Part 1: Introduction to Computers and Programming
Chapter 1: Machines, Input/Output, Storage, and Logic
Introduction
This chapter is about computers and how they work. It is an overview of a great many concepts
like input/output, logic, storage, instructions, machine language, etc. All of these concepts are written for novice
computer users. There will be no
questions, exams, or assignments at the end of this chapter. You are directed simply to read and get into da
groove! Throughout programming you will
come back to these things over and over again so there will be plenty of time
for quizzing you about your knowledge of them.
At this point I just want to break the ice.
Simple Example: Digital Clock
You know how to control a computer at a high level, user input. You click this,
type that, or push this there and the computer responds to your action. But
what is really happening when you do these things? A computer program is
receiving your input as it requested and dealing with it as it sees fit. There
are four things computers and any useful machines have: input, output,
storage, and logic. These are the things that help them do the
tasks they are meant for. To simplify things I'm going to simplify the machine
we're speaking of. Rather than a computer, I'm going to use a digital clock:
I/O
Immediately you can recognize that the clock has input through it's hour and
minute buttons. No input would be complete without some way of acknowledging
it, output. In the case of our clock, we have a LED screen capable of
displaying a 24-hour clock. Input/output seem to go hand in hand wherever you
look. When something goes in, something comes out. In lew
of this, the two things are usually globbed together
under one term: "i/o", an abbreviation for input/output.
Storage:
Temporary and Permanent
We know that the clock has a i/o, but where does it keep the current
time? The time seen is simply a display of the data where the clock keeps the
current time. Internally this machine has some place to store this data.
This is, obviously, known as storage.
When the power to the clock is shut off, the time resets. This means the data
for the time is in temporary storage. In computery
terms, our version of temporary storage might be defined as data
storage which is reset, blanked, or invalid when necessary power is lost or
shut off. Sounds complex, but just think: we take out the battery and the
clock goes dead and when we put it back in the clock is at
Philosophically, nothing is permanent. In our terms, permanent storage
would simply mean the opposite of temporary storage. I would define it
as data storage which can only be destroyed or change by explicit tampering
or unforseen accidents. Our clock has no permanent
storage, but I think you get the idea. I'll explain a computer's version of
this later on.
Any given storage, regardless of permanence, will have one of two
different access privileges: read-only or read/write (I've never
heard of write-only, but it may exist in some 3rd world country).
Storage that is read-only can never be changed while read/write
storage can. It's that simple.
And lastly the concept of storage could be described simply in terms of i/o
from a storage machine/device. Say our clock's storage is a
little teenie circuit inside.
Logic
Our machine can access input, display output, and keep a current
time in its storage; but how does it bring all these things together and
control them? Logic of course. You can think of logic as instructions
that encompass reading input, writing output, and making decisions.
Notice that I didn't mention storage. Again, that aspect is enveloped in
i/o. Because input from storage and output to storage
is simply implicit. Say, for example we have a jar of pennies for storage. I
would rather say add a penny to the jar than output a penny and feed
it as input to the jar. That two-way, give/take transaction is implied.
Logical
instructions for machines of any kind are very explicit and finite. There is no creative meanduring. Let me give an analogy. Say you write some directions to get to your
home from some known freeway. Now, if
you gave these to a computer it would act them out one by one in perfect
sequence and to the letter. A human,
especially a man, might try to take short-cuts.
That’s what instructions are on a machine. A list of actions to be carried out … it’s
that simple! What you do when you program is write or alter those
instructions for your specific purpose.
Now that I've rambled a bit, let's look at the logic for our clock.
- If the hour button is pressed, increase the current time by one
hour.
- If the minute button is pressed, increase the current time by one
minute.
- If a second has passed, update the current number of seconds.
- If the number of seconds is greater than or equal to 60, increase
the current time by one minute and reset the number of seconds.
- Update the display with the current time.
Even though I can sum up the clock’s
logic in five points, implementing it can be fairly complex. First of all, how does the clock know when a
second has passed? A lot of these
lower-level details may seem superfluous, but sometimes you’ll need to know
them. In the context of simply warming
you to programming, I’ll gloss over super-specifics like these.