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Real Time UNIX can be viewed as a technological double helix, with two independent strands, now intertwining, but eventually diverging; the one evolving into a superset of UNIX as a distributed OS, the other becoming not only a standard feature on every general-purpose UNIX platform, but ultimately embedded into the base fabric of our society's computerization.
REAL TIME
From beginnings in industrial automation and laboratory instrumentation, Real Time has migrated to telecommunications, C3I, simulation, point-of-sale and OLTP, because it responds to external asynchronous events within a predictable, and fast, timeframe.
The better Real Time achieved its results, the more it became a dedicated, proprietary technology, like a one-sport athlete. If it ever wanted to become a team player it had to find a way to associate itself with others striving for similar, but more universal goals. UNIX looked like the only game in town.
With its timesharing philosophy UNIX seems the ideal team sport. And it brings other important advantages: an advanced OS, networking, user interface, standards, portability, and off-the-shelf applications, along with considerable baggage--30 Mbytes of disk space before loading applications, and four Mbytes of memory without X-windows.
Timesharing is where the crunch comes in. While it makes UNIX a successful development system, its structure lacks many of the necessities for Real Time: prioritizing, preemption, predictability, contiguous file system and closeness to hardware. UNIX also carried the overhead of a fully-functional operating system. While these deficiencies would seem daunting, UNIX lucked out anyway--the alternatives were antithetical to the concept of an open environment.
The challenge was how to combine UNIX and Real Time symbiotically. A number of options have been explored and proved successful. A full-function UNIX OS could be modified, for example, to improve its responsiveness to Real Time requirements. Or an additional executive could be incorporated at the board level to off-load event-handling from a controlling...