEXEC.BAT files so that your computer uses its memory most effectively.

You can take Understanding two different paths to configure your memory with MemMaker. One is to follow the Express Setup. The other path you can choose is the Custom Setup.

Express Setup is the easier path to choose. You have little interaction with Express Setup, other than pressing Enter when MemMaker prompts you to do so. MemMaker searches through the upper memory area to find open memory addresses. It then sorts device drivers and TSRs that you load in memory to see the optimum loading order. Finally, it updates your CONFIG.SYS and AUTOEXEC.BAT files for two reasons. MemMaker ensures that HIMEM.SYS and EMM386.EXE load to manage memory and that the DOS=UMB directive is in CONFIG.SYS to provide the link to upper memory blocks. In addition, MemMaker inserts DEVICEHIGH and LOADHIGH before the device drivers and TSRs that load in upper memory.

Custom Setup is similar to Express Setup in that it scans the upper memory area for open address space, sorts device drivers and TSRs for optimal order, and updates your AUTOEXEC.BAT and CONFIG.SYS files. As you might expect, however, you can customize how MemMaker performs these tasks. The following list shows the elements you can customize using Custom Setup:

Now that you understand what MemMaker can do to optimize your computer's memory, you are ready to use MemMaker. The following two sections explain how to use MemMaker with Express Setup and Custom Setup.

The FCBS configuration command enables you to use programs written for DOS 1.1; some DOS users find FCBs indispensable. FCB is an acronym for file control block. FCBs serve as one way a program can access a file. This method of file access was used by DOS 1.1 to communicate with programs. Later versions of DOS borrow a Unix-like method for controlling files, called handles (discussed in “Using the FILES Command” in this chapter). Although FCBs can be used with any version of DOS, only DOS 2.0 and higher can use handles.

The syntax for the FCBS command in the CONFIG.SYS file is as follows:

FCBS = maxopen

The maxopen parameter is a number between 1 and 255 that sets the maximum number of unique FCBs that programs can open at one time. The default number is four. You don't need to use this command in CONFIG.SYS unless you have a program that was designed to work with DOS 1.1 and the program cannot open all the required files (a message to this effect appears). In this case, use the FCBS command to increase the number of FCBs that can be open at one time.

You pay a small price in RAM to use the FCBS command. For each number greater than four that maxopen exceeds, DOS uses about 40 bytes.

FILES is the configuration command used in DOS 2.0 and higher to allow for Unix-like file handling. Unix and later versions of DOS use a file handle (a number corresponding to the filename) instead of file control blocks to access files. You never have to deal with file handles directly. Each application program gives the operating system the name of the file or device you want to use. The operating system gives the program a handle, and the program uses that handle to manipulate the file or device.

To include the FILES command in CONFIG.SYS, use the following syntax:

FILES = n

The n parameter is a number (8, which is the default, through 255) that determines the number of files that can be open at one time during a DOS session. Each additional file beyond eight increases the size of DOS by 39 bytes.

If you do not specify the FILES command, DOS starts with eight file handles and immediately takes five handles for the standard devices, leaving only three handles for your programs. This number is almost never large enough for applications you are likely to run. On most systems, increase the number of handles to 20 or 30.

The LASTDRIVE configuration command informs DOS of the maximum number of disk drives on your system. Generally, LASTDRIVE is a command used with networked computers or with the pretender commands (such as SUBST).

If you do not use the LASTDRIVE command, DOS assumes that the last disk drive on your system is one more than the number of physical drives and RAM disks you are using. If your LASTDRIVE command specifies a letter corresponding to fewer drives than the number physically attached to your computer or created as RAM disks, DOS ignores the command. The LASTDRIVE command enables you to tell DOS how many disk drives, real or apparent, are on your system, including network drives and directories (if any) and drives created with the SUBST command.

If you want to use the LASTDRIVE command in CONFIG.SYS, use the following syntax:

LASTDRIVE = x

The x parameter is the letter for the last disk drive on your system. The letters A through Z in upper- or lowercase are acceptable.

