y and is extremely fast in terms of access speed. It is, however, volatile: When the PC is turned off, or power is lost, whatever information is stored in RAM disappears. When basic computer skills courses tell students to "save often," this is the reason. When a computer user is writing a document in a word processor, or populating a spreadsheet, or manipulating a digital pho- tograph, the file lives in RAM until the person saves, at which time it is written to main memory, which is nonvolatile as we'll see in a moment. Modern systems typically have a minimum of 512 Megabytes (MB) of RAM. It amazes me to look across my office at my first Mac, which I pur- chased in 1986. It has a 40 MB hard drive, 640 KB of RAM, and a proces- sor that doesn't run-it dribbles. Yet, it runs the same office applications I use today (although much reduced in terms of bells and whistles) and boots in seconds, compared to my current, very powerful XP machine. Secondary Memory Secondary memory has become a very popular line of business in the evolving PC market. It provides a mechanism for long-term storage of data files and is nonvolatile-when the power goes away, the information it stores does not. Secondary memory tends to be a much slower medium in terms of access time than main memory because it is usually mechan- ical in nature, whereas main memory is solid-state. Consider, for exam- ple, the hard drive shown in Figure 5-4, which chose a hot July day to fail, right in the middle of writing this book (and yes, I had backed up often). That's why the disk platters are exposed. Notice that the drive comprises three platters and an armature upon which are mounted read/write heads similar to those used in cassette decks of yore. Figure 5-5 is a schematic diagram that more clearly illustrates a bit more how hard drives actually work. The platters, typically made of aluminum and cast to extremely exacting standards, are coated with iron oxide Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Co