The directives in this file configure DOS for use with devices and applications in the system. The CONFIG.SYS directives also set up the memory managers in the system. After processing the CONFIG.SYS file, DOS proceeds to load and execute the command shell specified in the SHELL line of CONFIG.SYS, or COMMAND.COM if there is no such line. The command shell in turn is responsible for processing the AUTOEXEC.BAT file.
CONFIG.SYS is composed mostly of name=value directives which look like variable assignments. In fact, these will either define some tunable parameters often resulting in reservation of memory, or load files, mostly device drivers and terminate-and-stay-resident programs (TSRs), into memory.
In DOS, CONFIG.SYS is located in the root directory of the drive from which the system was booted.
Some versions of DOS will probe for alternative filenames taking precedence over the default CONFIG.SYS filename if they exist:
While older versions of Concurrent DOS 3.2 to 4.1 did not support CONFIG.SYS files at all, later versions of Concurrent DOS 386 and Concurrent DOS XM, as well as Multiuser DOS, System Manager and REAL/32 will probe for CCONFIG.SYS (with "C" derived from "Concurrent") instead of CONFIG.SYS. Some versions of Multiuser DOS use a filename of CCONFIG.INI instead, whereas REAL/32 is known to look for MCONFIG.SYS. These operating systems support many additional and different configuration settings (like INIT_INSTALL) not known under MS-DOS/PC DOS, but they are stored in the binary repository named CCONFIG.BIN rather than in CCONFIG.INI. Both files are typically modified through a configuration utility named CONFIG.EXE only.
Under DR DOS 3.31, PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02 and higher, a file named DCONFIG.SYS (with "D" derived from "DR DOS"), if present, will take precedence over CONFIG.SYS. Since DR DOS 6.0 this was used in conjunction with disk compression software, where the original boot drive C: would become drive D: after loading the compression driver (and the "D" in the file name came in handy as well), but it is commonly used to help maintain multiple configuration files in multi-boot scenarios.
In addition to this, OpenDOS 7.01 and DR-OpenDOS 7.02 will look for a file named ODCONFIG.SYS, whereas some issues of DR-DOS 7.02 and higher will instead also look for DRCONFIG.SYS.
Further, under DR DOS 6.0 and higher, the SYS /DR:ext command can be used to change the default file extensions. For example, with SYS /L /DR:703 the written Volume Boot Record would look for a renamed and modified IBMBIO.703 system file (instead of the default IBMBIO.COM) and the IBMBIO.703 would look for IBMDOS.703 and [D]CONFIG.703 (instead of IBMDOS.COM and [D]CONFIG.SYS), so that multiple parallel sets of files can coexist in the same root directory and be selected via a boot-loader like LOADER, supplied with Multiuser DOS and DR-DOS 7.02/7.03. The SHELL directive is enhanced to provide means to specify alternative AUTOEXEC.BAT files via /P[:filename.ext] and in this specific scenario, COMMAND.COM will accept file extensions other than ".BAT" as well (both features are also supported by 4DOS). Under DR DOS 6.0 and higher, the CONFIG.SYS directive CHAIN=filespec can be used to continue processing in the named file, which does not necessarily need to reside in the root directory of the boot drive. DR-DOS 7.02 and higher optionally support an additional parameter as in CHAIN=filespec,label to jump to a specific :label in the given file.
DR-DOS 7.03 and higher support a new SYS /A parameter in order to copy the corresponding CONFIG.SYS and AUTOEXEC.BAT files along with the system files.
FreeDOS implements a similar feature with its FDCONFIG.SYS configuration file. RxDOS 7.24 and higher use RXCONFIG.SYS instead.PTS-DOS uses CONFIG.PTS.
Both CONFIG.SYS and AUTOEXEC.BAT can be found included in the root folder of Windows 95, and Windows 98 boot drives, as they are based on DOS. Typically, these files are left empty, with no content.
