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1996-03-27
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The Binary Number System
converted by: Unreal
All digital computers are basically collections of switches. Each
switch has two possible positions: open, "0", or off; and closed, "1", or
on. The two positions form the basis of the binary (or two-valued: 0, 1)
number system. Otherwise, the computer uses numbers as we do in familiar
decimal system.
-= Binary Numbers =-----------------------------------------------------------
Any number can be represented in binary as well as in decimal
form. For example, the number 1,985 expressed in binary form is
11111000001. As is the case with decimal numbers, we can interpret a
binary number as the sum of a series of powers of the base number. For
example, in decimal notation:
(p = to the power of)
1,985 = 1 x 10p3 + 9 x 10p2 + 8 x 10p1 + 5 x 10p0
that is,
1,985 = 1 x 1,000 + 9 x 100 + 8 x 10 + 5 x 1
Similarly, in binary notation:
11111000001 = 1 x2p10 + 1 x 2p9 + 1 x 2p8 + 1 x 2p7 + 1 x 2p6 + 0
x 2p5 + 0 x 2p4 + 0 x 2p3 + 0 x 2p2 + 0 x 2p1 + 1 x 2p0
or, in decimal notation:
11111000001 = 1 x 1,024 + 1 x 512 + 1 x 256 + 1 x 128 + 1 + 64 +
0 x 32 + 0 x 16 + 0 x 8 + 0 x 2 + 1 x 1
Numbers can be used to represent nonnumerical quantities, such as
letters of the alphabet and punctuation marks. A standard code is often
used to assign specific patterns of binary numbers to printable
characters.
-= Logic Circuits =-----------------------------------------------------------
Binary numbers can also be used to represent the results of
logical operations. For example, if 1 represents TRUE and 0 represents
FALSE, we can represent all logical functions (except "maybe") by
sequences of binary numbers. We can then arrange the circuits of a
computer to make logical tests on statements given to the machine. These
logic circuits enable the CPU (central processor unit) to react to an
incoming instruction or piece of data.
For example, an important feature of the computer (and of the
human mind) is the ability to decide between two alternatives. Suppose the
computer must decide who in a group of people are at least 21 years of
age. The computer examines the item of data labeled "age" for each member
of the group. Each time it examines the age, a logic circuit in the CPU
compares the binary number for the age with the binary number for 21. If
the age number is equal to or greater than 21, the circuit produces a 1,
for TRUE. If the age number is less than 21, the circuit produces a 0.
The type of circuit used for logical comparisons is called a gate
because the circuit acts to pass on 1's, like a gate in a fence, only for
the logical conditions for which it is set. there are two basic kinds of
gate: AND and OR, representing the two basic kinds of logical decision to
be made. In the simplest form, each gate has two inputs and one output.
An AND gate produces a 1 at its output only if both of its inputs are also
1. An OR gate produces a 1 at its output if either or both of its inputs
are 1's. A third kind of gate, called an XOR gate (exclusive OR), produces
a 1 at its output only if one input but not the other is a 1. In other
words, and AND gate produces 0's unless both inputs are 1's; an OR gate ;
produces 0's only if both inputs are 0; an XOR gate demands one of each.
To see how this works, suppose a computer is in charge of a baking
a roast in a microwave oven. The owner of the oven programs it to stop
cooking the roast when either the preset time has elapsed or the
thermometer in the roast reads 140 degrees F. The logic gate used by the
computer in the oven for this task is an OR gate. At the start of the
cooking process, neither the timer output nor the thermometer output
satisfies the conditions set in the oven (timer output greater than or
equal to 140). Therefore , the OR gate will produce a 0 at its output,
since both inputs are 0. At some point one or both of the conditions will
be met and the OR gate will produce a 1. thereby shutting down the oven.
Now suppose the chef, knowing it is possible to get thermometer
reading that is too high if the thermometer is touching the bone in the
roast, sets the oven to stop cooking when the thermometer has at least
reached a certain point and the proper cooking time has elapsed. The
difference between this and the previous situation is that an AND gate is
used in the microwave computer; the gate produces the required 1 only when
both the temperature has reached 140 degrees F. and the roast has cooked
for 30 minutes. All the complex logical operations of much more
sophisticated computers can be reduced to combinations of logic-gate
operations much like those described.
______________________________________________________________________________
Logic Gate Illustration:
* A
* A * B ┌───┐ ┌───┘ *───┐ ┌───┐
┌────┘ *───┘ *────│ │ <- Bell ┌─────│ * B │───│ │
│ └───┘ │ └───┘ *───┘ └───┘
─┼─ │ ─┼─ │
┌───┐ <- Battery │ ┌─┴─┐ │
└───┘ │ └───┘ │
─┼─ │ ─┼─ │
└───────────────────────┘ (a) └───────────────────────┘ (b)
A
A B ┌───┐ ┌───────*───┐ ┌───┐
┌────────*───────*────│ │ ┌─────│ B │───│ │
│ └───┘ │ └───────*───┘ └───┘
─┼─ │ ─┼─ │
┌───┐ │ ┌─┴─┐ │
└───┘ │ └───┘ │
─┼─ │ ─┼─ │
└───────────────────────┘ (c) └───────────────────────┘ (d)
Logic Gates can be though of as doorbell circuits: (a) an AND gate, where
the two switches A and B are connected in series and both must be close to
the ring bell; (b) an OR gate, where the two switches are connected in
parallel and either must be closed to ring the bell; (c) a NAND (not AND)
gate, where both switches must be open for the bell to ring; (d) a NOR
(Not OR) gate, where either switch must be open for the bell to ring.