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Hardware Emergency Room:   Memory
 
Contents: RAM
Upgrading Memory
Memory Problems

RAM - Working Memory Storage
All the data that your system uses and works with during operation are stored in its RAM.  In order for your computer to use information stored on a drive, it must be read into working memory storage, which is made up of RAM chips.  This memory is temporary because the data and programs are there only as long as the system is powered. 

RAM Module
dimm
(Click to Enlarge)

When a system is shut down properly (as opposed to being powered off while still in Windows) any changes made to the data are copied back to disk.  This is one reason why it is important to shut down your system gracefully by using the shut down command and then waiting for the "It's now safe to turn off  your computer" message before turning it off.
RAM is a very complicated and highly technical subject.  An in-depth study is not possible here, but we can touch on the most salient points that you will need to know if you are troubleshooting RAM problems.
The traditional RAM type is called DRAM (dynamic RAM).  The other type is SRAM (static RAM).  While SRAM can respond much faster than DRAM, the latter is cheaper and is becoming faster all the time.  Types of DRAM include:
  • FPM (Fast Page Mode) - The RAM traditionally used for PC's prior to the introduction of EDO RAM.  Mounted in SIMM modules of 2, 4, 8, 16, or 32 MB.  Typically found in 70 ns (nano seconds) and 60 ns (faster) versions.  You can not mix different speeds on the same Pentium motherboard.
  • ECC (Error Correcting Code) - often used for servers.
  • EDO (Extended Data Output) - Faster than FPM, it is usually sold as 60 ns.
  • SDRAM (Synchronous DRAM) - The first DRAM to synchronize itself to the timing of the CPU.  With today's faster CPUs this type of RAM provides significant speed improvement over EDO.  Found as 168 pin DIMMs.
The type of RAM you can install on your system is controlled by the motherboard and its chip sets, and must match their specifications.  In addition, RAM chips come in different sizes, which must also match the motherboard's requirements.
In the early days, memory chips were installed individually, then someone figured out to put the individual chips on an expansion card.  These cards were called SIMM (single inline memory module) modules, and at first they had only a few MB of RAM on a 30-pin card.  Later, as their capacity increased, so did the number of pins (from 30 to 72 to 168).  SIMMs can have 2 or 4 or 8 chips on each side, and can be double-sided.

SIMM Module
simm
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Many early computers required that SIMM modules be installed in pairs, in what were called memory "banks" of sockets, which would hold two modules.
SDRAM are made on DIMM (dual Inline memory module) modules.  They have a 168-pin edge connector and require modern motherboards.  Unlike SIMMs, you can install one module at a time, and their advantage is significantly increased speed.  Modern systems can support from 4 to 8 DIMM sockets.

DIMM Module
dimm
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A quick test of whether you are looking at a SIMM or a DIMM is to count the number of notches at the bottom.  SIMMs have one notch, DIMMs have two.  SIMM slots are white and held in place by small clips at both ends.  DIMM slots are black and have much larger clips on both ends.  DIMMs are inserted upright, with the clips snapping up to lock the module into place.  SIMMs are inserted at a 45 degree angle and tilted forward until they are upright and the clips snap into place.

SIMM Insertion
insertsimm
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Upgrading Memory

PC memory speeds range from about 10ns to 200ns.  When you replace a failed memory module, you must install a module of the same type and speed, or faster.  Substituting a faster module, however, doesn't necessarily improve your system's performance because it will operate the new memory at the slower speed. 

In addition, it is important to know if your memory sockets are tin-plated or gold-plated, and to install memory that is of the same metal as the socket.  Mixing the plating can result in eventual memory failure.
Adding memory to a system is an excellent way to improve the performance of your computer, and as memory prices get cheaper, adding memory is cost-effective as well.  To do this you can either add memory to empty slots on your motherboard, or replace the current memory with higher-capacity modules.   

 

If you need help determining how much memory your system currently holds, you can simply reboot your computer and watch for the POST memory test, which counts up your memory and displays the count as it does so.   Or you can right click on the My Computer icon on your desktop, go to Properties, and look at your System Properties, which displays the current memory near the bottom of the window. System Properties
controlpanel.jpg (34458 bytes)
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Always follow the motherboard manufacturer's recommendations on which memory you can install.  In addition, some chip sets and BIOS can not support more than 64MB of RAM, adding more than that will actually decrease your computer's performance.
Memory must also be installed in banks, and some systems require multiple memory chips (generally two or four SIMMs) in a given bank.  On other systems (for example, Pentiums and above) a single DIMM represents an individual bank, and they can be added or removed one at a time.  In general, modern machines have from 2 to 4 memory banks, and a bank does not work if it is only partially filled.  The documentation for your computer's motherboard describes its particular requirements in detail.
The physical orientation and numbering of SIMMs or DIMMs on a motherboard is not standardized across manufacturers.  Use your system documentation to identify memory bank layout.
It is critical to prevent electrostatic discharge when you install memory modules, and to install them according to the accompanying instructions.  Make sure your system is turned off and unplugged from its power source.  Memory modules are keyed (notched) to ensure proper installation in the expansion socket.  Never use force to install a module - if it does not slip easily into place and lock, it may not be aligned properly.
On older systems you may have to run the CMOS Setup to add the new amount of memory (modern systems will do this automatically).  When you power up the first time watch your Power On Self Test to make sure the memory test agrees with the total amount of memory you now have installed.

