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 working prototype."
The second advisor, a computer programmer, 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 it
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 v8.3'
appears on the screen. (UNIX 8.3 should be out by the time
the product gets to 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 wisely had the computer programmer beheaded,
and they all lived happily ever after.
This message was sent on 19 Jul 1996