CONFIG.SYS is a text file containing DOS (Disk Operating System) commands that tell the operating system how the computer is initially set up. More specifically, CONFIG.SYS commands specify memory device drivers and programs that control hardware devices; enable or disable system features; and set limits on system resources.

CONFIG.SYS loads at startup before the autoexec.bat (automatic execute.batch program) file. Because CONFIG.SYS is a text file, it is editable in any text-editing program.