CONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar 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.
Read more about CONFIG.SYS: Usage, CONFIG.SYS Directives, Examples, Issues, Dual Booting DOS and Windows 9x, OS/2 / NT