Programs

A 1970s punched card containing one line from a FORTRAN program. The card reads: "Z(1) = Y + W(1)" and is labelled "PROJ039" for identification purposes.

In practical terms, a computer program may run from just a few instructions to many millions of instructions, as in a program for a word processor or a web browser. A typical modern computer can execute billions of instructions per second (gigahertz or GHz) and rarely make a mistake over many years of operation. Large computer programs consisting of several million instructions may take teams of programmers years to write, and due to the complexity of the task almost certainly contain errors.

Errors in computer programs are called "bugs". Bugs may be benign and not affect the usefulness of the program, or have only subtle effects. But in some cases they may cause the program to "hang"—become unresponsive to input such as mouse clicks or keystrokes, or to completely fail or "crash". Otherwise benign bugs may sometimes may be harnessed for malicious intent by an unscrupulous user writing an "exploit"—code designed to take advantage of a bug and disrupt a program's proper execution. Bugs are usually not the fault of the computer. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design.^[15]

In most computers, individual instructions are stored as machine code with each instruction being given a unique number (its operation code or opcode for short). The command to add two numbers together would have one opcode, the command to multiply them would have a different opcode and so on. The simplest computers are able to perform any of a handful of different instructions; the more complex computers have several hundred to choose from—each with a unique numerical code. Since the computer's memory is able to store numbers, it can also store the instruction codes. This leads to the important fact that entire programs (which are just lists of instructions) can be represented as lists of numbers and can themselves be manipulated inside the computer just as if they were numeric data. The fundamental concept of storing programs in the computer's memory alongside the data they operate on is the crux of the von Neumann, or stored program, architecture. In some cases, a computer might store some or all of its program in memory that is kept separate from the data it operates on. This is called the Harvard architecture after the Harvard Mark I computer. Modern von Neumann computers display some traits of the Harvard architecture in their designs, such as in CPU caches.

While it is possible to write computer programs as long lists of numbers (machine language) and this technique was used with many early computers,^[16] it is extremely tedious to do so in practice, especially for complicated programs. Instead, each basic instruction can be given a short name that is indicative of its function and easy to remember—a mnemonic such as ADD, SUB, MULT or JUMP. These mnemonics are collectively known as a computer's assembly language. Converting programs written in assembly language into something the computer can actually understand (machine language) is usually done by a computer program called an assembler. Machine languages and the assembly languages that represent them (collectively termed low-level programming languages) tend to be unique to a particular type of computer. For instance, an ARM architecture computer (such as may be found in a PDA or a hand-held videogame) cannot understand the machine language of an Intel Pentium or the AMD Athlon 64 computer that might be in a PC.^[17]

Though considerably easier than in machine language, writing long programs in assembly language is often difficult and error prone. Therefore, most complicated programs are written in more abstract high-level programming languages that are able to express the needs of the computer programmer more conveniently (and thereby help reduce programmer error). High level languages are usually "compiled" into machine language (or sometimes into assembly language and then into machine language) using another computer program called a compiler.^[18] Since high level languages are more abstract than assembly language, it is possible to use different compilers to translate the same high level language program into the machine language of many different types of computer. This is part of the means by which software like video games may be made available for different computer architectures such as personal computers and various video game consoles.

The task of developing large software systems presents a significant intellectual challenge. Producing software with an acceptably high reliability within a predictable schedule and budget has historically been difficult; the academic and professional discipline of software engineering concentrates specifically on this problem.

Example

A traffic light showing red

Suppose a computer is being employed to drive a traffic signal at an intersection between two streets. The computer has the following three basic instructions.

1. ON(Streetname, Color) Turns the light on Streetname with a specified Color on.
2. OFF(Streetname, Color) Turns the light on Streetname with a specified Color off.
3. WAIT(Seconds) Waits a specifed number of seconds.
4. START Starts the program
5. REPEAT Tells the computer to repeat a specified part of the program in a loop.

   Comments are marked with a // on the left margin. Assume the streetnames are Broadway and Main.

      START
   

      //Let Broadway traffic go
     OFF(Broadway, Red)
     ON(Broadway, Green)
     WAIT(60 seconds)
   

      //Stop Broadway traffic
     OFF(Broadway, Green)
     ON(Broadway, Yellow)
     WAIT(3 seconds)
     OFF(Broadway, Yellow)
     ON(Broadway, Red)
   

      //Let Main traffic go
     OFF(Main, Red)
     ON(Main, Green)
     WAIT(60 seconds)
   

      //Stop Main traffic
     OFF(Main, Green)
     ON(Main, Yellow)
     WAIT(3 seconds)
     OFF(Main, Yellow)
     ON(Main, Red)
   

      //Tell computer to continuously repeat the program.
     REPEAT ALL
   

   With this set of instructions, the computer would cycle the light continually through red, green, yellow and back to red again on both streets.

   However, suppose there is a simple on/off switch connected to the computer that is intended to be used to make the light flash red while some maintenance operation is being performed. The program might then instruct the computer to:

      START
   

      IF Switch == OFF then: //Normal traffic signal operation
     {
     //Let Broadway traffic go
     OFF(Broadway, Red)
     ON(Broadway, Green)
     WAIT(60 seconds)
   

      //Stop Broadway traffic
     OFF(Broadway, Green)
     ON(Broadway, Yellow)
     WAIT(3 seconds)
     OFF(Broadway, Yellow)
     ON(Broadway, Red)
   

      //Let Main traffic go
     OFF(Main, Red)
     ON(Main, Green)
     WAIT(60 seconds)
   

      //Stop Main traffic
     OFF(Main, Green)
     ON(Main, Yellow)
     WAIT(3 seconds)
     OFF(Main, Yellow)
     ON(Main, Red)
   

      //Tell the computer to repeat this section continuously.
     REPEAT THIS SECTION
     }
   

      IF Switch == ON THEN: //Maintenance Mode
     {
     //Turn the red lights on and wait 1 second.
     ON(Broadway, Red)
     ON(Main, Red)
     WAIT(1 second)
   

      //Turn the red lights off and wait 1 second.
     OFF(Broadway, Red)
     OFF(Main, Red)
     WAIT(1 second)
   

      //Tell the comptuer to repeat the statements in this section.
     REPEAT THIS SECTION
     }
   

   In this manner, the traffic signal will run a flash-red program when the switch is on, and will run the normal program when the switch is off. Both of these program examples show the basic layout of a computer program in a simple, familiar context of a traffic signal. Any experienced programmer can spot many software bugs in the program, for instance, not making sure that the green light is off when the switch is set to flash red. However, to remove all possible bugs would make this program much longer and more complicated, and would be confusing to nontechnical readers: the aim of this example is a simple demonstration of how computer instructions are laid out.