What Is a Computer?
A computer is a machine that can add, subtract, multiply, divide, and perform other mathematical and logical functions on numbers. A computer recognizes some numbers as numbers, and it also can recognize words by translating the words into numbers. At one time, you could talk to a computer only in numbers, but modern computers can accept words on-screen, translate them into command numbers, and then execute them.
The heart of a computer is the machine that does the addition, subtraction, multiplication, and division, and also moves data from one location to another. This is called the central processing unit (CPU), because it processes the data. In personal computing, the computer itself frequently is named after the CPU. A computer that uses a CPU called the 80286 is sometimes called a 286. Machines that use the 80386 usually are sold as 386 machines, and those that use the 80486 CPU are called 486 machines. The arrival of 80586 chips heralded a name change. These are called Pentiums. Will 80686 chips be called Hexiums?
To be useful, the computer also must have a way to be given the numbers to process (the input) and a method of presenting results to the user (the output). The input to a computer usually is entered with a keyboard. Other input devices include bar code readers, optical character readers (OCR), scanners, and mice. The output from a computer usually is displayed on a monitor (or screen) or printed on paper or can be written out to a file on a disk.
To perform large calculations, the computer needs some place to store temporary or intermediate results. This temporary storage for the computer frequently is called main memory, primary memory, or primary storage and usually is stored inside the main box of the computer. Think of the memory storage area inside the computer as a giant scratch pad for programs. Early personal computers had as little as 4 kilobytes (4KB) of memory. (A kilobyte is a bit more than 1,000 bytes; a byte can store one character of data, such as A, X, or @.) The personal computer boom brought the price of memory down to the point that computers now commonly sell with a starting memory of 4 or 8 megabytes (8MB) and can be upgraded to 32MB or more. (A megabyte is about a million bytes.)
To save results so that they can be reused, the computer needs some place to store information on a permanent or long-term basis. The problem with main memory is that it needs to have power all the time. When a computer is switched off, the contents of main memory are lost. The computer needs something that will retain information even when power is switched off. This is the task of secondary storage. Secondary storage is permanent and continues to function even after power is gone. Secondary storage comes most commonly in the form of diskettes (or floppies), hard disks, and tapes. Data is recorded on diskettes, hard drives, and tapes in a manner similar to the way that music is stored on a music tape cassette. A CD-ROM (compact disc-read-only memory) is another type of secondary storage. It is most commonly used as a permanent storage device and contains data that cannot be modified; such as the text of encyclopedias and dictionaries, or a complete listing of businesses in a country. There are more expensive devices available that can write to a CD-ROM that can be used for the initial storage of the dictionary, encyclopedia, or whatever.
The central processing unit requires that any program to be run and any data to be processed must be in main memory. Whenever you run a program, it is loaded from the secondary storage device (disk) into main or primary memory before it is executed. Whenever you work on data, such as editing a file, the file is first loaded into main memory from the disk drive (secondary storage). The editing is done directly in main memory and then must be saved back to disk. The central processing unit can neither execute a program directly from disk nor manipulate data directly on the disk.
What Is a Program?
A computer is an incredibly stupid device. It doesn't do anything unless and until it is told to do so.
When a computer is first switched on, the CPU starts looking through main memory for an instruction. You can think of it as being in a perpetual state of readiness.
The computer is designed so that a small portion of main memory is permanent; it retains its contents even when the power is switched off. This permanent memory is placed at the location where the CPU begins searching for its first instruction after powering up. Consequently, it finds this permanent instruction immediately. This permanent area of memory contains a sequence of instructions that the computer executes on power-up. The instructions look something like this:
· Test the monitor.
· Test the keyboard.
· Test all of the main memory and display the results on the monitor.
· Test any other devices that need to be tested, including disk drives.
· Load the operating system program from secondary storage (the disk) into main memory. The operating system is a master program that controls a computer's basic functions and allows other programs to access the computer's resources, such as the disk drives, printer, keyboard, and screen. (In practice, this step is a little more complicated, but the principle is correct.) For an MS-DOS-compatible computer, the operating system is MS-DOS (Microsoft Disk Operating System).
· Jump to the first, previously loaded instruction at the beginning of the MS-DOS operating system.
From this point on, the CPU is executing instructions within the MS-DOS operating system.
If you could read the first few instructions of the MS-DOS operating system in English, they might look something like what you see in Table
The first few instructions of the MS-DOS operating system as they might appear in English.
| Instruction Number | Instruction | Comment |
| 001 | Display the prompt | Put the > prompt on the screen. |
| 002 | Wait for a keypress | |
| 003 | Was a key pressed? | |
| 004 | If not, GO TO 002 | If no key was pressed, go back and try again. |
| 005 | Get the key value | |
| 006 | Save the key value | Store the value in main memory. |
| 007 | Display the key value | |
| 008 | Was the value (ENTER)? | Determine whether the Enter key was pressed, signaling the end of command input. |
| 009 | If not, GO TO 002 | The user is still typing, so keep getting keypresses. |
| 010 | GO TO do the command | The user pressed Enter, so jump to the instruction that will try to execute the command. This part of the program is not shown. |
The set of instructions is written in English to represent the steps of a program. The program is executed so quickly that you see no visible delay between typing the key and seeing it appear on-screen, even though the action of saving the key value occurs between the keystroke and the display.
