Since 1960 the complexity of the integrated circuits, i.e. the number of electronic elements on one chip, continued to double every year. Today we haven't yet seen any significant deviation. from this exponential law. Nor are there any signs that the process is slowing down. The technology is still far from the fundamental limits imposed by the laws of physics: further miniaturization is less likely to be limited by the laws of physics than by the laws of economics.
The culmination of all these advancements was the microprocessor, which has become virtually synonymous with microelectronics, but should not be confused with it.The microprocessor emerged in consequence of the progress of the microcalculators.
As we know, the electronic calculator in all but the latest versions uses hardwired logic. The arithmetic functions, or the operating program instructions, are embedded in the chips while the application program is in the user's head — his instructions yield the desired calculations.
M.E. Hoff, a young Intel Company engineer, envisaged a different way of employing the new electronic capabilities of the calculator. In 1969 he found himself in charge of a project that Intel took on for Busicom, a Japanese calculator company.
Busicom wanted Intel to produce calculator'chips of Japanese design. The logic circuits were spread around eleven chips and the complexity of the design would have taxed Intel capabilities — it was then a small company. Hoff saw a way to improve on the Japanese design by making a bold technological leap. The fact is Intel had pioneered in the development of semiconductor memory chips to be used in large computers. In the intricate innards of a memory chip, Hoff knew, it was possible to store a program to run a minuscule computing circuit.
In his preliminary design, Hoff condensed the layout onto three chips. He put the computer's "brain", its central processing unit, on a single chip of silicon. That was possible because the semiconductor industry had developed a means of inscribing very complex circuits on tiny surfaces. A master drawing, usually 500 times as large as the actual chip, is reduced photographically to microminiature size. The photo images are then transferred to the chip by the-technique similar to photoengraving.
Hoff's Central Processing Unit (CPU) on a chip became known as the microprocessor. The CPU comprised a logic unit, an arithmetic unit and a control unit. To the microprocessor Hoff a-ttactied two memory chips, one to move data in and out of the CPU and one to provide the program to drive the CPU. Hoff now hadf in hand a rudimentary general-purpose computer' (microcomputer) that could not only run a complex calculator, but also control, for example, an elevator or a set of traffic lights, or a wasliing-mnchine, or a multifunction digital watch, and perform a great many other tasks, depending on its program only.
So the microprocessor is an integrated circuit which hai the properties and fulfils the role of a complete central processing unit of a computer. This means that the circuit does not just react in a fixed, pre-programmed way to an input signal to produce an output signal. The main feature of the microprocessor is that its response and its logic can be altered. In other words, the microprocessor can be programmed in different ways rather than react in one pre-programmed way only.
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For logic and systems designers the appearance of the microprocessor brought with it a dramatic change in lln' way they employed electronics. They could now replace all those rigid hardwired logic systems with microcomputers because they could store program sequences in the labyrinthine circuits of I he memory chips instead of using individual logic chips and discrete components to implement the program. Engineers could thus substitute program code words for hardware parts.
It took about three years before the first devices reached the market but in the meantime about a hundred different microprocessors had become available. As with all microelectronic products, the capabilities of microprocessors advanced rapidly and the sophistication of circuits increased day by day.
After other Intel engineers who took over the detailed design work got through with it, Hoff's invention contained 2250 microminiaturized transistors on a chip slightly less th;m one-sixth of an inch long and one-eighth of an inch wide, and each of those microscopic transistors was roughly equal to an ENIAC vacuum tube. Intel labelled the microprocessor chip 4004 and the whole microcomputer MCS-4 (microcomputer system-4). Despite its small size, the 4004 just about matched ENIAC computational power. It also matched the capability of an IBM machine of the early 1960s whose central processing unit (CPU) took up the space of an office desk.
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