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

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 recognized:

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 non-composite versions.
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 cause problems.
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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