Keeping Parity in RAM

Q: I want to add RAM to my system. How can I tell whether I need parity RAM or not? Also, some salespeople have recommended something called logic parity. What is that, and how does it differ from regular parity?

A: Parity is an old-fashioned technique for checking the contents of memory for errors. For every byte of RAM (eight bits), a ninth bit is added for parity checking. When a memory address is read by the computer, the parity bit is compared to another parity bit calculated by the PC. If these two parity bits match, the data is assumed good. If they don't match, a fatal error is generated and the system halts to prevent further data corruption.

You can distinguish between parity and nonparity RAM by looking at the memory description on the SIMM. For example, you can tell that a 30-pin 2Mx9 SIMM has a parity bit because there are nine bits (x9) instead of eight; a 2Mx8 SIMM has no parity bit. Likewise, a 72-pin 4Mx36 SIMM also contains parity RAM because its 36 bits include four eight-bit bytes plus four bits left over, or one extra bit for each byte. As a rule, SIMMs labeled x8 or x32 lack parity bits, while SIMMs labeled x9 or x36 have them.

There are several ways to tell whether your current system is using parity memory or not. First, check the system manual for the manufacturer's recommendations for RAM upgrades. If you don't have the manual handy, run your PC's CMOS Setup and look under Advanced Chipset Setup for a memory-related entry such as Parity. If parity checking is enabled, you should add parity RAM. If it's disabled, you could add nonparity RAM.

Another alternative is to check the part number on the SIMM board (not on the SIMM's memory chips) against the online database of a major memory manufacturer such as Kingston (www.kingston.com) or PNY (www.pny.com). When you enter the part number of your SIMM, the database will return a matching part and description that should include a parity designation. If you don't have access to the Internet, any computer store that sells memory should be able to help.

As for regular versus logic parity (also called fake parity), there is indeed a difference. Logic parity relies on a parity generator chip instead of memory tests. The chip provides parity bits that always match memory's contents. In effect, logic parity lies to the motherboard. It only pretends to check parity, but provides no real protection for your system. I don't recommend it.

Generally you can use parity RAM in a nonparity motherboard and vice versa, but only if you disable parity checking via the PC's CMOS Setup or a motherboard jumper.

The Scoop on DIVX

Q: I'm considering an upgrade to DVD-ROM, but I'm confused about something called DIVX. What is this? Is it part of the DVD-ROM specification?

A: DIVX means Digital Video Express. It is a proposed extension to the existing DVD player standards already in place. For now, DIVX is an issue only with commercial DVD players that connect to TV sets, not with DVD-ROM drives installed in PCs. But since a PC already contains the elements essential to supporting DIVX, DIVX could be available for your PC in the near future.

According to Creative Labs, DIVX is the brainchild of consumer electronics retailer Circuit City and the law firm of Ziffren, Brittenham, Branca & Fischer. The DIVX approach requires users to have a special DIVX player--cum--modem and a dial-up connection. A DIVX player costs about $100 more than an ordinary DVD player.

Rather than renting a DVD movie, you buy a DIVX DVD movie for, say, $4.99. Once you insert a DIVX disc in the DIVX player, you can play the movie only during the next 48 hours. And once the DIVX player registers the movie online, you can't play it past that initial 48 hours unless you pay for an additional viewing or to permanently unlock the disc. Attempting to break the dial-up connection will render the player unusable until it's reconnected to the DIVX online service.

Normal DVD players can't read DIVX discs because DIVX uses instructions they don't recognize. To use DIVX, you need a player that's DIVX compatible. I haven't heard of any DIVX-ready DVD-ROM drives for the PC yet. But who knows? DIVX could find its way to your desktop sooner than you think.

Classic Hard Drive Problems

Q: I just installed a 420MB Conner hard drive as my PC's D: drive. The C: drive is a 520MB Maxtor. Installing programs on my D: drive often crashes the system, scrambling data on C:. The drives share the same data cable connected to an old IDE interface card. Do you know what's wrong?

A: The rules of adding hard drives have changed very little over the years. Start by checking the physical installation of both drives. The power and data cables should be snugly connected to the drives and controller. Check the drive jumpers to verify that the C: drive is set as the master (or master with slave present), and the D: drive is set as the slave. Check CMOS Setup to verify that you've entered the correct setup parameters for both drives. Finally, confirm that both drives have been partitioned and formatted correctly.

If the problem persists, you may have incompatible IDE drives, a rare but documented problem. In the old days, IDE drives from different manufacturers did not always work properly when combined in certain master/slave relationships. Try making drive D: the master and C: the slave, reversing the drive settings in CMOS Setup. You won't need to reverse their places on the data cable, but you will have to make sure that drive D: is bootable.

Fixing a Finicky Mouse

Q: Every so often, my Logitech mouse will not initialize at boot time. Why?

A: You didn't tell me how the mouse is connected (serial or PS/2) or what operating system you're using, but there are several things you can try. First, make sure the cable from the mouse to the PC is properly connected. If you're using a serial mouse, make sure the serial cable is screwed into the PC's female serial port. Check inside the PC to verify that the ribbon cable from the mouse port to the motherboard is also secure.

Next, check the mouse driver. If your PC is running DOS, you can usually add a command-line switch to the mouse driver (referenced in either CONFIG.SYS or AUTOEXEC.BAT) that will force your PC to detect the mouse at a specific port. For example, a /z switch may force detection at the PS/2 port. If you're using any version of Windows, download the latest driver for your mouse from Logitech (www.logitech.com) and install it.

If the mouse still goes undetected, it may be defective. See if plugging in a different mouse solves the problem.

New Video Card, No DVD Movies

Q: I upgraded my video adapter, and now my DVD movies won't play. Is the card bad or incompatible with DVD?

A: Neither. When you installed your DVD drive, you also installed a DVD-compliant MPEG video driver. When you later installed the new video card, you apparently replaced the DVD-compliant MPEG driver with a new one that doesn't support DVD. The solution? Simply reinstall the MPEG drivers that came with your DVD drive.

© 1998 Stephen J. Bigelow. All rights reserved. Stephen J. Bigelow is publisher of The PC Toolbox troubleshooting news letter. Visit the newsletter site at www.dlspubs.com. E-mail questions to him directly at sbigelow@cerfnet.com.

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