A cautionary tale. . . . . . .
 
  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.

Index