The software is in the holes - The Computing Universe: A Journey through a Revolution - page 42

Suppose there is a big company that employs many salespeople to sell its
products and stores a great deal of information about the sales force in a big
filing system consisting of cards in cabinets. Let's say that our file clerk knows
how to get information out of the filing system. The data are stored in the
filing cabinets on “Sales” cards, which contain the name, location, number
of sales, salary, and other information about each salesperson. Suppose we
want the answer to a simple question: “What are the total sales in the state of
Washington?”
The instructions for the clerk could be:
Take out a “Sales” card.
If “Location” says Washington , then add the number of “Sales” to a running
count called “Total.”
Put the card back.
Take out the next “Sales” card and repeat.
This set of instructions looks fine, but what do we do if our file clerk does not
know what is meant by keeping a “running count”? In this case we need to pro-
vide the clerk with more detailed instructions on exactly how to do that task.
We therefore provide the clerk with a new card called “Total,” and our more
detailed “program” now reads:
Take out the next “Sales” card.
If “Location” says Washington , then take out the “Total” card.
Add the sales number to the number on the card.
Put the “Total” card back.
Put the “Sales” card back.
Take out the next “Sales” card and repeat.
In a modern computer, of course, the data would not be stored on cards and the
machine does not physically take out a card. Instead, the computer reads the
stored information from a memory register , a storage place in its main memory.
Similarly, the computer can write from such a register to a “card” without actu-
ally physically taking out a card and putting it back.
To go any further with our analogy, we need to specify more precisely how
our file clerk carries out the basic set of operations. One of the most elemen-
tary operations is that of transferring information from the cards that the clerk
reads to some sort of scratch pad or working area where the clerk can do the
arithmetic. We can do this by specifying exactly what our “Take” and “Replace”
card operations mean:
“Take card X” means that the information on card X should be written on the
scratch pad.
“Replace card Y” means that the information on the pad should be written
on card Y.
We now need to instruct the clerk exactly how to check if the location on
card X was Washington . The clerk will need to do this for each card, so he
needs to remember Washington from one card to the next. One way to do this
is to have Washington written on another card we shall call C. The instruc-
tions are now: