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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

Microcomputers versus Mainframe Computers

Microcomputers cost With this general idea of what a computer is, it is instructive to compare
less and occupy less a general-purpose mainframe computer and a microcomputer. A
space than the main- microcomputer is just a small computer, typically thousands of times smaller
frame computers com- than the large, general-purpose mainframe computers used by banks and large
monly used by corporations. Microcomputers cost much less than mainframes, and their
governments and large computing power and speed is only a fraction of that of a mainframe. A
businesses. However, typical mainframe computer costs from tens of thousands of dollars to
microcomputers operate millions of dollars and is capable of hundreds of thousands of arithmetic
more slowly and are less operations per second (additions, subtractions, multiplications, and divisions).
accurate in mathematical A microcomputer costs from a little less than $1,000 to $15,000 and can
operations. perform several thousand operations per second. More important for
mathematical calculations than the speed of the operation is the accuracy of
the operation. Mainframe computers use up to 64 bits to obtain high accuracy
when doing arithmetic. The decimal equivalent for the largest number that
can be represented using 64 bits is roughly 10 to the 19th power (1 followed
by 19 zeros). A typical engine control microcomputer does arithmetic using
only 16–32 bits. The largest decimal number that can be represented in 8 bits
is 127, if one of the bits is used as a sign bit to indicate whether the number is
positive or negative.

Programs
A program is a set of A program is a set of instructions organized into a particular sequence to
steps (instructions) in a do a particular task. The first computers were little more than fancy calculators.
logical order. The com- They did only simple arithmetic and made logic decisions. They were
puter follows these steps programmed (given instructions) by punching special codes into a paper tape
to perform a given task. that was then read by the machine and interpreted as instructions. A program
containing thousands of instructions running on an early model machine
might require yards of paper tape. The computer would process the program by
reading an instruction from the tape, performing the instruction, reading
another instruction from the tape, performing the instruction, and so on until
the end of the program. Reading paper tape was a slow process compared with
the speed with which a modern computer can perform requested functions. In
addition, the tape had to be fed through the computer each time the program
was run, which was cumbersome and allowed for the possibility of the tape
wearing and breaking.
To minimize the use of paper tape, and to increase computational
efficiency, a method was invented to store programs inside the computer. The
program is read into a large electronic memory made out of thousands of data
latches (flip-flops), one for each bit, that provide locations in which to store
program instructions and data. Each instruction is converted to binary

UNDERSTANDING AUTOMOTIVE ELECTRONICS 101