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Each time you boot up your computer, DOS searches the root directory of the drive from which it was started for a file named CONFIG.SYS. If this file is located, DOS executes all of the commands it contains much like a batch file (although this file is executed long before an AUTOEXEC.BAT file), according to the values assigned by special configuration commands. If DOS does not find this file in the root directory, it supplies its own default values for all of the configuration commands it requires.

The CONFIG.SYS File is used, then, only when you need to set new values for particular configuration commands. The parameters that can be modified in the CONFIG.SYS File include:

Each of these parameters is explained in this appendix, along with its associated commands.


Creation of CONFIG.SYS File

You can create the CONFIG.SYS File using the COPY command, EDLIN (the line editor supplied with DOS), or any other editor or word processor that can save its text in ASCII format. If you create it with a word processor, then save it in ASCII or text format and copy it to the root directory. To put the commands that you enter into commission, you must reboot your computer (Ctrl-Alt-Del, or turn the power off and on ).

To create the CONFIG.SYS File using the COPY command, use the following model:


Before creating a CONFIG.SYS File for a floppy or your hard disk, make sure that you are in the root directory (\) before you enter this command.

After typing this command and pressing the Enter key, you simply type in each configuration command to be processed when the CONFIG.SYS File is executed. After typing in each command, you terminate the line by pressing the Enter key.

Once you have entered all of the configuration command statements you wish to have in this file, you save it on disk (in the current directory) by pressing Ctrl-Z (you can also accomplish this by pressing F6 on IBM PCs and most compatibles) and the Enter key.


BREAK Status

Normally, DOS checks for the BREAK key (Ctrl-Break) only when performing standard input/output or print operations because the default for the BREAK command is off. If you want DOS to check for the BREAK key under more circumstances, set the status to on by entering


as a line in the CONFIG.SYS File. Setting the status to on in this file will allow you to abort program operations that produce few or no standard device operations (such as running a compiler). For more information on the BREAK command, refer to its reference entry in the main text of the book.


BUFFERS Parameter

A disk buffer is a specific block of RAM that DOS uses to store temporarily data that is being read or written to a disk. The default number of buffers maintained by DOS is 2. In version 3.3, the number of buffers is set according to these criteria:

Only if none of these apply to your computer system is the default number of buffers set to 2. Each buffer that is added with the BUFFERS command uses up an additional 528 bytes of memory.

To open additional buffers, you use the BUFFERS command followed by the equal sign and the number of buffers to use in the CONFIG.SYS File. For example, to increase the number of buffers from 2 to 10, you would enter

	  BUFFERS = 10

in a line of the CONFIG.SYS File.

Generally, performance is enhanced when running application programs when you increase the number of buffers from the default of 2. However, there is a trade-off that can occur when you have somewhere between 10 and 20 buffers open (depending upon the type of application that you are using). At that point, it may take DOS as much time to locate data in a particular memory buffer as it would to get the data from disk.

Many DOS application programs, such as WordStar 2000 and dBASE III PLUS, require many more than 2 buffers in order to run. During installation of such software, the application's installation program checks the root directory to make sure first that the CONFIG.SYS exists and, if it does, to check the number of buffers open with the BUFFERS command. If the number is less than 20 (or the BUFFER command is not used in the file), the program will then automatically enter a BUFFERS command or edit an existing one to read as follows:

	  BUFFERS = 20


COUNTRY Parameter

You can use the COUNTRY command in a CONFIG.SYS File to change the currency, date, or time format or the collating sequence to match that preferred by a foreign country. The syntax of the COUNTRY command is

	  COUNTRY = xxx,[yyy],[d:]COUNTRY.SYS


	  xxx	    Specifies the country code (see SELECT).
	  yyy	    Specifies the code page of the desired
                    country (in DOS 3.3, a country may have 
                    different information depending on the code 
                    page selected- -see Appendix C of the DOS 
                    reference for a listing of the codes).

Note that if you have used the SELECT command, DOS will have already created a CONFIG.SYS File that includes the COUNTRY configuration command. To make changes to the COUNTRY codes, you must then edit the contents of this file using either EDLIN or a word processor that can read ASCII files.


Devices, Drivers for

DOS automatically loads device drivers for standard input/output devices, printers, and storage devices such as floppy and fixed disk drives. Therefore, you need to use the DEVICE configuration command in the CONFIG.SYS File only when you need to install additional nonstandard devices (such as a mouse or an 8-inch floppy disk drive). The driver files for nonstandard devices are supplied by the device manufacturer.

To install a new device driver, you enter the DEVICE command followed by the equal sign and the name of the file that contains the driver on a line in the CONFIG.SYS File.

