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When John von Neumann, first suggested storing a sequence of
instructions - a program - in the same memory as the data, it
was a revolutionary idea. According to his article the First
Draft of a Report on the EDVAC, written in 1945, the central
processing unit or CPU conssted of three main parts:
- the Arithmetical-Logic unit.
- the Control unit.
- the Memory,
Fifty-two years later, nearly all CPUs still have a von Neumann
architecture. The first CPU to appear on a single chip was the
Intel 4004. Introduced in 1971 the 4004 powered one of the first
portable electronic calculators. The first CPU to make it into
a home computer was the Intel 8080, a complete 8-bit computer
on one chip, introduced in 1974. In 1979 the Intel 8088 appeared
on the market and was used in 1982 to power the first IBM PC.
The underlying principles of all computer processors are the
same. They all take signals in the form of 0s and 1s
(thus binary signals), manipulate them according to a set of
instructions, and produce output in the form of 0s and 1s. The
voltage on the line at the time a signal is sent determines
whether the signal is a 0 or a 1. On a 3.3-volt system, an application
of 3.3 volts means that it's a 1, while an application of 0
volts means it's a 0.
Processors work by reacting to an input of 0s and 1s in specific
ways and then returning an output based on the decision. The
decision itself happens in a circuit called a logic gate,
each of which requires at least one transistor, with
the inputs and outputs arranged differently by different operations.
The fact that today's processors contain millions of transistors
gives one a slight hint as to just how complex the logic system
is.
The CPU's logic gates work together to make decisions using
Boolean logic, which is based on the algebraic system
established by mathematician George Boole. The main Boolean
operators are AND, OR, NOT, and NAND
(not AND); many combinations of these are possible as well.
- AND gates outputs a 1 only if both its inputs were
1s.
- An OR gate outputs a 1 if at least one of the inputs
was a 1.
- the NOT gate takes a single input and reverses it,
outputting 1 if the input was 0 and vice versa.
- NAND gates are very popular, because they use only
two transistors instead of the three in an AND gate yet provide
just as much functionality. In addition, the processor uses
gates in combination to perform arithmetic functions; it can
also use them to trigger the storage of data in memory.
Intel's first claim to fame lay in its high-level integration
of all the processor's logic gates into a single complex processor
chip - the Intel 4004 - released in late 1971.
This was 4-bit microprocessor, intended for use in a calculator.
It processed data in 4 bits, but its instructions were 8 bits
long. Program and data memory were separate, 1KB and 4KB respectively.
There were also sixteen 4-bit (or eight 8-bit) general purpose
registers. The 4004 had 46 instructions, using only 2,300 transistors
in a 16-pin DIP and ran at a clock rate of 740kHz (eight clock
cycles per CPU cycle of 10.8 microseconds).
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