Inside Computers - Bits and Pieces

        Your computer successfully creates the illusion that it
        contains photographs, letters, songs, and movies. All it
        really contains is bits, lots of them, patterned in ways
        you can't see.  Your computer was designed to store just
        bits - all the files and folders and different kinds of
        data are illusions created by computer programmers.

        (Hal Abelson, Ken Ledeen, Harry Lewis, in "Blown to Bits")
   

Basically, computer instructions perform operations on groups of bits.  A bit is either on or off, like a lightbulb.  Figure 1.1_a shows an open switch and a lightbulb that is off - just like a transistor in a computer represents a bit with the value: zero.  Figure 1.1_b shows the switch in the closed position and the lightbulb is on, again just like a transistor in a computer representing a bit with the value: one.

Figure 1.1_a	Figure 1.1_b

A microprocessor, which is the heart of a computer, is very primitive but very fast.  It takes groups of bits and moves around their contents, adds pairs of groups of bits together, subtracts one group of bits from another, compares a pair of groups, etc... - that sort of stuff.
Inside a microprocessor, at a very low level, everything is simply a bunch of switches, also known as bits - things that are either on or off!  Time to expand on how this is done; first let's explore how groups of bits can be used to form numbers.