The SHELL command was originally implemented to enable programmers to replace the DOS command interpreter (COMMAND.COM) with other command interpreters. The SHELL command is more commonly used, however, to perform the following two functions:

The syntax for the SHELL command is as follows:

SHELL = filespec parameters

The filespec parameter is the path and filename of the command processor and should be COMMAND.COM if you are using the standard DOS command processor. The SHELL command doesn't take any other parameters or switches, but you can add command-line parameters or switches available for use with the command processor. The parameters for COMMAND.COM are explained in the next few paragraphs.

When used from the command line, COMMAND loads a copy of the command processor into memory. A common use of COMMAND is as a parameter of the SHELL command. The syntax for COMMAND is as follows:

COMMAND d:path device /E:size /P /C string /K:filename /MSG

The d:path parameter specifies the disk drive and path that contain the command processor if it is not located in the root directory. Always use this parameter when including COMMAND in the SHELL configuration command. This parameter has the additional effect of setting an environment variable named COMSPEC, which informs DOS and other programs of the location and name of the current command processor.

/E:size is an optional switch that sets the environment space. The size parameter is a number between 160 and 32,768 that denotes the amount of memory reserved for the environment. (If you do not specify a multiple of 16, DOS rounds the size parameter up to the next highest multiple of 16.) By default, DOS reserves 256 bytes for the environment.

The /P switch instructs DOS to load the command processor permanently. Without the /P switch, DOS loads COMMAND.COM only temporarily into memory. When you are using COMMAND with the SHELL command in CONFIG.SYS, be sure to use the /P switch.

The /C switch and string parameter work together. This combination causes DOS to load the command processor, execute any command represented by string, and then unload the command processor.

The /MSG switch tells DOS to store all its error messages in memory rather than read them from the disk. This feature can speed operation. More importantly, if you are running a system that has only floppy disks, you sometimes remove the disk that contains COMMAND.COM from the disk drive. Without the /MSG switch, DOS cannot access error messages contained on disk within the COMMAND.COM file itself. Use this switch only if you are running DOS from floppy disks. You also must use the /P switch any time you use the /MSG switch.

The /K parameter tells DOS to run a program or batch file. Use this switch only when you're running COMMAND from the DOS prompt and not as part of the SHELL command in CONFIG.SYS.

The DOS Setup program adds the following command to the default CONFIG.SYS file:

SHELL=C:DOSCOMMAND.COM C:DOS/P

This configuration command tells DOS that COMMAND.COM is the command interpreter and that it is located in the DOS directory on the C drive. The /P switch causes the command interpreter to be loaded permanently, not temporarily, in memory.

The preceding SHELL command enables you to place a copy of COMMAND.COM in C:DOS and delete the copy in the root directory. This practice helps you maintain a clean root directory and protects COMMAND.COM from being replaced by an older version that might be on a floppy disk you are copying. If you accidentally copy the disk to the root directory, you don't overwrite the current version of COMMAND.COM.

Occasionally, you create such a long PATH command in AUTOEXEC.BAT that you fill the available environment space, causing DOS to display the following message:

Out of environment space

If this message appears, use COMMAND with the SHELL command and the /E switch to specify a larger environment space. The following command used in CONFIG.SYS, for example, increases the environment to 384 bytes:

SHELL=C:DOSCOMMAND.COM /E:384

If you already have a SHELL command in CONFIG.SYS, you can add the /E switch. Combining the two preceding SHELL commands, for example, you can include the following command in CONFIG.SYS:

SHELL=C:DOSCOMMAND.COM C:DOS/P /E:384

The INSTALL configuration command enables you to load memory-resident programs from within CONFIG.SYS. In versions of DOS before 4.0, you had to load these programs from the DOS prompt or through a batch file, such as AUTOEXEC.BAT. You can save several kilobytes of memory by loading a program from CONFIG.SYS with INSTALL rather than from the command line or a batch file as an executable program. DOS 4.0 and later versions support loading any of the following programs by using INSTALL:

→ For more information about these programs, see Appendix F,“ Command Reference,” p. 583.

The following is the syntax for using INSTALL in CONFIG.SYS:

INSTALL = filespec parameters

The filespec parameter is the path and filename information of the utility you want to load, whereas parameters specifies any parameters or switches required by the utility you want DOS to load.