The DOS diskette includes two device drivers, ANSI.SYS and VDISK.SYS. Version 3.3 has added three more device drivers:DISPLAY.SYS, PRINTER.SYS, and DRIVER.SYS.


The ANSI.SYS driver file extends cursor control and keyboard reassignments. These extended functions make it easier for software to control cursor positioning, display messages on the screen, set colors, and reassign keyboard functions. Some software applications, such as SuperKey, require that ANSI.SYS be loaded by theCONFIG.SYS file in order to run.

To add this driver, you enter

	  DEVICE = [d:][path]ANSI.SYS

on a line of your CONFIG.SYS File, where

d:path Specifies the drive and path.


The VDISK.SYS driver allows you to set up a virtual or RAM disk that performs the functions of a physical disk drive. A virtualdisk, however, is installed in RAM and, therefore, disappears when power is interrupted to the computer system. You can install more than one virtual disk using the VDISK.SYS driver, depending upon the amount of RAM available to your system.

The syntax of the DEVICE configuration command when using the VDISK.SYS driver is as follows:

	  DEVICE = [d:][path]VDISK.SYS [comment][bbb]

Before explaining each optional parameter, consider the following example that uses all of the options:

	  DEVICE = C:\DOS\VDISK.SYS buffer size=256
	  sector size=512 directory entries=128 /E

The options are as follows:

	  d:path	 Tells DOS where the VDISK.SYS file is
                         located (C:\DOS in the example).
	  bbb		 Sets the size of the virtual disk in
                         kilobytes. If you do not specify this 
                         parameter, DOS uses a default of 64K. 
                         You can enter a value between 1K and the 
                         amount of memory available to your 
                         computer. Notice that you can also add 
			 an optional [comment] explaining the
                         value. In the example, the comment 
			 buffer size= precedes the value in
	  sss		 Sets the sector size in bytes. The
                         default of 128 bytes is used by DOS if 
                         this parameter is omitted or an 
                         inaccurate value is entered. Allowable 
                         values for the sector size are 128, 256, 
                         or 512 bytes. The sector size parameter 
                         can also be preceded by an optional 
                         comment. In the example, the comment 
			 sector size= has been added.
	  ddd		 Sets the number of directory entries
                         that the virtual disk can hold (one 
                         directory entry per file copied to the 
                         virtual disk). The default is 64, and 
                         you can enter a value between 2 and 512. 
                         However, DOS may automatically adjust 
                         the value you enter when installing the 
                         virtual disk. The value is increased to 
                         the nearest sector boundary (as set by 
                         the sector size). It is decreased if the 
                         size of the virtual disk (as set by the 
                         buffer size) is too small to accommodate 
                         the file allocation table, the 
                         directory, and two additional sectors. 
                         If the directory size reaches 1 and 
                         these files still cannot be 
                         accommodated, you will receive an error 
                         message and the virtual disk will not be 
	  comment	 You may enter an optional comment before
			 the [ddd] parameter. In the example, the
			 comment directory entries= has been
          /E             Tells DOS to install the virtual disk in 
			 extended memory AM at or beyond 1
                         megabyte). This parameter can only be 
                         used with a personal computer that has 
                         extended memory, such as the IBM PC AT 
                         or PS/2 machines (computers equipped 
                         with an add-on board such as the Intel 
                         Above Board support expanded instead of 
                         extended memory). When you add the /E 
                         parameter, the virtual disk buffer is 
                         established in extended memory while the 
                         device driver is installed in 
                         conventional memory. Extended memory up 
                         to 4 megabytes may be used for a single 
                         virtual disk.
	  :m		 Specifies the maximum number of sectors
			 (as specified by the sss parameter) of
                         data that are transferred to the virtual 
                         disk at one time. The permissible values 
                         are 1 through 8, with 8 being the 
                         default value.

When a virtual disk is established in extended memory, interrupt servicing is suspended during data transfers. In some situations, this can result in some interrupts being lost. If this happens, you should install the virtual disk in conventional memory. If the problem is resolved, you can then resinstall the virtual disk in extended memory with a smaller [:m] value.

When you establish a virtual disk, DOS assigns it the next available drive letter specification. For example, if your computer has two floppy disk drives, A and B, the virtual disk will be given C as the drive letter specification. If you have a single fixed disk, C, the virtual disk will be given D as the drive letter specification.


The DISPLAY.SYS device driver allows you to use code page switching on the EGA and IBM PS/2 displays and the IBM Convertible LCD screen. This device driver is included only in version 3.3 of DOS, which supports code page switching (see the CHCP and NLSFUNC command reference entries), and it is used only when you need to switch from the standard for U.S. symbols to new code pages containing international symbols.

