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 
asses."
"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 Pentium-II, 700 MHz, with 128 MB of memory, a 
4.3 GB hard disk, and a 1280x1024 resolution 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 had the computer scientist thrown in the moat, and they all lived 
happily ever after.
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