Q: Can I mix different-capacity RAM modules?

A: No problem for SDRAM (PC100 and PC133) and DDR RAM. You can fill your PC's memory slots with virtually any combination of different-capacity modules as long as they're the same type. In fact, mixing different-capacity modules is the best way to upgrade without removing the old RAM from your PC.

You can't mix SDRAM and DDR modules, though. They operate at different voltages and communicate with the system in different ways. Motherboards are designed for either one or the other.

Rambus (or RDRAM) memory is different. You can install a single RIMM in some older motherboards based on the Intel 820 chip set. But for more-recent motherboards that use the Intel 840, 850, and 850E chip sets, RIMMs must be installed in pairs. And RIMM capacities and speeds must match.

Q: Can I mix memory modules of different speeds?

A: Yes. But all modules must be the same type. It's okay to add higher-speed modules; they just won't run at their full speed. The newest Rambus systems using the Intel 850E chip set with a 533-MHz frontside bus require 800-MHz or 1.066-GHz modules with an access speed of 40 nanoseconds. And these modules are incompatible with earlier Rambus systems.

Q: Do I need to fill all the memory slots?

A: Not with SDRAM or DDR RAM. But with Rambus, every slot usually must be filled. You can buy continuity modules (about $10 each) that fill Rambus slots without adding memory.

If your PC is really long in the tooth (over five years old), it probably uses SIMMs, which must be installed in pairs. Most motherboards that use SIMMs have four RAM sockets, but only two need to be filled.

Q: Can I mix modules from different manufacturers?

A: Yes, as long as they're the same RAM type. Don't believe dealers who say it can't be done. They want you to buy the RAM they have available, which is usually from one manufacturer.

Q: What's ECC RAM, and should I buy it?

A: Error-Correcting-Code RAM modules have special circuitry that corrects memory errors on the fly. They're used primarily in servers and high-end workstations. ECC SDRAM will work in non-ECC motherboards, but you won't get the benefit of the ECC circuitry, so it doesn't make sense to pay the extra cost.

Q: I just filled my memory slots with 768MB of RAM. Now my system doesn't work right and locks up.

A: You're running into a problem with Windows or the limitations of your PC's BIOS. Windows 95, 98, 98 SE, and Me aren't designed to work with more than 512MB of RAM. (Windows NT, 2000, and XP don't have this limitation.) If you encounter this problem, check out Microsoft's product support.

Also, some older motherboards aren't designed to take 256MB and 512MB RAM modules. Check your manual or your PC manufacturer's Web site; sometimes a BIOS upgrade solves the problem.

Q: What is registered memory? Can I use it?

A: Used in certain servers and high-end workstations, registered memory uses special timings and isn't compatible with the unbuffered (also known as unregistered) memory used in most motherboards.

Q: What is the "CL" that I see listed in memory specifications, especially with premium-priced, "high-performance" memory?

A: Time to get a bit technical. CL stands for CAS (Column Access Strobe) Latency, which is the amount of time that the PC will wait before retrieving data from the RAM. CL2 modules need the PC to wait two CPU clock cycles before getting data; CL3 modules need a delay of three clock cycles. CL2 memory is more expensive than CL3, but your motherboard must support the faster modules. The CL speed is programmed into the memory module and is automatically detected by the setup program's SPD (Serial Presence Detect) when you boot the system. If the PC detects a CL2 memory module and can use it, the speed will be set automatically. A CL2 memory module will work in motherboards that support CL3, but not at its maximum speed. In most cases, you don't need to be concerned about CL when you're purchasing memory, as long as you order modules specifically designed for your PC's motherboard.