You might be able to use INSTALL with some memory-resident non-DOS programs. Do not use INSTALL, however, to load a memory-resident program that uses environment variables or shortcut keys or that uses COMMAND.COM. The program you install with this command must have the extension .COM or .EXE.

The REM configuration command is equivalent to the REM batch file command. This command enables you to insert remarks into your CONFIG.SYS file. You can leave notes to yourself (or others) explaining what particular lines do. Such documentation in a CONFIG.SYS file is especially helpful if you use non-DOS device drivers for your hardware. You also can temporarily remove a CONFIG.SYS statement by prefacing the statement with a REM command. After you test the new configuration, you can return easily to the old configuration by simply removing the REM command.

The syntax for the REM command is as follows:

REM remarks

The remarks parameter can be any string of characters that fits on a single line in the CONFIG.SYS file.

The SWITCHES configuration command enables you to set any of four options that control how four of DOS's features operate. The syntax of the SWITCHES command is as follows:

SWITCHES = /K /W /N /F

You can specify one or more of these four switches in a single SWITCHES command.

The /K switch turns off the enhanced keyboard functions. This command works like the ANSI.SYS /K switch. Some software cannot work with the enhanced keyboard. Use this command to disable the enhanced keyboard so that the software functions properly. If you use the SWITCHES=/K command in CONFIG.SYS and also install ANSI.SYS as a device driver, add the /K switch to the DEVICE=ANSI.SYS line as well.

The /W switch specifies that the WINA20.386 file has been moved to a directory other than the root directory. Use this switch if you moved WINA20.386 to another directory and are using Windows in enhanced mode.

Normally, you can use the F5 or F8 keys during the first two seconds of booting to bypass some or all the commands in CONFIG.SYS or AUTOEXEC.BAT. The /N switch tells DOS to ignore F5 or F8 during boot.

Normally, DOS displays this message at the beginning of the boot process and then pauses for two seconds before continuing:

Starting MS-DOS

The /F switch tells DOS to skip the two-second pause.

As you already know, Ctrl+Break and Ctrl+C are helpful but not foolproof panic buttons you can use to stop commands. The response to a Ctrl+Break or Ctrl+C is not instantaneous. Although only an “Oh, no” second might pass from the time you press the panic button until DOS responds, you still have time to wonder why DOS takes so long to respond. The reason is that DOS is busy doing other things most of the time and looks for Ctrl+Break only at intervals. You can use the BREAK command in CONFIG.SYS to tell DOS when to check for this key sequence. BREAK does not enable or disable the Break key; the BREAK command only controls when DOS checks for the Break key.

The syntax for the BREAK command can be either of the following:

BREAK=ON

BREAK=OFF

The default setting for this command is OFF.

If you use the command BREAK=ON in CONFIG.SYS, DOS checks to see whether you pressed Ctrl+Break whenever a program requests some activity from DOS (performs a DOS function call). If you use the command BREAK=OFF, DOS checks for a Ctrl+Break only when DOS is working with the video display, keyboard, printer, or asynchronous serial adapters (the ports at the back of the computer).

Because DOS manages disks in a logical rather than a strictly physical way, DOS can pretend that a disk's identity is different from the disk's name. DOS provides the following three commands that pretend that a disk's identity has changed:

→ For detailed information on these commands, see Chapter 11, “ Controlling Your Environment,” p. 293.

DOS provides other commands to control devices and report system information, which are briefly discussed in this section.

→ These commands are explained in greater detail in Appendix F,“Command Reference,” beginning on p. 583.

The SET command displays the current environment settings and enables you to make new variable assignments.

The PRINT command enables you to print text files on your printer while you continue to do other PC work. This “background” printing can be a great timesaver if your applications don't have a similar feature.

The MODE command is a multifaceted device-control command. MODE can establish the height and width of your screen's lines and characters and control the speed of your serial ports. MODE can redirect the output from a parallel printer port to a serial port. You also can use MODE in association with code-page support for international character sets on the PC. You might want to browse through the MODE section of the “Command Reference.