storage device that reads and writes data to disks. DOS: Disk operating system. The first operating system for IBM personal computers, written by Microsoft and often called MS-DOS. Using simple commands, it told the computer what to do. Windows has now replaced DOS on PCs. The operating system allows programs to run on the computer. floppy disk: A type of portable data storage. The first floppy disk measured eight inches diagonally. The next version measured 5.25 inches and was capable of holding three times as much storage. The 3.5-inch disk, which was encased
