The main memory in a computer is made of RAM, which stands for Random Access Memory. The term "random access" means the same thing: that the data in the memory can be accessed in any order.
RAM is made of capacitors, which are devices that store small electrical charges. Each capacitor stores one bit. If the capacitor is mostly charged, it's considered a 1 bit, and if it's close to empty, it's a 0. There are two categories of RAM. Dynamic RAM uses capacitors that must be recharged periodically. Static RAM uses capacitors that hold their charge indefinitely. Static RAM is faster because it doesn't have to waste time with the periodic recharge, but dynamic RAM is much cheaper to make. Because main memories are so large, they use dynamic RAM.
RAM is divided into cells of memory called words, which may be a single byte (eight bits) or multiple bytes. Each word's location in RAM is specified by a unique address, which is just a whole number that starts from 0. For example, if a computer system has 256 megabytes of RAM, then it has 268,435,456 bytes. Thus, on this system the addresses range from 0 to 268,435,455. If a word on this system is two bytes, for instance, then only the even numbers in that range are legitimate addresses.
The capacitors in RAM are wired into grids. Think of a piece of graph paper with a capacitor wherever the lines cross. The lines on the graph would be wires called "control lines." Although every control line has many capacitors on it, if one vertical line and one horizontal line are selected, the lines cross in only one place. In the same way, the capacitors are wired to respond only when both of their control lines are activated.
When a user requests the execution of a program, the computer brings the program from where it is stored into main memory. Any temporary data the program needs is also stored in main memory. Storing the programs and data in main memory is necessary because storage devices like hard drives are very slow compared to the CPU. The main memory acts as a temporary "scratch pad" where the currently active program and data can be kept for quick access.
As the program executes, the CPU may need to access the devices previously discussed. For example, suppose a user opens a word processing program, such as Microsoft Word. This action brings the program into main memory. Throughout the execution of the program, data is sent to the monitor for display. Also throughout the execution, the CPU must read the instructions of the program from main memory. Suppose the user opens an existing document for editing with Word.
To do so, he or she clicks the mouse on a menu item that reads "Open," then types the name of the file using the keyboard or clicks the file name from a list of Word files. Now the computer must access the hard drive again to load the file into main memory. The user edits the file, by using the keyboard and the mouse, then prints it by accessing the printer, and then saves the file, which stores it back on the hard drive.Booting
The previous discussion assumes the operating system is already running so that the user has a way to select a program to run. But what starts the operating system? At first it seems like a chicken-and-egg problem. If the operating system is used to tell the computer what program to run next and the operating system is itself a program, what tells the computer to run anything when it's first turned on?
The process by which a computer starts and executes the operating system is called booting. This is short for "bootstrapping," which means to lift oneself up by one's own bootstraps, as the expression goes, which is analogous to the seemingly impossible task the computer has when it starts.
The secret ingredient is called the BIOS, which stands for Basic Input/Output System. This is a set of small programs stored in ROM. Recall that ROM, as in CD-ROM, means read-only memory. Here it refers to memory that is accessed like RAM but is "hard-wired"; that is, it can't be changed.
When a computer's power is turned on, the BIOS acts like a drill sergeant waking up the troops. It initializes all the devices in the system and performs some self-diagnostic tests. Then it performs its most important task, which is loading the operating system. Actually, the BIOS isn't capable of loading the operating system on its own. Instead, it retrieves a program from a special location on the primary hard drive called the boot sector. This short program is part of the operating system, and its job is to retrieve the rest of the operating system into memory.
When you turn on a computer, you often see a few seconds of plain-looking text, perhaps with a simple logo for the manufacturer of the computer. During this time, the BIOS is running. A few seconds later the screen displays the logo of the operating system. That's the program from the boot sector. Finally the full operating system is loaded and you can begin running other programs.
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02122011
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