<P> Transistor - based computers had several distinct advantages over their predecessors . Aside from facilitating increased reliability and lower power consumption, transistors also allowed CPUs to operate at much higher speeds because of the short switching time of a transistor in comparison to a tube or relay . The increased reliability and dramatically increased speed of the switching elements (which were almost exclusively transistors by this time), CPU clock rates in the tens of megahertz were easily obtained during this period . Additionally while discrete transistor and IC CPUs were in heavy usage, new high - performance designs like SIMD (Single Instruction Multiple Data) vector processors began to appear . These early experimental designs later gave rise to the era of specialized supercomputers like those made by Cray Inc and Fujitsu Ltd . </P> <P> During this period, a method of manufacturing many interconnected transistors in a compact space was developed . The integrated circuit (IC) allowed a large number of transistors to be manufactured on a single semiconductor - based die, or "chip". At first, only very basic non-specialized digital circuits such as NOR gates were miniaturized into ICs . CPUs based on these "building block" ICs are generally referred to as "small - scale integration" (SSI) devices . SSI ICs, such as the ones used in the Apollo Guidance Computer, usually contained up to a few dozen transistors . To build an entire CPU out of SSI ICs required thousands of individual chips, but still consumed much less space and power than earlier discrete transistor designs . </P> <P> IBM's System / 370, follow - on to the System / 360, used SSI ICs rather than Solid Logic Technology discrete - transistor modules . DEC's PDP - 8 / I and KI10 PDP - 10 also switched from the individual transistors used by the PDP - 8 and PDP - 10 to SSI ICs, and their extremely popular PDP - 11 line was originally built with SSI ICs but was eventually implemented with LSI components once these became practical . </P> <P> Lee Boysel published influential articles, including a 1967 "manifesto", which described how to build the equivalent of a 32 - bit mainframe computer from a relatively small number of large - scale integration circuits (LSI). At the time, the only way to build LSI chips, which are chips with a hundred or more gates, was to build them using a MOS process (i.e., PMOS logic, NMOS logic, or CMOS logic). However, some companies continued to build processors out of bipolar chips because bipolar junction transistors were so much faster than MOS chips; for example, Datapoint built processors out of transistor--transistor logic (TTL) chips until the early 1980s . At the time, MOS ICs were so slow that they were considered useful only in a few niche applications that required low power . </P>

Central processing unit (cpu) parts definition & function