MemTest86 Technical Information
Windows Boot-disk Creation
Before you can use Mestest86 on a Windows system it must first be installed on a CD-ROM, USB flash drive or floppy disk. To create a CD-ROM a system with the capability of creating a CD-Image from an ISO file is required.
Create a boot-able CD-ROM:
Create a boot-able USB Flash drive:
Create a boot-able floppy:
Linux Boot-disk Creation
MemTest86 is a stand alone program and can be loaded from a bootable CD-ROM, USB disk, floppy disk or a Linux disk partition. It is recommended that Linux users download and install pre-compiled packages to create bootable media. Advanced users may wish to build from source and optionally make source code changes.
Create a boot-able CD-ROM:
Create a boot-able USB Flash drive:
Create a boot-able floppy:
Apple Mac Boot-disk Creation
See this forum post for details on how to prepare a bootable CD on an Apple Mac for RAM testing.
MemTest86 has a limited number of online commands. The on-line commands do not work with some USB keyboards. Online commands provide control over error report modes, test selection and test address range. A help bar is displayed at the bottom of the screen listing the available on-line commands.
Command Description: ESC Exits the test and does a warm restart via the BIOS. c Enters test configuration menu Menu options are: (1) Test Selection (2) Address Range (3) Error Report Mode (4) CPU Selection Mode (5) Refresh Screen (6) Restart Test (7) Miscellaneous Options SP Set scroll lock (Stops scrolling of error messages) Note: Testing is stalled when the scroll lock is set and the scroll region is full. CR Clear scroll lock (Enables error message scrolling)
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Memtest has three options for reporting errors. The default is an error summary that displays the most relevant error information. The second option is reporting of individual errors. In BadRAM Patterns mode patterns are created for use with the Linux BadRAM feature. This slick feature allows Linux to avoid bad memory pages. Details about the BadRAM feature can be found at http://home.zonnet.nl/vanrein/badram
The error summary mode displays the following information:
Error Confidence Value: A value that indicates the validity of the errors being reported with larger values indicating greater validity. There is a high probability that all errors reported are valid regardless of this value. However, when this value exceeds 100 it is nearly impossible that the reported errors will be invalid. Lowest Error Address: The lowest address that where an error has been reported. Highest Error Address: The highest address that where an error has been reported. Bits in Error Mask: A mask of all bits that have been in error (hexadecimal). Bits in Error: Total bit in error for all error instances and the min, max and average bit in error of each individual occurrence. Max Contiguous Errors: The maximum of contiguous addresses with errors. ECC Correctable Errors: The number of errors that have been corrected by ECC hardware. Errors per DIMM slot: Error counts are reported for each memory module installed in the system. Use the Show DMI Memory Info runtime option for detailed memory module information. Test Errors: On the right hand side of the screen the number of errors for each test are displayed.
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Troubleshooting Memory Errors
Please be aware that not all errors reported by MemTest86 are due to bad memory. The test implicitly tests the CPU, L1 and L2 caches as well as the motherboard. It is impossible for the test to determine what causes the failure to occur. However, most failures will be due to a problem with memory module. When it is not, the only option is to replace parts until the failure is corrected.
Once a memory error has been detected, determining the failing SIMM/DIMM module is not a clear cut procedure. With the large number of motherboard vendors and possible combinations of memory slots it would be difficult if not impossible to assemble complete information about how a particular error would map to a failing memory module. However, there are steps that may be taken to determine the failing module. Here are four techniques that you may wish to use:
Sometimes memory errors show up due to component incompatibility. A memory module may work fine in one system and not in another. This is not uncommon and is a source of confusion. In these situations the components are not necessarily bad but have marginal conditions that when combined with other components will cause errors.
Often the memory works in a different system or the vendor insists that it is good. In these cases the memory is not necessarily bad but is not able to operate reliably at full speed. Sometimes more conservative memory timings on the motherboard will correct these errors. In other cases the only option is to replace the memory with better quality, higher speed memory. Don't buy cheap memory and expect it to work reliably. On occasion "block move" test errors will occur even with name brand memory and a quality motherboard. These errors are legitimate and should be corrected.
We are often asked about the reliability of errors reported by Mestest86. In the vast majority of cases errors reported by the test are valid. There are some systems that cause MemTest86 to be confused about the size of memory and it will try to test non-existent memory. This will cause a large number of consecutive addresses to be reported as bad and generally there will be many bits in error. If you have a relatively small number of failing addresses and only one or two bits in error you can be certain that the errors are valid. Also intermittent errors are without exception valid. Frequently memory vendors question if MemTest86 supports their particular memory type or a chipset. MemTest86 is designed to work with all memory types and all chipsets.
All valid memory errors should be corrected. It is possible that a particular error will never show up in normal operation. However, operating with marginal memory is risky and can result in data loss and even disk corruption. Even if there is no overt indication of problems you cannot assume that your system is unaffected. Sometimes intermittent errors can cause problems that do not show up for a long time. You can be sure that Murphy will get you if you know about a memory error and ignore it.
