MIDAS (operating system)
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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. It was designed for the 8080/Z80 microprocessors, 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, 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. 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.
- 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.
- Duncan, Ray (1988). The MS-DOS Encyclopedia - version 1.0 through 3.2. Microsoft Press. ISBN 1-55615-049-0.