PC DOS 2.0

PC DOS 2.x

Later, a group of Microsoft programmers (primarily Paul Allen, Mark Zbikowski and Aaron Reynolds) began work on PC DOS 2.0. Completely rewritten, DOS 2.0 added subdirectories and hard disk support for the new IBM XT, which debuted in March 1983. A new 9-sector format bumped the capacity of floppy disks to 360 KB. The Unix-inspired kernel featured file handles in place of the CP/M-derivative file control blocks and loadable device drivers could now be used for adding hardware beyond that which the IBM PC BIOS supported. BASIC and most of the utilities provided with DOS were substantially upgraded as well. A major undertaking that took almost 10 months of work, DOS 2.0 was more than twice as big as DOS 1.x, occupying around 28 KB of RAM compared to the 12 KB of its predecessor. It would form the basis for all Microsoft consumer-oriented OSes until 2001, when Windows XP (based on Windows NT) was released.