The configuration file for DOS systems. Whenever a DOS computer boots up, it reads the CONFIG.SYS file (if it exists) and executes any commands in it. The most common commands are BUFFERS= and FILES=, which enable you to specify the buffer size and the number of files that can be open simultaneously. In addition, you can enter commands that install drivers for devices.

Following is an example of a CONFIG.SYS file.

DEVICE=C:\DOS\HIMEM.SYS
DEVICE=C:\DOS\EMM386.EXE RAM
BUFFERS=15,0
FILES=30
DOS=HIGH,UMB
LASTDRIVE=E
STACKS=9,256

In this example, the DEVICE parameter loads the HIMEM.SYS and EMM386.EXE device drivers. HIMEM.SYS manages Extended memory. The EMM386.EXE with the RAM option allows access to the upper memory area and acts like Expanded memory.
The BUFFERS statement assigns 15 buffers for transferring information to and from disk drives and no look-ahead buffers which are used to hold data in anticipation of use. This is the recommended setting when using SMARTDRV, which provides file caching using available Extended memory.
FILES indicates that 30 files can be open simultaneously on this system.
The DOS=HIGH,UMB runs DOS in high memory and allows access to the upper memory area.
The LASTDRIVE command reserves space for five drives, which will be labeled A through E.
The STACKS setting indicates that nine stacks will handle the hardware interrupts (this is the
default) and each stack will be 256 bytes in size.