Under FlexOS, CONFIG.SYS is a binary file defining the resource managers and device drivers loaded.
:(DR DOS 5.0 and higher only)
Defines labels in CONFIG.SYS as jump targets for CHAIN, DRSWITCH, GOTO, GOSUB and SWITCH directives.
;(DOS 6.0 and DR DOS 6.0 and higher)
Similar to REM, but can be used also for in-line comments (following other CONFIG.SYS directives). See COMMENT. (Under MS-DOS/PC DOS the inline-comment is not ignored if put in double-quotes.)
Used to lead in in-line comments in addition to the ; directive.
?(DR DOS 3.41 and higher, Embedded DOS[nb 2] and FreeDOS only)
Displays a Yes/No query and (since DR DOS 5.0) optional message to ask the user for confirmation to execute the following CONFIG.SYS directives. (FreeDOS does not support optional messages, but optional conditions in conjunction with boot menus defined with MENU and MENUDEFAULT.) (DOS 6.0 and higher supports a similar feature by adding a ? after the corresponding CONFIG.SYS directive, f.e. DEVICE? or DEVICE?= instead of ?DEVICE. This variant is also supported by DR DOS.)
Unconditionally execute the following CONFIG.SYS directive.
Specifies if programs can be aborted on errors.
ACCDATE(MS-DOS 7.0 and higher only, ignored under DR-DOS 7.02 and higher)
 Configures which drives should store file access date stamps in the reserved area of directory entries in FAT file systems.
Conditional execution on AT-compatible machines only.
SHELLHIGH(DR-DOS 7.02 and higher and PTS-DOS and FreeDOS only)
Same as SHELL, but supports an additional SIZE parameter to define the size of the pre-allocation for COMMAND.COM to relocate its resident portion into the HMA (with /MH). Can be useful to avoid HMA fragmentation in conjunction with third-party shells which cannot load into the HMA. SHELLHIGH under PTS-DOS tries to load the shell into UMBs instead.)
Specifies the state of the keyboard modifiers like Shift Lock, Num Lock, Scroll Lock or Insert mode.
Specifies if files should be closed on Ctrl+Alt+Del.
Specifies if POST should be skipped on reboots.
STACK(ignored under DR‑DOS 3.31-5.0 and 7.02 and higher only)
Compatibility dummy - DR DOS 3.31 - 6.0 were mostly reentrant and had a different stacks layout.
STACKS(DOS 3.2 and Novell DOS 7 and higher, ignored under DR DOS 6.0 and PTS-DOS)
Specifies the number of stacks for hardware interrupts.
STACKSIZE(Embedded DOS 6-XL only)
Default stack size allocated for new threads.
STACKSHIGH(MS-DOS 7.0 and DR-DOS 7.02 and higher, and FreeDOS only)
Same as STACKS, but explicitly loads buffers into upper memory.
STRING(MS-DOS 3.0 only, ignored under DR-DOS 7.02 and higher)
Specifies the timeouts in CONFIG.SYS single-stepping and with SWITCH, GETKEY and ?, and the default answers for Yes/No queries and SWITCH selections. PTS-DOS only supports the timeout value.
The system can still boot if this file is missing or corrupted. However, this file, along with AUTOEXEC.BAT, is essential for the complete bootup process to occur with the DOS operating system. These files contain information that is used to customize the operating system for personal use. They also contain the requirements of different software application packages. A DOS system would require troubleshooting if either of these files became damaged or corrupted.
If CONFIG.SYS does not contain a SHELL directive (or the file is corrupt or missing), DOS typically searches for COMMAND.COM in the root directory of the boot drive. If this is not found, versions of DOS before 6.0 will not start up. MS-DOS 6.0/PC DOS 6.1 and Novell DOS 7 and higher will instead display a prompt to enter the path and filename of a command processor. This recovery prompt is also displayed when the primary command processor is aborted due to faults or if it is exited deliberately. (In the case of COMMAND.COM, the internal EXIT command is disabled only when the shell was started with /P.) This also provides limited means to replace the shell at runtime without having to reboot the system.
