What does it do?
Parameter memory is a special separate area of memory which is used to store some of your Mac’s most important settings when it isn’t running. When you shut down your Mac, all the contents of the main and video memories are erased, because those memory chips need power to keep the contents ‘alive’. Parameter memory (parameter RAM, or PRAM) is so small that it requires very little power (which comes from a special battery) to keep it going. How does it work?
When you alter the sound volume, for instance, in the Monitors & Sound control panel, that setting is saved to parameter memory and retained when the Mac is next shut down. PRAM and your Mac’s internal clock are powered by a half-size AA lithium battery in most models. The battery is obvious on most computer motherboards once you have removed your Mac’s cover. (Don’t do this if your Mac has an integral monitor, as it can store lethally high voltages for many days after disconnection from the mains.) The iMacs PRAM is accessed by sliding out the motherboard from the bottom rear (see Vol 15 No 10, p59). The battery has a plastic cage covering it, which can be eased off after levering it free from one end. What can go wrong with it?
There are two common problems with PRAM: the battery goes flat, or the PRAM contents become corrupted. Flat batteries must be replaced, generally three or more years after the computer was made. Failure to remember the date correctly is an early sign of a flat battery. An inability to start up, with a black screen but all power lights working, is an indication that the battery is completely flat. A lithium-thionyl chloride battery, in half-AA size delivering 3.6v, is the correct replacement for most models, and most clones use different batteries. If you replace your own battery, take great care to insert the new one the right way round. Corruption of the PRAM contents can cause peculiar effects, such as resetting the clock to a date in the 1950s, or even preventing successful startup. Hold down command-option-p-r during startup until you hear a second startup chime to ’zap’ the PRAM back to defaults and fix such problems. How can I upgrade it?
Parameter memory is soldered to the motherboard, and can’t be upgraded or expanded. Further info
Most good Mac books describe the PRAM and battery replacement, while Web sites worth visiting include a good introduction to the PRAM with links at www.macshare.com/pram.html and Apple’s trouble-shooting pages on PRAM and startup problems at til.info.apple.com/techinfo.nsf/artnum/n16473 | 