The syntax of the DISPLAY.SYS DEVICE configuration command is as follows:

	  DEVICE = [d:][path]DISPLAY.SYS CON[:]=

The options are as follows:

	  d:path    Specifies the drive letter and path that
                    contain the DISPLAY.SYS file.
	  type	    Specifies the display adapter type. You can
                    use MONO, CGA, EGA, and LCD. Use EGA if you 
                    have an IBM PS/2 display (VGA is not yet 
                    fully supported).
	  hwcp	    Specifies the code page. Permissible values
                    are 437, 850, 860, 863, and 865 (refer to 
                    Appendix C of the DOS documentation for a 
                    description of these code page values).
	  n	    Specifies the number of prepared code pages
                    that can be supported. This must be a value 
                    between 0 and 12 (refer to the table in the 
                    DOS documentation for the DISPLAY.SYS command 
                    to determine this value).
	  m	    Specifies the number of subfonts supported by
                    each code page (refer to the table in the DOS 
                    documentation for the DISPLAY.SYS command to 
                    determine this value).

Note that if you are using ANSI.SYS with DISPLAY.SYS, the DEVICE = ANSI.SYS statement must precede the configuration statement DEVICE = DISPLAY.SYS in the CONFIG.SYS File.


The PRINTER.SYS device driver allows you to use code page switching on the IBM Proprinter Model 4201 and the IBM Quietwriter III Model 5202. Like the DISPLAY.SYS file, this device driver is included only in version 3.3 of DOS, which supports code page switching (see the CHCP and NLSFUNC command reference entries), and it is used only when you need to switch from the standard for U.S. symbols to new code pages containing international symbols.

The syntax of the PRINTER.SYS DEVICE configuration command is as follows:

	  DEVICE = [d:][path]PRINTER.SYS LPT#[:]=

The options are as follows:

	  d:path	      Specifies the drive letter and path
                              that contain the PRINTER.SYS file.
	  LPT#		      Specifies the printer device. It
                              can be entered up to three times 
                              (for LPT1, LPT2, and LPT3). You can 
                              substitute PRN for LPT in the 
                              command line.
	  type		      Specifies the type of printer use.
                              You can choose between 4201 (IBM 
                              Proprinter) or 5202 (IBM 
                              Quietwriter III). 
	  (hwcp1,hwcp2,...)   Specifies the code page that is
                              built into the hardware. The 
                              permissible values are 437, 850, 
                              860, 863, and 865 (refer to the 
                              PRINTER.SYS command in the DOS 
                              documentation for an explanation of 
                              how these code page values are 
                              applied to the two printer types).
	  n		      Specifies the number of additional
                              code pages that can be prepared. 
                              This value determines the number of 
                              buffers that PRINTER.SYS will set 
                              up to hold the code pages being 
                              prepared. The maximum number that 
                              can be specified is 12.


The DRIVER.SYS statement in the CONFIG.SYS File allows you to access and use a disk device by referring to a logical drive letter. The syntax used when adding this to the CONFIG.SYS File isas follows:

	  DEVICE = DRIVER.SYS /D:ddd[/T:ttt][/S:ss][/H:hh]

The options are as follows:

	  /D:ddd    Specifies the physical drive number between 0
                    and 255. The first physical diskette drive 
                    (drive A) has the value 0. The second 
                    physical diskette drive (drive B) has the 
                    value 1. The third physical diskette drive 
                    (must be external) has the value 2. The first 
                    fixed drive has the value 128 and the second 
                    has the value 129.
	  /T:ttt    Specifies the number of tracks per side
                    between 1 and 999 (default: 80).
	  /S:ss     Specifies the number of sectors per track
                    between 1 and 99 (default: 9).
	  /H:hh     Specifies the number of drive heads between 1
                    and 99 (default: 2).
          /C        Specifies that changeline support is required 
                    (only used on computers such as the IBM PC AT 
                    that support diskette changeline).
          /N        Specifies that the physical device is a non-
                    removable block device (such as a fixed 
	  /F:f	    Specifies the device type (form factor). The
		    value of the f parameter is determined as
		    Device		Value
		    160K/180K		0
                    320K/360K           0
                    1.2 megabytes       1
                    720K or others      2
                    1.44 megabytes      7

Note that the DEVICE=DRIVER.SYS configuration statement is not used to drive fixed (hard) disks. To set a logical drive letter for a fixed disk, use the SUBST command (see the SUBST reference entry for more information).

To find out the logical drive letter assigned by DOS to the device driver for a particular computer configuration and value of /D:, refer to the table included in the DOS documentation under the reference entry for the DEVICE command.



