www.cyberslayer.co.uk

 


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 omelette classes."

"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 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 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.