PC Hardware in a Nutshell, 3rd Edition [Electronic resources] نسخه متنی

5.8 Troubleshooting Memory Installation and Operation

Once installed and
configured, memory seldom causes problems. When problems do occur,
they may be as obvious as a failed RAM check at boot or as subtle as
a few corrupted bits in a datafile. The usual symptom of memory
problems is a kernel panic under Linux or a blue-screen crash under
Windows. Unfortunately, that occurs so often with Windows that
it's of little use as a diagnostic aid. When
troubleshooting memory problems, always do the following:

Use standard antistatic precautions. Ground yourself before you touch
a memory module.

Remove and reinstall all memory modules to ensure they are seated
properly. While you're doing that,
it's a good idea to clean the contacts on the memory
module. Some people gently rub the contacts with a pencil eraser.
We've done that ourselves, but memory manufacturers
recommend against it because of possible damage to the contacts.
Also, there is always the risk of a fragment from the eraser finding
its way into the memory slot, where it can block one or more
contacts. Better practice is to use a fresh dollar bill, which has
just the right amount of abrasiveness to clean the contacts without
damaging them.

Although we have never used it, many people whom we respect recommend using Stabilant-22, a liquid contact enhancer. You'll probably keel over from sticker shock when you see the price of this stuff, but a drop or two is all that's needed, and a tiny tube lasts most people for years (http://www.stabilant.com/).

Before assuming memory is the problem,
check all internal cables to ensure none is faulty or has come loose.

The next steps you should
take depend on whether you have made any changes to memory recently.

5.8.1 ... When You Have Not Added Memory

If you suspect memory problems but have not added or reconfigured
memory (or been inside the case), it's unlikely that
the memory itself is causing the problem. Memory does simply die
sometimes, and may be killed by electrical surges, but this is
uncommon because the PC power supply itself does a good job of
isolating memory and other system components from electrical damage.
The most likely problem is a failing power supply. Try one or both of
the following:

If you have another system, install the suspect memory in it. If it
runs there, the problem is almost certainly not the memory, but the
power supply.

If you have other memory, install it in the problem system. If it
works, you can safely assume that the original memory is defective.
More likely is that it will also fail, which strongly indicates power
supply problems.

If you have neither another system nor additional memory, and if your
system has more than one bank of memory installed, use binary
elimination to determine which modules are bad. For example, if you
have two modules installed (one per bank), simply remove one module
to see if that cures the problem. If you have four identical modules
installed (one per bank), designate them A, B, C, and D. Install only
A and B and restart the system. If no problems occur, A and B are
known good and the problem must lie with C and/or D. Remove B and
substitute C. If no problems occur, you know that D is bad. If the
system fails with A and C, you know that C is bad, but you
don't know whether D is bad. Substitute D for C and
restart the system to determine if D is good.

If you haven't enough banks to allow binary
elimination, the best solution is to remove the modules, wrap them if
possible in a static-safe bag (the pink plastic that most components
arrive in), and take them to a local computer store that has a memory
tester.

MS-DOS, Windows 3.X, and Windows 9X do not stress memory. If you install Windows NT/2000/XP or Linux, memory errors may appear on a PC that seemed stable. People often therefore assume that they did something while installing the new OS to cause the errors, but that is almost never the case. Such errors almost always indicate a real problem with physical memory. The memory was defective all along, but the more forgiving OS simply ignored the problem.

5.8.2 ... When You Are Adding Memory

If you experience problems when adding memory, note the following:

If a DIMM appears not to fit, there's good reason.
SDR-SDRAM DIMMs have two notches whose placement specifies 3.3V
versus 5V and buffered versus unbuffered. DDR-SDRAM DIMMs have a
keying notch in a different location. If the DIMM notches
don't match the socket protrusions, the DIMM is of
the wrong type.

If the system displays a memory mismatch error the first time you
restart, that usually indicates no real problem. Follow the prompts
to enter Setup, select Save and Exit, and restart the system. The
system should then recognize the new memory. Some systems require
these extra steps to update CMOS.

Verify the modules are installed in the proper order. Unless the
motherboard documentation says otherwise, fill banks sequentially
from lowest number to highest. Generally, install the largest module
in Bank 0, the next largest in Bank 1, and so on. A few systems
require the smallest module be in Bank 0 and larger modules
sequentially in higher banks.

If the system recognizes a newly installed module as half actual size
and that module has chips on both sides, the system may recognize
only single-banked or single-sided modules. Some systems limit the
total number of "sides" that are
recognized, so if you have some existing smaller modules installed,
try removing them. The system may then recognize the double-side
modules. If it doesn't, return those modules and
replace them with single-sided modules.

A memory module may not be defective, but still be incompatible with
your system. For example, many 486s treat three-chip and nine-chip
SIMMs differently, although they should theoretically be
interchangeable. Some 486s use only three-chip SIMMs or only
nine-chip SIMMs. Others use either, but generate memory errors if you
have both types installed.

A memory module may not be defective, but still be incompatible with
your current configuration. For example, if you install a CAS3 PC133
DIMM in a 133 MHz FSB Pentium III motherboard that is configured to
use CAS2 timing, the system will almost certainly generate memory
errors.