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 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
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
"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,
"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
The king wisely had the computer scientist beheaded, and they all
lived happily ever after.