This document provides an overview of the Disk Operating System (DOS). It discusses the history and versions of DOS, which was first created by Microsoft in 1981 for IBM computers. It then describes some key aspects of DOS, including that it is a single-user operating system, and how the boot process loads DOS files from secondary memory to primary memory. It also discusses files and directories structure in DOS, including file naming conventions, directories, drives, and important system files used by DOS like IO.SYS, MSDOS.SYS, COMMAND.COM, AUTOEXEC.BAT and CONFIG.SYS.