Once upon a time, in an absolute monarchy 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 electrical engineer, answered first. "It is a toaster."

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.

Wait 'til 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. Would-be diners

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 80586 with 16MB of memory, a

1.2GB hard disk, and a SuperVGA 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 scientist beheaded,

and the kingdom lived happily ever after.