MIDAS (operating system)

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"M-DOS" redirects here. For DR MDOS, see Multiuser DOS. For the multitasking MS-DOS, see MS-DOS 4.0 (multitasking). For other uses, see MDOS (disambiguation).
"My DOS" redirects here. For the similarly named Atari operating system, see Wordmark Systems MyDOS.

MIDAS (Microsoft Interrupt Driven Asynchronous System (aka My DOS), MDOS or M-DOS as it was named before 1980) refers to an operating system that was designed by Marc McDonald of Microsoft in 1979.[1][2] It was designed for the 8080/Z80 microprocessors,[1][2] popular due to CP/M at the time.

The Microsoft Disk Operating System was designed to use terminal input/output and offered the following features:

  • Terminal interrupts for input and output
  • Ability to turn output terminal interrupts on and off without sending garbage characters
  • Clock interrupts used for MDOS/MIDAS scheduling and time accounting
  • Disk interrupts taking advantage of multiple sector I/O
  • 8,[2] 10, 12, and 16-bit File Allocation Table


MDOS/MIDAS included many systems calls (more than 65). Its interrupt driven design enabled the system software to use idle I/O time (when slow devices are being accessed) to do other tasks. It was designed to use structures (similar to structures in the C language) to keep tabs on every user and every task. Thus a high priority operation could stop a user task and restart it without losing anything. MDOS/MIDAS was designed to occupy only 9 kilobytes for its code and 2-3 kilobytes for its data.[citation needed] Multi-user installations would need additional 50 bytes/user and 550-600 bytes per terminal. Upgrading the single user version to the multi-user would be no different from a program.

MDOS/MIDAS was designed to run on any 8080 or Z80 processor (and even support the 8086 and the Z8000 processors at a later stage). It used data blocks such as a File Data Block (FDB) handling device independent I/O, a Unit Data Block (UDB) for physical disk I/O, a Structure Data Block (SDB) for logical data I/O and a Line Data Block (LDB) for Terminal I/O.

Microsoft abandoned MIDAS when they bought a Unix license to release as XENIX. The unrelated QDOS/86-DOS from Seattle Computer Products borrowed the FAT concept from Standalone Disk BASIC-86 and later became MS-DOS, shipped as PC DOS with the IBM PC in 1981/1982.

See also[edit]


  1. ^ a b Manes, Stephen; Andrews, Paul (1993). Gates: How Microsoft's Mogul Reinvented an Industry—and Made Himself the Richest Man in America. Doubleday. ISBN 0-385-42075-7. 
  2. ^ a b c Duncan, Ray (1988). The MS-DOS Encyclopedia - version 1.0 through 3.2. Microsoft Press. ISBN 1-55615-049-0.