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| Troubleshooting the System Memory | Memory
Not Recognized ]
Not all of the memory that I have installed in the PC is being recognized. The system
thinks I have megabytes less memory in the PC than I do
Explanation: When the PC is booted up, it does not show that it is detected all of
the memory in the machine. Several megabytes, usually an amount equal to the size of a
memory module or half a memory module, are missing. For example, you may have two 8 MB
SIMMs in the PC but the system is only reporting 8 MB of system memory.
Diagnosis: This is almost always caused by using the wrong type of memory,
incorrect installation, or incorrect configuration. Putting the modules into the wrong
sockets, forgetting to change jumpers that may be present, using unsupported technology or
unsupported module sizes are common reasons that memory would be not detected.
Recommendation: Most of the usual causes of memory not being recognized can also be
causes of more general memory failures, although certain potential causes are more likely
to be at fault in one situation than the other. Assuming that this is a new PC build or
upgrade, you may want to look here to troubleshoot the memory
as a general failure. This is especially so if the system isn't booting or is exhibiting
strange behavior. Below I have several of the causes of memory problems from the general
failure section, modified and rearranged so the more likely candidates of memory
non-recognition are listed first. I have also excluded some of the causes of outright
memory failure that are not generally found when you have memory simply not being
- 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 not.
- 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.
- One of the most common causes of unrecognized memory is using an unsupported module
size. 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 modules are composite and will not work
in some machines. Consult your motherboard manual.
- A partial bank of memory, if it is not the only bank in the PC, is often ignored by the
system (although some systems will refuse to boot in this situation). 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. If you put memory into all of the
sockets on such a machine, you may find some of it ignored. Consult your motherboard
documentation for exact guidelines matching what you are using, and look on the
motherboard for any jumpers that control memory configuration. Try using only one bank of
memory at a time.
- 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 some systems will only
recognize them as being half of their true size. Ask for the SIMMs to be replaced with
- 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).
- 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 some machines can cause the memory not to be recognized.
- 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 and DIMM SDRAM memory may be ignored.
- 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.
- 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
- 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 does not work 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 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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