COMPUTER MEMORY

What is memory?

The basic purpose of memory is to keep a record of information for a period of time. One of the really noticeable things about human memory is that it's extremely good at forgetting. That sounds like a major defect until you consider that we can only pay attention to so many things at once. In other words, forgetting is most likely a clever tactic humans have evolved that helps us to focus on the things that are immediately relevant and important in the endless clutter of our everyday lives—a way of concentrating on what really matters. Forgetting is like turning out old junk from your closet to make room for new stuff.

The two types of memory:

1. Internal memory:

RAM and ROM:

The chips that make up a computer's internal memory come in two broad flavors known as RAM (random access memory) and ROM (read-only memory). RAM chips remember things only while a computer is powered on, so they're used for storing whatever a computer is working on in the very short term. ROM chips, on the other hand, remember things whether or not the power is on. They're preprogrammed with information in the factory and used to store things like the computer's BIOS (the basic input/output system that operates fundamental things like the computer's screen and key board). RAM and ROM are not the most helpful names in the world, as we'll shortly find out, so don't worry if they sound baffling. Just remember this key point: the main memory inside a computer is based on two kinds of chip: a temporary, volatile kind that remembers only while the power is on (RAM) and a permanent, nonvolatile kind that remembers whether the power is on or off (ROM).

   2. External memory:

   Auxiliary memory:

The most popular kinds of auxiliary memory used in modern PCs and hard drives and CD/DVD ROMs. But in the long and fascinating history of computing, people have used all kinds of other memory devices, most of which stored information by magnetizing things. Floppy drives (popular from about the late-1970s to the mid-1990s) stored information on floppy disks. These were small, thin circles of plastic, coated with magnetic material, spinning inside durable plastic cases, which were gradually reduced in size from about 8 inches, through 5.25 inches, down to the final popular size of about 3.5 inches. Zip drives were similar but stored much more information in a highly compressed form inside chunky cartridges. In the 1970s and 1980s, microcomputers (the forerunners of today's PCs) often stored information using cassette tapes, exactly like the ones people used back then for playing music. You might be surprised to hear that big computer departments still widely use tapes for backing up data today, largely because this method is so simple and inexpensive. It doesn't matter that tapes work slowly and sequentially when you're using them for backups, because generally you want to copy and restore your data in a very systematic way—and time isn't necessarily that critical.

Going back even further in time, computers of the 1950s and 1960s recorded information on magnetic cores (small rings made from ferromagnetic and ceramic material) while even earlier machines stored information using relays (switches like those used in telephone circuits) and vacuum tubes (a bit like miniature versions of the cathode-ray tubes used in old-style TV).