Buses, interrupts and parallel hardware

In 1967 Gene Amdahl, one of the architects of the IBM System/360, gave a short talk at the AFIPS Spring Joint Computer Conference that is still quoted in every course on parallel computing. His point was arithmetic: the part of a job that cannot be parallelised caps the speed-up, however many processors you add.

For decades the warning seemed academic, because single processors kept getting faster. In 2001 IBM's Power4 put two processor cores on one chip, a first by IBM's own account, and within a few years the whole industry had followed. Amdahl's arithmetic became everyday engineering.