 At startup, a text file called CONFIG.SYS is read from the boot disk.The contents of the file decides, which memory the DOS will use to keep the bulk of its code and drivers (the choice is between the extended memory and two areas of the conventional 64K), how many files can be opened at a time, the size of the buffer cache and its location in the memory (same choices as for the DOS code), the default environment variables, and which drivers to load.Apart from the "main" one, there can be up to 9 alternative configuration files (or 19 as of v.4.45).