Since the MS-DOS 7.0 and higher COMMAND.COM executable is incompatible with DR-DOS, but typically resides in the root of drive C: in dual-boot scenarios with DR-DOS, DR-DOS 7.02 and higher no longer allow to bypass SHELL directives in (Ctrl+)F5/F7/F8 "skip"/"trace"/"step" modes. (Some later issues added (Ctrl+)F6 to reinvoke the former F5 "skip" behaviour in order to allow recovery from problems with invalid SHELL arguments as well.) Also, if no SHELL directive could be found when skipping CONFIG.SYS processing via (Ctrl+)F5 (and also with (Ctrl+)F7/F8, when the default file extension has been changed with SYS /DR:ext), the user is prompted to enter a valid shell file name before trying to load COMMAND.COM from the root. Pressing ↵ Enter without specifying a file will assume the former default.
Depending on the version, the size of the CONFIG.SYS file is limited to a few kilobytes under MS-DOS/PC DOS (up to 64 KB in most recent versions), whereas the file's size is unlimited under DR-DOS. This is because the former operating systems (since DOS 3.0) will compile the file into some tokenized in-memory representation before they sort and regroup the directives to be processed in a specific order (with device drivers always being loaded before TSRs), whereas DR-DOS interprets the file and executes most directives line-by-line, thereby giving full control over the load order of drivers and TSRs via DEVICE and INSTALL (for example to solve load order conflicts or to load a program debugger before a device driver to be debugged) and allowing to adapt the user interaction and change the flow through the file based on conditions like processor types installed, any type of keys pressed, load or input errors occurring, or return codes given by loaded software. This becomes particularly useful since INSTALL can also be used to run non-resident software under DR-DOS, so that temporary external programs can be integrated into the CONFIG.SYS control flow.
In MS-DOS/PC DOS 2.0 through 4.01, the length of the SHELL line was limited to 31 characters, whereas up to 128 characters are possible in later versions. DR-DOS even accepts up to 255 characters. CONFIG.SYS directives do not accept long filenames.
Dual booting DOS and Windows 9x
When installing Windows 95 over a preexisting DOS/Windows install, CONFIG.SYS and AUTOEXEC.BAT are renamed to CONFIG.DOS and AUTOEXEC.DOS. This is intended to ease dual booting between Windows 9x and DOS. When booting into DOS, they are temporarily renamed CONFIG.SYS and AUTOEXEC.BAT. Backups of the Windows 95 versions are made as CONFIG.W40 and AUTOEXEC.W40 files.
When Caldera DR-DOS 7.02/7.03 is installed on a system already containing Windows 95, Windows' CONFIG.SYS and AUTOEXEC.BAT retain those names. DR-DOS' startup files are installed as DCONFIG.SYS (a name already used in earlier versions of DR DOS) and AUTODOS7.BAT.
OS/2 / Windows NT
OS/2 uses the CONFIG.SYS file extensively for setting up its configuration, drivers and environment before the graphical part of the system loads.
In the OS/2 subsystem of Windows NT, what appeared as CONFIG.SYS to OS/2 programs was actually stored in the registry.
There are many undocumented or poorly documented CONFIG.SYS directives used by OS/2.
^The filename of the DOS configuration file CONFIG.SYS was derived from an identically named file used by the operating system DX-85M for the Durango F-85 family of computers introduced in 1978 - five years before the advent of MS-DOS 2.0/PC DOS 2.0 in 1983. Both files are text files and use various directives to configure the system and load drivers, however, the syntax differs.
