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| Troubleshooting the System Memory | Apparent
I have a new or recently-upgraded system and my system memory appears to have failed or
is not working in general
Explanation: There is an apparent failure of the memory on a newly built or
upgraded system. This means either that diagnostic tests or troubleshooting procedures
have identified the memory as being suspected of being bad, or the system memory is one
possible cause of an unknown system problem.
Diagnosis: There are three common categories of causes of memory problems on a new
system. The first is improper configuration, or using the wrong type of memory. The second
is incorrect installation. The third is hardware failure of the memory itself; since there
is so much cheap junk being sold to people who only look at cost when making a purchasing
decision, it is more common than ever to end up with defective RAM.
- First, check for possible general problems associated with
new systems. One of these could be causing the memory to appear to be bad when it is
- If you happen to have a duplicate set of memory modules available that are just like the
ones you are trying to diagnose, then try swapping the spare modules in to see if the
problem goes away. If it does, put the original memory back in to see if the problem
returns. If it returns, you can feel very confident that the original memory itself was
bad. If you don't have any spare memory for this sort of test, just continue with the
- Make sure to carefully check the memory modules, which sometimes appear to be inserted
correctly when they are not. Make sure they are all sticking up at the correct height from
the motherboard. Ensure that the modules have been pushed all the way into their sockets
and that the spring clips have snapped into position properly to hold them.
- Check for loose connections within the PC.
- Check the inside of the case for possible overheating
- If you have just installed a new 32-bit operating system where before you were just
using DOS, this may bring a memory problem to the surface. When this happens, the bad
memory was probably there the whole time, but DOS is much more forgiving of bad memory
than Windows 95, Windows NT or other 32-bit operating systems are.
- If you installed used memory, or installed into an older motherboard, the problem may be
dirt either on the modules or in the sockets. Try removing the modules and gently cleaning
the contacts with a soft cloth. If the motherboard is in an existing system then it has
had a chance to collect dust in the sockets. Clean the dust away using either compressed
air or a PC vacuum.
- If you are using SIMMs, ensure that you have installed a full bank of memory. For a 486-class
motherboard using 30-pin SIMMs, you must have 4 identical SIMM modules. For a
Pentium-class motherboard using 72-pin SIMMs, you must have 2 identical SIMMs. For DIMM
memory a single DIMM is a bank so this does not apply.
- Some 486-class motherboards that have both 30-pin and 72-pin SIMM sockets have
restrictions on the extent to which both can be used. Some Pentium and later motherboards
that have both SIMM and DIMM sockets can also have restrictions. For example, you may only
be able to use either one set of sockets or the other, or you may only be able to fill
some of the sockets depending on what you are using. Consult your motherboard
documentation for exact guidelines matching what you are using.
- Make sure that you have used the correct sockets. Motherboards have multiple sockets and
putting modules in the incorrect ones will often cause problems. For example, most Pentium
motherboards have four SIMM sockets, which make up two banks. If you put the SIMMs in the
middle two sockets then you have accidentally put memory into half of the first bank and
half of the second bank, and the PC will not boot. Putting the memory into the full second
bank instead of the first won't work on many motherboards either (but it will on some).
- Ensure that the size of memory modules you have selected is supported. Some motherboards
will not support certain sizes of modules; in particular, 2 MB, 8 MB and 32 MB SIMMs are
generally composite and will not work in
some machines. Consult your motherboard manual.
- Watch out for 16 MB non-parity SIMMs that have chips on both sides of the SIMM
(parity SIMMs usually do). Some of these are showing up on the market that are actually
composite (even though 16 MB SIMMs aren't supposed to be) and they will not work properly
in a system expecting non-composite 16 MB SIMMs only. Ask for the SIMMs to be replaced
with non-composite versions.
- Check the technology of the memory you are
using. Whether a motherboard supports memory types such as EDO or SDRAM for example,
depends on the chipset used on the motherboard, as well as how the motherboard itself was
implemented. Using EDO memory in many older machines will cause them to lock up.
