Taking a Detour on a Two-Lane Road ........................................................
Within the electronic brain of your computer, data moves along from place to
place on an 8-, 16-, or 32-lane expressway that is somewhat confusingly called
a bus (or data path). There’s plenty of open roadway, and access is limited to
well-managed on- and off ramps.Information in the computer is made up of words that bring together groups
of bits(binary digits). The original personal computers used words made of
eight bits; a package of data of that size is called a byte.Modern PCs and lap-
tops move words made up of 16 bits, or 2 bytes, from point to point on the
internal bus. (And just for the record, current versions of Intel and AMD
microprocessors juggle words of 32 and even 64 bits internally.)Let me get back to the automotive metaphor: Think of a 16-bit computer
word moving from the microprocessor to the video display adapter as a
convoy. On the motherboard, these 16 bits move alongside each other on
tiny wires; they depart at the same time and arrive at their destination at the
same time (give or take a few millionths of a second here and there). The
words move along to the drumbeat of a computer’s clock. In computing
terms, this is called a parallelcircuit.You might think that a parallel circuit would be a very fast and efficient way to
move large amounts of data from one place to another, and in most instances
you would be right. The first few generations of PCs used thick parallel cables
with bundles of 25 or so wires (8 wires for a one-byte word of data outgoing,
8 wires for a one-byte incoming word, plus additional lines for grounding and
specialized signals) to communicate with devices like printers.But when it came to other forms of communication, like using a modem to
venture out onto a telephone line to reach the Internet, a parallel pathway
would not work because in many ways the phone system of the 21stcentury
is little changed from Thomas Edison’s original design: two wires, one in each
direction. (Take a look at the connector for the phone on your desk. For a
single-line phone there’s one red and one green wire; a two-line phone adds a
yellow and black pair.)And so (back to automotive imagery one last time, I promise) the parallel super-
highway that exists within a computer has to suddenly squeeze all of its traffic
into a single lane in each direction for what is called serialcommunication. The
computer’s 8- or 16-bit words have to line up one behind the other instead of
alongside each other. And slowing things down just a little bit more, the com-
puter has to find a way to mark the beginning and end of each word. It does so
by adding a start and a stop bit, and just for good measure most systems also
add some form of error checking that allows the computer to reject a word that
seems to have a misspelling.240 Part IV: Failing to Communicate