CONFIG.SYS

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CONFIG.SYS is a plain text startup file used by DOS and OS/2 to set boot options that can’t be changed after the system starts. Each line is a directive or a DEVICE line that loads a driver or a terminate-and-stay-resident (TSR) program. It first appeared with DOS 2.0, and as Windows evolved, its use declined.

Where it sits and how it runs
- The file is kept in the root of the boot drive and is read during boot. After processing CONFIG.SYS, the system loads the shell specified by the SHELL directive (or COMMAND.COM if none) and that shell then runs AUTOEXEC.BAT.
- The file is ASCII text and is edited with a text editor.

Boot behavior and recovery
- The system can boot if CONFIG.SYS is missing or corrupted, but certain problems may require recovery.
- If there’s no valid SHELL line, DOS looks for COMMAND.COM in the root. Some older DOS versions may fail to boot, while newer ones may prompt for a command processor path.

Loading order and differences
- In MS-DOS/PC DOS, CONFIG.SYS could be small (a few KB; newer versions allow up to about 64 KB).
- DR-DOS interprets CONFIG.SYS line by line, supports INSTALL, and lets you customize load order (drivers, TSRs) and flow based on conditions. This gives more control and can integrate nonresident software.
- Directives in CONFIG.SYS don’t support long filenames.

Windows and OS history
- Windows 95/98 still used CONFIG.SYS and AUTOEXEC.BAT in the boot drive root, but they’re often empty. Windows Me doesn’t use CONFIG.SYS at all and environment variables are stored in the Registry.
- For dual-boot setups, Windows could rename the files to CONFIG.DOS and AUTOEXEC.DOS, with backups like CONFIG.W40.
- DR-DOS uses its own startup files (for example, DCONFIG.SYS and AUTODOS7.BAT) in Windows/DOS mixes.
- OS/2 uses CONFIG.SYS to configure options and drivers before the graphical system starts; OS/2 in Windows NT stores similar settings in the Registry.