- If using SDRAM, make sure you are using the right type. SDRAM comes in 2-clock and
4-clock varieties, and some motherboards require only one kind or the other. Some
motherboards also require SDRAM that has the "serial presence detect" EEPROM on
it. See this section for more details.
- Support for EDO memory is not found in
386 and earlier machines. 486 class machines fall into three categories: some will allow
it and use it properly as EDO (these usually have a BIOS parameter to enable it); some
will allow it to be used but will use it as regular FPM memory; and some will not work
with it at all.
- Some 430HX motherboards come with DIMM sockets. Most DIMM form factor memory is SDRAM,
but SDRAM will not work in these boards because the 430HX chipset does not support it.
These slots are intended for DIMM EDO memory.
- If your system uses DIMMs, make sure that you are using the right kind. DIMMs come in
different voltages, and buffered and unbuffered versions.
- Many motherboards that support both DIMM and SIMM memory will malfunction if both are
used on the board at once. This is because most DIMMs require 3.3 volt power, while SIMMs
run at 5 volts. When both are present, the DIMMs are fed 5 volts and problems can result.
Try the system with only one type of memory.
- If you have an older motherboard, especially in a 486 or earlier system, there is a
chance that your motherboard requires jumpers to be set when adding memory to the PC. If
this is the case then failing to change the jumpers may result in either the memory not
being detected, or in incorrect operation.
- Some systems require a special
BIOS setting to be enabled when using more than 64 MB of memory. If you are trying to
use more than 64 MB, check for one of these settings and enable it if necessary.
- Mismatches between different banks of memory can cause problems. For example, some
systems only like when all the memory in the machine is identical, even in different
banks. If you use memory that is of different speed or if one bank is using FPM and another EDO memory, your motherboard may not be happy.
Many newer systems allow this with no problem, but some older ones do not. You may need to
try changing around the memory configuration. Try to see if the machine works with either
bank installed but not both together. If this is the case, then the two banks of memory
- Some PCs use proprietary, special
modules; for example IBM's PS/2 systems. Using industry standard memory in a machine
that requires special modules, or vice-versa, will cause problems.
- Older 386 and 486 motherboards use 30-pin SIMMs. These SIMMs generally come in two
versions: 9-chip SIMMs or 3-chip SIMMs. The 9-chip version uses all the same DRAM chips,
while the 3-chip version uses chips of different sizes. Some PCs do not work properly with
the 3-chip SIMMs; you need to consult your motherboard manual or technical support for the
system. See here for more.
- There could be a problem with the relative speed of the memory modules compared to the timing settings (memory access timing
or wait states) that were entered in the BIOS setup program. Double-check the speed of the
modules you are using to make sure it is fast enough. Some PCs will work with slower
memory modules, but you may need to increase wait states or slow down the memory timing. See here for more.
- Some motherboards will work with EDO memory installed, but only if EDO support is
specifically enabled through a BIOS parameter. You may need to boot the system with
regular fast-page mode memory to get into the BIOS setup, change the setting to EDO, and
then shut the machine down and replace with the EDO memory.
- There could be something wrong the memory modules themselves. Note that bad memory will
often pass the BIOS memory test at boot time, and will also often pass the tests performed
by those small module testers that many vendors use. Those tests are very superficial and
will not catch all memory problems. If you can, try the modules in another PC that uses
the same kind of memory. If you have performed all the checks listed in the points above,
and the memory works in another PC, the memory itself may very well be bad. Try to replace
the memory and see if the problem goes away.
- There could be a problem with the power supply, though this is unusual. A bad power
supply can cause strange memory errors that crop up because the memory is not getting
- Check in this section that discusses memory problems with
existing systems, for more possible ideas that could account for the failure (though they
are less likely).
- There could be a motherboard problem. If double-checking all the settings and replacing
the memory does not fix the problem, there may be a bad motherboard or a problem with how
it is configured. Troubleshoot it here.
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