The King's Toaster

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 microcontroller, I would write a simple program that
reads
the darkness knob and quantizes 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 tum 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 tum 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 s s, '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 multitasking, 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 microcontroller!)." The king had the computer scientist
thrown in the moat, and they all lived happily ever after.