Date: Tue, 30 Jul 96 17:04:01 +0100 From: Mike Chaloner
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 quantises its position to one of 6
shades of darkness, from snow white to coal black. The program would use
that darkness level as an 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 recognised 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. Specialise this class
into subclasses: grains, pork and poultry. The specialisation 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
"The ham and cheese omelette 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 should 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
"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 Pentium with 32MB of memory, a 500MB
hard disk and 17inch SVGA 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 microcontroller!)."
The king wisely had the computer scientist beheaded, and they all lived
happily ever after.