tation, it was the introduction of the Burroughs B5000 in 1961 that captured the idea that ultimately proved to be the way forward: disjoint CPUs concurrently executing different instruction streams, but sharing a common memory. In this regard (as in many) the B5000 was at least a decade ahead of its time. But it was not until the 1980s that the need for multiprocessing became clear to a wider body of researchers, who over the course of the decade explored cache coherence protocols (for example, the Xerox Dragon and DEC Firefly), prototyped parallel operating systems (for example, multiprocessor Unix running on the AT&T 3B20A), and developed par-

allel databases (for example, Gamma at the University of Wisconsin).

In the 1990s, the seeds planted by researchers in the 1980s bore the fruit of practical, shipping systems, with many computer companies (for example, Sun, SGI, Sequent, Pyramid) placing big bets on symmetric multiprocessing. These bets on concurrent hardware necessitated corresponding bets on concurrent software: if an operating system cannot execute in parallel, not much else in the system can either. These companies came to the realization (independently) that their operating systems must be rewritten around the notion of concurrent execution. These rewrites took place in

the early 1990s and the resulting systems were polished over the decade. In fact, much of the resulting technology can today be seen in open source operating systems like OpenSolaris, FreeBSD, and Linux.

Just as several computer companies made big bets around multiprocessing, several database vendors made bets around highly parallel relational databases; upstarts like Oracle, Teradata, Tandem, Sybase and Informix needed to use concurrency to achieve a performance advantage over the mainframes that had dominated transaction processing until that time. ⁵ As in operating systems, this work was conceived in the