DATA STORAGE

intelligent and swift as a computer’s memory may be, RAM has one fatal flaw. It is a will-o’­the-wisp. With a few exceptions, all memory chips lose the information stored in them once you turn off the computer. All the work you’ve put into figuring out next year’s budget, creating ac­count billings, or writing the great American sitcom will vanish if the electricity constantly stoking the RAM chips’ transistors falters for even a fraction of a second.

Fortunately, there are several ways to provide permanent storage for a computer’s programs and the work they generate—storage that stays intact even when the power is turned off. The most common form of permanent storage is magnetic disks—both the floppy and hard variety. Magnetic storage is also used in the form of tape drives—a method of permanent storage that’s been around almost as long as the first computers. Gaining popularity are optical devices that use lasers to store and retrieve data. And flash memory, nonvolatile chips that, unlike their more com­mon RAM chip cousins, don’t lose their contents when you turn off your PC, is becoming increasingly popular for digital cameras and other devices.

Floppy disks are universal, portable, and inexpensive but lack both large capacity and speed, al­though improvements on the technology, such as the Zip drive and Super Drive, greatly increase the capacity of removable floppies. Hard disks are, for now, the best all-around storage medium. They store and retrieve data quickly, have the capacity to save several volumes of data, and are inexpensive on a cost-per-megabyte basis. The most recent hard disks are removable and portable, although they don’t hold as much data as conventional hard drives. Tape drives provide virtually endless off-line storage at low cost, but they are too slow to use as anything other than a backup medium.

Optical storage serves PC users who need to store enormous quantities of data. CD-ROM drives pack up to 650 megabytes of data on a disc identical to the laser compact discs that play music, and CD-ROM discs are cheap to produce. The newest innovation, DVD (digital versatile disk), stores, theoretically, up to 17 gigabytes of data, the equivalent of a couple of dozen CDs. Both CD and DVD provide both permanent and rewriteable storage, and each has a place in dif­ferent situations. DVD-RAM has the potential to replace both the floppy and CD-ROM drives. Common CD-ROM and DVD are read-only media, which means that you can only use the data that was stored on them when they were created; you can’t erase or change it. Along with magneto-optical and flopptical drives, computer users have a choice of cheap, handy solutions to unlimited removable storage with access times fast enough for everyday duty.

Three types of memory chips retain their information when you turn off your computer. EPROMs (for Erasable Programmable Read-Only Memory) are found in every personal computer. They are the chips that store the code and BIOS information needed to boot your PC.  EPROMs are slow, and their data can be changed only by first exposing them to ultraviolet light. The memory chip that remembers your PC’s hardware configuration each time it boots is called the CMOS. That stands for complementary metal oxide semiconductor, a bit of trivia you can safely forget. The CMOS is powered by a separate battery you’ll have to change someday if you

 keep your PC long enough. Flash RAM chips, which combine the writeability and much of the speed of conventional RAM chips, have their own integrated power supplies and retain data when the main power source is turned off, promise to be in common use in the future and may turn out to be the ideal permanent storage medium. But for now, they are too expensive to completely replace hard disks. You’ll find them most often in PC cards  and digital cameras .

Despite the different technologies behind these methods of storage, they all have in common a similar notation for recording data and a similar system for filing that information so that it can be found again. Permanent data storage is similar in concept to paper filing systems. Paper files may be handwritten or typed, but they are in the same language. And just as paper files thrown willy-nilly into file cabinets would be impossible to retrieve easily and quickly, electronic files must be stored, too, in an orderly and sensible system and in a common language. In this part of the book, we’ll look at how various forms of permanent storage solve the task of saving data so that it can easily be found again, and how different storage devices write and retrieve that data.