|
Once upon a time, in a kingdom not far from here, a king summoned two of his advisors for
a test. He showed them both a shiny metal box with two slots in the top, a control
knob, and a lever. What do you think this is?
One advisor, an engineer answered first. It is a toaster, he said.
The king asked, how would you design an embedded computer for it?
The engineer replied, Using a four bit micro controller, I would write a simple
program that reads the darkness knob and quantifies its position to one of 16 shades of
darkness, from snow white to coal black. The program would use that darkness level
as the index to a 16-element table of initial timer values. Then it would turn on
the heating elements and start the timer with the initial value selected from the
table. At the end of the time delay, it would turn off the heat and pop up the
toast. Come back next week, and I'll show you a working prototype.
The second advisor, a computer scientist, immediately recognized the danger of such
short-sighted thinking. He said Toasters don't just turn bread into toast,
they are also used to warm frozen waffles. What you see before you is really a
breakfast food cooker. As the subjects of your kingdom become more sophisticated,
they will demand more capabilities. They will need a breakfast food cooker that can
also cook sausage, fry bacon, and make scrambled eggs. A toaster that only makes
toast will soon be obsolete. If we don't look to the future, we will have to
completely redesign the toaster in just a few years.
With this in mind, we can formulate a more intelligent solution to the problem.
First, create a class of breakfast foods. Specialize this class into
subclasses: grains, pork, and poultry. The specialization process should be
repeated with grains divided into toast, muffins, pancakes, and waffles; pork divided into
sausage, links, and bacon; and poultry divided into scrambled eggs, hard-boiled eggs,
poached eggs, fried eggs, and various omelet classes. The ham and cheese omelet class is
worth special attention because it must inherit characteristics from the pork, dairy, and
poultry classes. Thus, we see that the problem cannot be properly solved without
multiple inheritance. At run time, the program must create the proper object and
send a message to the object that says, cook yourself The semantics of this message
depend, of course, on the kind of object, so they have a different meaning to a piece of
toast than to scrambled eggs.
Reviewing the process so far, we see that the analysis phase has revealed that the primary
requirement is to cook any kind of breakfast food. In the design phase, we have
discovered some derived requirements. Specifically, we need an object-oriented
language with multiple inheritance. Of course, users don't want the eggs to get cold
while the bacon is frying, so concurrent processing is required, too.
We must not forget the user interface. The lever that lowers the food lacks
versatility, and the darkness knob is confusing. Users won't buy the product unless
it has a user-friendly, graphical interface. When the breakfast cooker is plugged
in, users should see a cowboy boot on the screen. Users click on it, and the message
booting UNIX v. 8.3? appears on the screen. (UNIX 8.3 should be out by the time the
product gets to the market.) Users can pull down a
menu and click on the foods they want to cook.
Having made the wise decision of specifying the software first in the design phase, all
that remains is to pick an adequate hardware platform for the implementation phase.
An Intel 80386 with 8MB of memory, a 30MB hard disk, and a VGA monitor should be
sufficient. If you select a multi tasking, object oriented language that supports
multiple inheritance and has a built-in GUI, writing the program will be a snap.
(Imagine the difficulty we would have had if we had foolishly allowed a hardware first
design strategy to lock us into a
four-bit micro controller!).
The king had the computer scientist thrown in the moat, and they all lived happily ever
after.
|
