Editing CONFIG.SYS (section)

== Usage ==
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-line interpreter|command shell]] specified in the <kbd>SHELL</kbd> 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 <kbd>[[key=value pair|name=value]]</kbd> 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 driver]]s and [[terminate-and-stay-resident program]]s (TSRs), into memory.

In DOS, CONFIG.SYS is located in the [[root directory]] of the drive from which the system was booted.

The filename is also used by {{ill|Disk Control Program|de}} (DCP), an MS-DOS derivative by the former East-German [[VEB Robotron]].<ref name="DCP_2016"/>

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<!-- in 1985/1986 --> did not support CONFIG.SYS files at all, later versions<!-- since when exactly? --> of [[Concurrent DOS 386]] and [[Concurrent DOS XM]]<!-- since 1987. What about Concurrent DOS 286? -->, as well as [[Multiuser DOS]], [[Datapac System Manager|System Manager]] and [[REAL/32]] will probe for '''CCONFIG.SYS'''<!-- really CCONFIG.SYS, not CCONFIG.INI as under CCI Multiuser DOS? --> (with "C" derived from "Concurrent") instead of CONFIG.SYS. Some versions of Multiuser&nbsp;DOS use a filename of '''CCONFIG.INI''' instead,<ref name="CCI_1997_PRINTDOC"/><ref name="CCI_1997_HELP"/> whereas <!-- probably other Multiuser DOS issues as well, and -->REAL/32<!-- 7.90--> is known to look for '''MCONFIG.SYS'''. These operating systems support many additional and different configuration settings (like <kbd>INIT_INSTALL</kbd>) not known under MS-DOS/PC&nbsp;DOS, but they are stored in the binary repository named '''CCONFIG.BIN''' rather than in '''CCONFIG.INI'''.<ref name="CCI_1997_PRINTDOC"/><ref name="CCI_1997_HELP"/> Both files are typically modified through a configuration utility named <kbd>CONFIG.EXE</kbd> only.<!-- This holds true at least for CCI Multiuser DOS 7.22 Gold. --><ref name="CCI_1997_PRINTDOC"/><ref name="CCI_1997_HELP"/>

Under [[DR&nbsp;DOS]]&nbsp;3.31, [[PalmDOS]]&nbsp;1.0, [[Novell&nbsp;DOS]]&nbsp;7, [[OpenDOS]]&nbsp;7.01, and DR-DOS&nbsp;7.02 and higher, a file named '''DCONFIG.SYS''' (with "D" derived from "DR&nbsp;DOS"), if present, will take precedence over CONFIG.SYS.<ref name="Paul_1997_NWDOSTIP"/><ref name="Caldera_1998_USER"/><ref name="Paul_2004_CHAIN"/><ref name="Paul_2017_DR"/> Since DR&nbsp;DOS&nbsp;6.0 this was used in conjunction with disk compression software, where the original boot drive&nbsp;C: would become drive&nbsp;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&nbsp;7.01 and DR-OpenDOS&nbsp;7.02 will look for a file named '''ODCONFIG.SYS''',<ref name="Paul_1997_OD-A3"/><ref name="Paul_2001_NWDOSTIP"/><ref name="Paul_2004_CHAIN"/> whereas some issues of DR-DOS&nbsp;7.02 and higher will instead also look for '''DRCONFIG.SYS'''.<ref name="Paul_2004_CHAIN"/>
Further, under DR&nbsp;DOS&nbsp;6.0 and higher, the <code>SYS&nbsp;/DR:ext</code> command can be used to change the default file extensions.<ref name="Paul_1997_OD-A3"/><ref name="Novell_1993_LOADER"/><ref name="Paul_2017_DR"/> For example, with <code>SYS&nbsp;/L&nbsp;/DR:703</code> 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.<ref name="Paul_1997_NWDOSTIP"/> The <kbd>SHELL</kbd> directive is enhanced to provide means to specify alternative AUTOEXEC.BAT files via <kbd>/P[:filename.ext]</kbd> and in this specific scenario, COMMAND.COM will accept file extensions other than "<code>.BAT</code>" as well (both features are also supported by [[4DOS]]).<ref name="4DOS_8.00_HELP"/> Under DR&nbsp;DOS&nbsp;6.0 and higher, the CONFIG.SYS directive <kbd>CHAIN=filespec</kbd> 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.<ref name="Paul_1997_NWDOSTIP"/><ref name="Paul_2004_CHAIN"/> DR-DOS&nbsp;7.02 and higher optionally support an additional parameter as in <kbd>CHAIN=filespec,label</kbd> to jump to a specific <kbd>:label</kbd> in the given file.<ref name="Paul_1997_OD-A3"/><ref name="Paul_2001_NWDOSTIP"/><ref name="Paul_2004_CHAIN"/>
DR-DOS 7.03 and higher support a new <code>SYS&nbsp;/A</code> parameter in order to copy the corresponding CONFIG.SYS and AUTOEXEC.BAT files along with the system files.<ref name="Paul_2017_DR"/>

[[FreeDOS]] implements a similar feature with its '''FDCONFIG.SYS''' configuration file. [[RxDOS]]&nbsp;7.24 and higher use '''RXCONFIG.SYS''' instead.<ref name="RxDOS724"/> [[PTS-DOS]] uses '''CONFIG.PTS'''.

Both CONFIG.SYS and AUTOEXEC.BAT can be found included in the [[Root directory|root folder]] of [[Windows&nbsp;95]], and [[Windows&nbsp;98]] boot drives, as they are based on DOS. Typically, these files are left empty, with no content.

[[Windows&nbsp;Me]] does not even parse the CONFIG.SYS file during the Windows boot process,<ref name="Microsoft_2000_Windows-ME"/> loading [[environment variable]]s from the [[Windows Registry]] instead:
:
: <kbd>HKLM\System\CurrentControlSet\Control\SessionManager\Environment</kbd>

Under [[FlexOS]]<!-- at least in FlexOS 286 1.13 as of 1986/1987 -->, CONFIG.SYS is a binary file defining the resource managers and device drivers loaded.