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 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. 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 wisely had the computer scientist beheaded, and they all lived
happily ever after.
WikiSree 