The highest drive specification letter that DOS 3 will recognize is drive E (three fixed drives attached: C, D, and E). If your system has more than this number of logical or physical drives attached, you must add a LASTDRIVE statement to your CONFIG.SYS File:



	  x    Specifies a letter between A and Z. If the drive
               letter you specify is less than the number of 
               drives attached to your system, DOS will ignore 
               the LASTDRIVE statement in the CONFIG.SYS File.

For example, if you are on a network and you have 15 drive volumes attached to the system, you would enter



FCBS Parameter

Some older application programs use file control blocks (FCBs) instead of the newer file handles to create, open, and delete files as well as to read from and write to files. When using these programs on a network with file sharing in use, you may have to increase the number of files that can be opened by FCBs. The default value used by DOS is 4 files.

To specify a new number of files that can be concurrently open by DOS, you use the FCBS command in the CONFIG.SYS File:

	  FCBS = x,y


	  x	    Specifies the total number of files that can
                    be opened by FCBs.
	  y	    Specifies the number of files protected from
                    automatic closure by DOS (the default is 0).

When file sharing is in use and an application program tries to open more than the total number of files, DOS closes the least-recently used file and opens the new file (excluding the files protected from automatic closure). If the program tries to read from or write to a file that has been closed by DOS, you will receive the following error message:

FCB unavailable

Abort, Fail?

To avoid such an error, you would add a FCBS command to your CONFIG.SYS File. For example, entering

	  FCBS = 10,5

would allow 10 FCB files to be open concurrently and would protect 5 of these from automatic closure.

When specifying the first parameter (the total number of FCB files), you can enter a value between 1 and 255. When specifying the second parameter (the number of files protected from automatic closure), you can enter a value between 0 and 255. (See also the SHARE command in the main text.)


FILES Parameter

By default, DOS allows up to 8 files (controlled by file handles, not FCBs) to be open concurrently. This number is insufficient to run several newer application programs as well as some DOS commands (such as XCOPY).

If you receive either the error message

Too many open files


Too many files open

you will have to use the FILES configuration command in the CONFIG.SYS File to increase the maximum number of files available to the entire system:

	  FILES = n


	  n	    Specifies the number of files that can be
                    opened at the same time (a value between 8 
                    and 255). However, be aware that the maximum 
                    number of files that a single process can 
                    have opened is set at 20.

To accommodate 15 different files being open at one time, you would enter

	  FILES = 15

as a line in the CONFIG.SYS File.


SHELL Parameter

Normally, DOS loads the command processor from the COMMAND.COM file during the boot sequence. If you have your own command processor, you can have it initialized and loaded at start-up in place of COMMAND.COM by using the SHELL configuration command.

The syntax of the SHELL command is

	  SHELL = [d:][path]filename [/E:xxxxx][/P]


	  d:pathfilename      Specifies the file name of the new
                              command processor (including its 
                              path, if it is not located in the 
                              root directory). Using COM-MAND.COM 
                              as the file name is a convenient 
                              way to increase the environment 
                              size under DOS 3.2 and 3.3.
	  /E:xxxxx	      Specifies the number of bytes for
                              the environment size (expressed as 
                              a base-10 integer between 160 and 
          /P                  Causes COMMAND.COM to remain loaded 
                              and to execute the AUTOEXEC.BAT 
                              file (if one exists).

Note that using the SHELL command to run a different command processor does not affect the COMSPEC command, which points to the name of the controlling processor (see SET in the reference entries). If you use the SHELL command in the CONFIG.SYS File to load a new command processor, you will also want to use the COMSPEC parameter in the SET command in the AUTOEXEC.BAT file to refer to it.


STACKS Parameter

DOS 3.3 includes a STACKS configuration command that allows you to override the default stack resources used by DOS. The syntax of this command is

	  STACKS = n,s


	  n	    Specifies the number of stack frames between
                    8 and 64.
	  s	    Specifies the size in bytes of each stack
                    frame between 32 and 512.

If the STACKS command is not included in the CONFIG.SYS File, the n and s parameters are set to 0 for the IBM PC, IBM PC XT, and the IBM Portable PC. For all other IBM personal computers, the default values are n = 9 and s = 128.

Every time a hardware interrupt occurs, DOS appropriates one stack frame from the stack pool. Once the interrupt has been processed, DOS returns the stack frame to the pool. If you experience stack overflow errors, you should use the STACKS command to increase the number of stack frames available to DOS.


file: /Techref/dos/config.htm, 31KB, , updated: 2005/5/28 13:07, local time: 2023/12/11 04:00,

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