Page 2: programming

This second page shows how computer programs transform a number crunching machine into a versatile tool that seems to think.

The earliest computer operators performed a desired calculation by wiring up the circuits appropriately, which was time consuming and inefficient. A Princeton University mathematician, John von Neumann described the concept of a stored program:

Machine instructions

One byte (eight bits) represents an instruction. For example, for IBM-PC compatible compuers:

10110000 00000111

Means move the number 7 into position A (note that binary 00000111 = 7)

00000011 11000011 (a two-byte long instruction)

Add the bits in location A to those in location B, placing the result in A.

Program code

When a computer program is run, machine language instructions are placed in memory with an instruction pointer set to that location. After each byte is executed the instruction pointer is advanced to the next step, and so on.

Consider the things a machine instruction can call for:

• Move the number that follows this instruction to location A.
• Add location A and B, placing the result in A.
• Jump to the instruction found at memory location so-and-so.
• Send the number 65 (which stands for capital A) to the printer, which causes the printer to print an "A".
• Move the number 66 (capital B) to memory location 47104 (or whatever memory location corresponds to the value of the upper left corner of the computer screen) to print a "B" to the screen.
• Decrement the value in location C by one.
• Jump to memory location so-and-so if the value in location C is zero.

Sounds exciting, huh? Well, it is. In particular, look at that last instruction again:

In IBM-PC language this is number 11100011, called JCXZ, or "Jump if CX is zero." The emphasis is on if!

The principle that the flow of instructions can branch conditionally is at the heart of the magic of computing. Now see what concepts can be represented in machine language code:

• If the person presses "Y" or "y" then write the word "Yes" on the screen. Otherwise, write "No."
• If the person presses "Y" or "y" then write "Oh, I'm sorry you're feeling badly." Otherwise, write "Well, that's good. Have you told someone you loved them today?"
• If the car fuel mix is too rich, make it leaner (electronic ignition in modern cars)
• If the bouncing ball hits the right wall, change its horizontal motion to the opposite direction
• If the new checking balance is less than $0, then write, "Sorry your account is overdrawn."
• For each of the 9 possible moves in tic-tac-toe, go through this list: If it gives us three-in-a-row, make it. Otherwise, if it blocks their three-in-a-row, make it. Otherwise, look at the next move, etc.

The point is, a programmer can write code (computer programming) to specify in advance what it should do, including writing to the screen, printing on paper, calculating numbers and saving them in memory. What is more, it can accept input from the user and react according to how it is programmed. That is how a computer ticks.

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