A text file containing special system configuration commands. It is found in the root directory on an MS-DOS computer, typically on drive C (the hard disk). It is read by MS-DOS at boot time, after the setup has been read from CMOS RAM and before running AUTOEXEC.BAT. It can be modified by the user.

Some example commands which CONFIG.SYS might contain are:

        DEVICE=C:\DOS\HIMEM.SYS /testmem:off

Load the extended memory manager.

        DEVICE=C:\DOS\EMM386.EXE RAM

Load the expanded memory manager.

        BUFFERS=10,0

Specify memory for disk buffers.

        FILES=70

Set the number of files that can be open at once.

        DOS=UMB

DOS is located in UppeMemoryBlock.

        LASTDRIVE=Z

Disk drives are A: to Z:.

        FCBS=16,0

Set the number of file control blocks.

        DEVICEHIGH /L:1,12048 =C:\DOS\SETVER.EXE

Report the DOS version to older programs.

        DOS=HIGH

DOS should maintain a link to UMB.

        COUNTRY=358,437 C:\DOS\COUNTRY.SYS

Set the country code for some programs.

        STACKS=9,256

Set dynamic stacks for hardware control.

        SHELL=C:\DOS\COMMAND.COM C:\DOS\ /E:1024 /p

Set the location of the command interpreter.