If you have a PC you probably have a file named CONFIG.SYS on the disk you use to start, or boot, the computer. A file that is automatically read at start up by the MS-DOS operating system used to load the DEVICE DRIVERS required by the various peripherals connected to the computer, and to set the correct configuration parameters for them. This file contains a variety of instructions used to customize the way your system works. Each time you start or restart the machine, the CONFIG.SYS file is read by the operating system (DOS or OS/2), which configures itself according to the instructions in the file. In DOS the typical CONFIG.SYS file starts with commands that tell the operating system how many files can be in use at anyone time, and how many buffers to create, like this:
The file may also have commands that load into memory one or more device drivers, which are little pieces of software that allow the system to communicate with and control some device like memory or a mouse.
DEVICE =C: \DOS\H IMEM. SYS
DEVICE =C \MOUSE\MOUSE. SYS
You can change the contents of the CONFIG.SYS with almost any word processor or text editor, since the file is just a text file. You do have to make sure to save your work as a plain ASCII file without any word processor-type formatting.
You don’t absolutely need a CONFIG.SYS file to use your computer, because DOS makes assumptions about how to set things up if you don’t tell it explicitly what to do. However, certain programs won’t run unless the correct settings are specified in CONFIG.SYS.