Memory: RAM and ROM

There are two types of memory in a computer: RAM and ROM. In the early days of personal computing, some vendors made their systems sound more impressive by adding these two totals. However, their function in computer systems is verydifferent.

The contents of RAM (random access memory ) can be accessed in any order and can change instantly. They are in constant flux as you start a computer; load its operating system and drivers for particular devices; load an application; create, store, change, and copy data; and shut down the computer.

Programs are loaded into RAM; until data is stored, it exists only in RAM (that's why you should save your work so often!). The "enemies" of data stored in RAM include

• System crashes and lockups

• User error ( forgetting to save before you close a program)

• Power failures

Because most types of RAM must receive a steady dose of electricity to keep its contents around, even momentary power failures can destroy its contents. Because all data must be created or changed in RAM before it's stored, you must make sure that RAM is working correctly. In Chapter 7, "RAM," you'll learn more about adding, configuring, and using RAM.

Flash memory , also called Flash RAM, is a special type of RAM that uses electricity to change its contents, but doesn't require electricity to maintain its contents. It is used in BIOS chips and in storage for digital music players and digital cameras .

ROM stands for read-only memory , meaning that although its contents, like those of RAM, can be accessed in any order, ROM's contents can't be changed by normal computer operations. So, what is ROM good for?

Because ROM's contents don't change when a system is powered down or restarted, it's the perfect storage place for firmware. As we saw earlier, firmware is the "software on a chip" used to control various devices in the computer. ROM isn't suitable for software storage, however, because its capacity is too limited for today's large programs. And, of course, ROM can't be used to store data files that are constantly changing.

The way that ROM chips have been made has changed several times over the years . Originally, ROMs contained a permanently etched pattern; later, ROMs were made of reprogrammable materials that could be changed through controlled ultraviolet light or electricity. Because the chip had to be removed from the motherboard for replacement or reprogramming, changing the contents of ROMs was difficult and inconvenient. If you think that opening up a single system to change its ROM chip is a pain, imagine performing the same job on dozens of PCs!

Fortunately, current ROMs can be reprogrammed with software. This process is called flashing the ROM and is performed with the BIOS firmware found on motherboards and in modems, among other devices. Want to learn more? The reasons for upgrading ROMs and the methods used are covered in Chapter 6.