^ abcdefat least General Software Embedded DOS-ROM 4.1 and Embedded DOS 6-XL
^ abcdPaul, Matthias R. (2017-08-14) [2017-08-07]. "The continuing saga of Windows 3.1 in enhanced mode on OmniBook 300". MoHPC - the Museum of HP Calculators. Archived from the original on 2018-05-01. Retrieved 2018-05-01. […] SYS […] /A Additionally copy [D]CONFIG.SYS and AUTOEXEC.BAT file(s). […] /B or /L Do not modify the Boot Sector in destination (with LOADER). […] /DR[:ext] Use other file extension for system files (BIN) (with LOADER). […]
^ abPaul, Matthias R. (2001-04-09). NWDOS-TIPs — Tips & Tricks rund um Novell DOS 7, mit Blick auf undokumentierte Details, Bugs und Workarounds [Tips & tricks for Novell DOS 7, with a focus on undocumented details, bugs and workarounds]. MPDOSTIP. Release 183 (in German) (3 ed.).
^ abcdPaul, Matthias R. (2002-04-06). "Re: [fd-dev] ANNOUNCE: CuteMouse 2.0 alpha 1". freedos-dev. Archived from the original on 2020-02-07. Retrieved 2020-02-07. […] DR-DOS 7.02+ has a [D]CONFIG.SYS directive to override the serial ports detected during System BIOSPOST, so tools like DEBUG […] are no longer required to update or change the port settings in segment 40h. […] insert a line to [D]CONFIG.SYS to make the port available to the System BIOS, DOS, and most drivers and applications: […] COM4=260,1 ; set I/O address for COM4 to 260h, timeout 1 […] Since old machines and DOS versions did not know about COM3 and COM4 at all, good software will try to use the data stored in segment 0040h […] Optionally hide the chosen mouse COM port from the system configuration to avoid any clobbering by old BASIC programs. […]
^Schneider, Stefanie (1994-06-17). "Das Ende von DOS ist nur noch eine Frage der Zeit" [The end of DOS is only a question of time]. Computerwoche (in German). Munich, Germany: IDG Business Media GmbH. Archived from the original on 2022-01-02. Retrieved 2022-01-02. […] So handelt es sich bei dem fuer Ende 1994, Anfang 1995 erwarteten MS-DOS 7.0 offenkundig um eine der grafischen Benutzeroberflaeche entledigte Version von Windows 4.0, die auch den Codenamen "Chicago" traegt. Dieses neue DOS soll der CW-Schwesterpublikation "PC-Welt" zufolge durch die neuen Parameter "standard" und "enhanced" in der Systemdatei Io.sys die leidigen Speicherbarrieren ueberwinden. […] (NB. The undocumented settings STANDARD and ENHANCED are optional parameters of the CONFIG.SYS directive DOS.)
^ abcdPaul, Matthias R. (2004-06-17). "Re: Random Lockups with DR-DOS 7.03". firstname.lastname@example.org; FidoNet conference: ALT_DOS. Archived from the original on 2019-04-28. Retrieved 2019-04-28. […] all MS-DOS versions prior to Windows 95 […] used a COM style COMMAND.COM file which has a special signature at the start of the file […] queried by the MS-DOS BIOS before it loads the shell, but not by the DR-DOS BIOS […] COMMAND.COM would […] check that it is running on the "correct" DOS version, so if you would load their COMMAND.COM under DR-DOS, you would receive a "Bad version" error message and their COMMAND.COM would exit, so DR-DOS would […] display an error message "Bad or missing command interpreter" (if DR-DOS was trying to load the SHELL= command processor after having finished CONFIG.SYS processing). In this case, you could enter the path to a valid DR-DOS COMMAND.COM (C:\DRDOS\COMMAND.COM) and everything was fine. Now, things have changed since MS-DOS 7.0 […] COMMAND.COM has internally become an EXE style file, so there is no magic […] signature […] to check […] thus no way for DR-DOS to rule out an incompatible COMMAND.COM. Further, their COMMAND.COM no longer does any version checks, but […] does not work under DR-DOS […] just crashes […] the PC DOS COMMAND.COM works fine under DR-DOS […]