FCBS (CONFIG.SYS directive)

CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system during boot. CONFIG.SYS was introduced with DOS 2.0.

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 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 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.

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