Jim Eck
Chicks dig the Mule
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.
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.