MemTest86 cannot diagnose many types of PC failures. For example a faulty CPU that causes Windows to crash will most likely just cause MemTest86 to crash in the same way.Back to top
Troubleshooting Memory Errors
The time required for a complete pass of MemTest86 will vary greatly depending on CPU speed, memory speed and memory size. MemTest86 executes indefinitely. The pass counter increments each time that all of the selected tests have been run. Generally a single pass is sufficient to catch all but the most obscure errors. However, for complete confidence when intermittent errors are suspected testing for a longer period is advised.Back to top
Memory Testing Philosophy
There are many good approaches for testing memory. However, many tests simply throw some patterns at memory without much thought or knowledge of memory architecture or how errors can best be detected. This works fine for hard memory failures but does little to find intermittent errors. BIOS based memory tests are useless for finding intermittent memory errors.
Memory chips consist of a large array of tightly packed memory cells, one for each bit of data. The vast majority of the intermittent failures are a result of interaction between these memory cells. Often writing a memory cell can cause one of the adjacent cells to be written with the same data. An effective memory test attempts to test for this condition. Therefore, an ideal strategy for testing memory would be the following:
It should be obvious that this strategy requires an exact knowledge of how the memory cells are laid out on the chip. In addition there is a never ending number of possible chip layouts for different chip types and manufacturers making this strategy impractical. However, there are testing algorithms that can approximate this ideal.Back to top
MemTest86 Test Algorithms
MemTest86 uses two algorithms that provide a reasonable approximation of the ideal test strategy above. The first of these strategies is called moving inversions. The moving inversion test works as follows:
This algorithm is a good approximation of an ideal memory test but there are some limitations. Most high density chips today store data 4 to 16 bits wide. With chips that are more than one bit wide it is impossible to selectively read or write just one bit. This means that we cannot guarantee that all adjacent cells have been tested for interaction. In this case the best we can do is to use some patterns to insure that all adjacent cells have at least been written with all possible one and zero combinations.
It can also be seen that caching, buffering and out of order execution will interfere with the moving inversions algorithm and make less effective. It is possible to turn off cache but the memory buffering in new high performance chips can not be disabled. To address this limitation a new algorithm I call Modulo-X was created. This algorithm is not affected by cache or buffering. The algorithm works as follows:
This algorithm accomplishes nearly the same level of adjacency testing as moving inversions but is not affected by caching or buffering. Since separate write passes (1a, 1b) and the read pass (1c) are done for all of memory we can be assured that all of the buffers and cache have been flushed between passes. The selection of 20 as the stride size was somewhat arbitrary. Larger strides may be more effective but would take longer to execute. The choice of 20 seemed to be a reasonable compromise between speed and thoroughness.Back to top
Individual Test Descriptions
MemTest86 executes a series of numbered test sections to check for errors. These test sections consist of a combination of test algorithm, data pattern and cache setting. The execution order for these tests were arranged so that errors will be detected as rapidly as possible. A description of each of the test sections follows:
Test 0 [Address test, walking ones, no cache]
Tests all address bits in all memory banks by using a walking ones address pattern.
Test 1 [Address test, own address, Sequential]
Each address is written with its own address and then is checked for consistency. In theory previous tests should have caught any memory addressing problems. This test should catch any addressing errors that somehow were not previously detected. This test is done sequentially with each available CPU.
Test 2 [Address test, own address, Parallel]
Same as test 1 but the testing is done in parallel using all CPUs and using overlapping addresses.
Test 3 [Moving inversions, ones&zeros, Parallel]
This test uses the moving inversions algorithm with patterns of all ones and zeros. Cache is enabled even though it interferes to some degree with the test algorithm. With cache enabled this test does not take long and should quickly find all "hard" errors and some more subtle errors. This is done in parallel using all CPUs.
Test 4 [Moving inversions, 8 bit pattern]
This is the same as test 3 but uses a 8 bit wide pattern of "walking" ones and zeros. This test will better detect subtle errors in "wide" memory chips.
Test 5 [Moving inversions, random pattern]
Test 5 uses the same algorithm as test 4 but the data pattern is a random number and it's complement. This test is particularly effective in finding difficult to detect data sensitive errors. The random number sequence is different with each pass so multiple passes increase effectiveness.
Test 6 [Block move, 64 moves]
This test stresses memory by using block move (movsl) instructions and is based on Robert Redelmeier's burnBX test. Memory is initialized with shifting patterns that are inverted every 8 bytes. Then 4mb blocks of memory are moved around using the movsl instruction. After the moves are completed the data patterns are checked. Because the data is checked only after the memory moves are completed it is not possible to know where the error occurred. The addresses reported are only for where the bad pattern was found. Since the moves are constrained to a 8mb segment of memory the failing address will always be less than 8mb away from the reported address. Errors from this test are not used to calculate BadRAM patterns.
Test 7 [Moving inversions, 32 bit pattern]
This is a variation of the moving inversions algorithm that shifts the data pattern left one bit for each successive address. The starting bit position is shifted left for each pass. To use all possible data patterns 32 passes are required. This test is quite effective at detecting data sensitive errors but the execution time is long.
Test 8 [Random number sequence]
This test writes a series of random numbers into memory. By resetting the seed for the random number the same sequence of number can be created for a reference. The initial pattern is checked and then complemented and checked again on the next pass. However, unlike the moving inversions test writing and checking can only be done in the forward direction.
Test 9 [Modulo 20, Random pattern]
Using the Modulo-X algorithm should uncover errors that are not detected by moving inversions due to cache and buffering interference with the algorithm.
Test 10 [Bit fade test, 2 patterns]
The bit fade test initializes all of memory with a pattern and then sleeps for a few minutes. Then memory is examined to see if any memory bits have changed. All ones and all zero patterns are used.Back to top