CMOS Setup
cmos
(Click to Enlarge)

A word of warning -- sometimes adding a lot of memory to a computer can increase the demand on the power supply, leading to power supply failure.  The only way to solve this problem and keep your new memory is to replace the power supply.

Memory Problems

Errors and faults in system memory can crash a system, make it behave erratically, or even stop you from installing an operating system.  Symptoms of bad memory can manifest themselves in many ways, including "Parity Check" errors, "201" errors, "164" errors, "Incorrect Memory Size" errors, "ROM" errors, "Memory Address" errors, and more. 

Although in modern systems the address of a fault is generally presented in the error message, it is usually impossible to correlate a specified bad address with a particular component, so trial and error testing is your best option.  Generally, when you suspect memory problems you can try the following:
  • If you have just installed a new peripheral device, check Device Manager to make sure there are no conflicts (look for a yellow exclamation mark or a red "X" symbol).  If there is no hardware conflict specified, take out all devices and reinstall them one at a time to determine the device that is causing the problem.
  • Check your memory by reseating each unit and restarting your system each time to be sure that each unit is inserted properly and working.
  • Check your CMOS Setup settings to make sure the correct amount of RAM has been entered.
  • Check your motherboard jumpers if necessary, especially when you add more memory, to make sure that bank 0 is defined properly, along with the number of active banks.
  • Check your memory speed to make sure that the RAM in your system is fast enough to support your CPU.  Never mix memory speeds within the same bank, and do not mix memory types.  It is inadvisable to mix parity and non-parity RAM.
  • Try a known, good memory unit by rotating it through each occupied memory socket in sequence.  If the error disappears when the known, good memory is in a given slot, then the memory that was displaced is likely bad.

Device Manager
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Conflict Symbols
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CMOS Setup
cmos
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If you see the number "164" displayed on the monitor:
  • This is a generic memory-size error - the amount of memory found during the POST does not match the amount of memory listed in the system's CMOS Setup.
  • Check your CMOS Setup settings to make sure the listed memory matches the actual memory amount.
  • Adjust the figure in CMOS Setup if necessary.
  • If CMOS Setup parameters do not remain in the system after re-powering the computer, try replacing the battery.
  • Note that some modern CMOS Setup routines automatically detect the amount of RAM so do not list it.  If this is the case, try recalculating memory by entering CMOS Setup, saving changes and then exiting.

CMOS Setup
cmos
(Click to Enlarge)

If you see an "Incorrect Memory Size" error message displayed on the monitor:
  • This message can be displayed if there is a CMOS Setup error, or if there is an actual memory failure.
  • Check your CMOS Setup as described above.
  • If CMOS Setup parameters are correct, try a known, good memory unit by rotating it through each occupied memory socket in sequence.  If the error disappears when the known, good memory is in a given slot, then the memory that was displaced is likely bad.
If you see a "ROM Error" message displayed on the monitor:
  • The POST system ROM test has failed.  You may need to replace the system BIOS ROM(s).
If you see a general "Memory address line failure" error or similar error on the monitor:
  • Try a known, good memory unit by rotating it through each occupied memory socket in sequence.  If the error disappears when the known, good memory is in a given slot, then the memory that was displaced is likely bad.
  • If, after rotating through all occupied memory the problem persists, you may have a motherboard problem.
You see a "Decreasing Available Memory" error message on the monitor:
  • This message indicates that a failure has been detected.  In order to allow the system to function, all memory after the failure has been disabled.
  • Check your memory by reseating each unit and restarting your system each time to be sure that each unit is inserted properly and working.
  • Try a known, good memory unit by rotating it through each occupied memory socket in sequence.  If the error disappears when the known, good memory is in a given slot, then the memory that was displaced is likely bad.
Reseating
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Many CMOS Setup programs allow you disable the "Quick" Power On Self Test to get a more detailed test that counts down your memory several times as different areas are tested. 

The terminology used to describe these test options differs between manufacturers.  Consult your documentation if you are unsure of the proper setting for the detailed POST on your system.

The full memory test can be very helpful in troubleshooting memory problems:

  • Disable your "Quick" POST or enable the "Full" POST (depending on your BIOS).
  • If you have just one memory module, run the full POST on it and note any POST errors.
  • If you have multiple modules, remove them, then run the full POST on each separate chip, rotating through all, and note any POST errors.  Remember you may need to rotate sets of multiple modules so that you have a complete bank.
Quick POST
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If Device Manager indicates that there is a memory conflict, you likely have two expansion boards that have overlapping ROM or RAM addresses.  In these cases typically neither board will function properly when both are enabled; but disabling one will often cause the other to work properly.

Expansion Card
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Memory conflicts can be a particularly difficult problem to troubleshoot, and you need to examine the documentation for each adapter to find out what memory addresses it uses and how to change them.  In order for the two boards to co-exist you will need to reconfigure one of the boards by either changing jumpers, switching settings or software-driver parameters. 
In a Plug-and-Play (PnP) system with PnP adapters, you may be able to use Device Manager's Edit Properties  to change memory usage on a conflicting board.  Edit Properties
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Back: CPU Problems
Next: Expansion Card Problems
 
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Disclaimer:  These pages are provided for information purposes only.  We cannot be held responsible for any damage you might inflict on your system while using the information contained herein.  We recommend you always refer any technical matter that is "over your head" to a qualified computer technician.
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2011 Double-Hammer Computer Services.  All rights reserved.
Last Updated: September 04, 2011

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