kpm: PRAM Battery

All at a sudden my PowerMac won't boot anymore: I get a flashing question mark in the middle of the screen.
I was told that I need to change the PRAM battery. But I don't have a PowerBook. My Mac is a desktop. It doesn't have batteries. Why should I change the battery then? And what is "PRAM"?

Answer

PRAM stands for Parameter RAM, which is also known as CMOS.
Every computer has a chip called RTC (= Real Time Clock). The RTC is a RAM chip and it appears just like one of the memory chips which you find on the RAM expansion modules (or sticks) you use inside your computer.
Usually is 64 or 128 byte in size.
The first 12-14 bytes are used up by a quatrz clock that runs all the time, whether or not the computer has power. The battery powers this clock. When the computer boots up, part of the process is to query the RTC to get the correct time and date. A little quartz clock like this might run for 5 to 7 years off of a small battery. Then it is time to replace the battery.
However, the quartz clock isn't the only software installed in that chip; other informations, like the number of floppy and hard disk drives, the hard disk drive type, which is the hard drive to boot the OS from, etc. are stored in it (the computer's Parameters or CMOS). If the CMOS RAM (the RTC chip) loses power (i.e.: because the backup battery is dead), all the informations stored in that CMOS chip is lost and the computer may not know anything about the hard disk configuration of your machine and therefore it cannot boot.
Also, when you install new software and, especially, new hardware, it is important to reset the Parameter RAM, so as to allow your computer to "know" about the recent changes you've brought about.

These are some instructions on how to change the battery for various PowerMac models; specific Beige G3 instructions are towards the end of the page.
If you think you've been having the same battery for more than 5 years, it is time to change it as for good precaution measure.
The kind of battery you need inside a Beige G3 PowerMac, looks just like this one (I'm not promoting the shop; it's just an example).

I hope not to have confused you and to have helped you understand better what PRAM is and why PowerMacs have batteries inside them.

Costa