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Memtest86+ is a free, open-source, stand-alone memory tester for x86 and x86-64 architecture computers. It provides a much more thorough memory check than that provided by BIOS memory tests.
It is also able to access almost all the computer's memory, not being restricted by the memory used by the operating system and not depending on any underlying software like UEFI libraries.
Memtest86+ can be loaded and run either directly by a PC BIOS (legacy or UEFI) or via an intermediate bootloader that supports the Linux 16-bit, 32-bit, 64-bit, or EFI handover boot protocol. It should work on any Pentium class or later 32-bit or 64-bit CPU.
Binary releases (both stable and nightly dev builds) are available on memtest.org.
Memtest86+ v6.00 was based on PCMemTest, which was a fork and rewrite of the earlier Memtest86+ v5, which in turn was a fork of MemTest-86. The purpose of the PCMemTest rewrite was to:
In the process of creating PCMemTest, a number of features of Memtest86+ v5 that were not strictly required for testing the system memory were dropped. In particular, no attempt was made to measure the cache and main memory speed, or to identify and report the DRAM type. These features were added back and expanded in Memtest86+ v6.0 to create a unified, fully-featured release.
Memtest86+ is released under the terms of the GNU General Public License version 2 (GPLv2). Other than the provisions of the GPL there are no restrictions for use, private or commercial. See the LICENSE file for details.
Build is only tested on a Linux system, but should be possible on any system using the GNU toolchain and the ELF file format. The tools required are:
To build a 32-bit image, change directory into the build32 directory and
type make. The result is a memtest.bin binary image file which can be
booted directly by a legacy BIOS (in floppy mode) or by an intermediate
bootloader using the Linux 16-bit boot protocol and a memtest.efi binary
image file which can be booted directly by a 32-bit UEFI BIOS. Either image
can be booted by an intermediate bootloader using the Linux 32-bit or 32-bit
EFI handover boot protocols.
To build a 64-bit image, change directory into the build64 directory and
type make. The result is a memtest.bin binary image file which can be
booted directly by a legacy BIOS (in floppy mode) or by an intermediate
bootloader using the Linux 16-bit boot protocol and a memtest.efi binary
image file which can be booted directly by a 64-bit UEFI BIOS. Either image
can be booted by an intermediate bootloader using the Linux 32-bit, 64-bit,
or 64-bit EFI handover boot protocols.
In either case, to build an ISO image that can be used to create a bootable
CD, DVD, or USB Flash drive, type make iso, The result is a memtest.iso
ISO image file. This can then be written directly to a blank CD or DVD, or
to a USB Flash drive, which can then be booted directly by a legacy or UEFI
PC BIOS.
Note that when writing to a USB Flash drive, the ISO image must be written
directly ('dumped') to the raw device, either by using the dd command or
by using a utility that provides the same functionality.
When using an intermediate bootloader, either the memtest.bin file or the
memtest.efi file should be stored in a disk partition the bootloader can
access, and the bootloader configuration should be updated to boot from
that file as if it were a Linux kernel with no initial RAM disk. Several
boot command line options are recognised, as described below. If using the
16-bit boot protocol, Memtest86+ will use the display in text mode (640x400).
If using the 32-bit or 64-bit boot protocols, Memtest86+ will use the display
in either text mode or graphics mode, as specified in the boot_params struct
passed to it by the bootloader. If in graphics mode, the supplied framebuffer
must be at least 640x400 pixels; if larger, the display will be centred. If
the system was booted in UEFI mode, graphics mode must be used.
For test purposes, there is also an option to build an ISO image that uses
GRUB as an intermediate bootloader. See the Makefile in the build32 or
build64 directory for details. The ISO image is both legacy and UEFI
bootable, so you need GRUB modules for both legacy and EFI boot installed
on your build system (e.g. on Debian, the required GRUB modules are located
in packages grub-pc-bin, grub-efi-ia32-bin and grub-efi-amd64-bin).
You may need to adjust some path and file names in the Makefile to match
the naming on your system.
The GRUB configuration files contained in the grub directory are there for
use on the test ISO, but also serve as an example of how to boot Memtest86+
from GRUB.
An intermediate bootloader may pass a boot command line to Memtest86+. The
command line may contain one or more options, separated by spaces. Each
option consists of an option name, optionally followed by an = sign and
one or more parameters, separated by commas. The following options are
recognised:
0x prefix (eg: 0xFEDC9000)Memtest86+ supports both the legacy keyboard interface (using I/O ports 0x60 and 0x64) and USB keyboards (using its own USB device drivers). One or the other or both can be selected via the boot command line, If not specified on the command line, the default is to use both if the system was booted in UEFI mode, otherwise to only use the legacy interface.
Older BIOSs usually support USB legacy keyboard emulation, which makes USB
keyboards act like legacy keyboards connected to ports 0x60 and 0x64. This
can often be enabled or disabled in the BIOS setup menus. If Memtest86+'s
USB device drivers are enabled, they will override this and access any USB
keyboards directly. The downside of that is that the USB controllers and
device drivers require some memory to be reserved for their private use,
which means that memory can't then be covered by the memory tests. So to
maximise test coverage, if it is supported, enable USB legacy keyboard
emulation and, if booting in UEFI mode, add keyboard=legacy on the boot
command line.
NOTE: Some UEFI BIOSs only support USB legacy keyboard emulation when you enable the Compatibility System Module (CSM) in the BIOS setup. Others only support it when actually booting in legacy mode.
Many USB devices don't fully conform to the USB specification. If the USB keyboard probe hangs or fails to detect your keyboard, try the various workarounds provided by the "usbinit" boot option.
NOTE: Hot-plugging is not currently supported by the Memtest86+ USB drivers. When using these, your USB keyboard should be plugged in before running Memtest86+ and should remain plugged in throughout the test.
Once booted, Memtest86+ will initialise its display, then pause for a few seconds to allow the user to configure its operation. If no key is pressed, it will automatically start running all tests using a single CPU core, continuing indefinitely until the user reboots or halts the machine.
At startup, and when running tests, Memtest86+ responds to the following keys:
Note that testing is stalled when scroll lock is enabled and the scroll region is full.
The configuration menu allows the user to:
In all cases, the number keys may be used as alternatives to the function keys (1 = F1, 2 = F2, ... 0 = F10).
The error reporting mode may be changed at any time without disrupting the current test sequence. Error statistics are collected regardless of the current error reporting mode (so switching to error summary mode will show the accumulated statistics since the current test sequence started). BadRAM patterns are only accumulated when in BadRAM mode.
Any change to the selected tests, address range, or CPU sequencing mode will start a new test sequence and reset the error statistics.
The error counts only mode just displays the total number of errors found since the current test sequence started.
The error summary mode displays the following information:
The individual error mode displays the following information for each error instance:
The BadRAM patterns mode accumulates and displays error patterns for use with
the Linux BadRAM feature. Lines are
printed in the form badram=F1,M1,F2,M2... In each F,M pair, the F
represents a fault address and the M is a bitmask for that address. These
patterns state that faults have occurred in addresses that equal F on all 1
bits in M. Such a pattern may capture more errors that actually exist, but
at least all the errors are captured. These patterns have been designed to
capture regular patterns of errors caused by the hardware structure in a terse
syntax.
The BadRAM patterns are grown incrementally rather than calculated from an overview of all errors. The number of pairs is constrained to ten for a number of practical reasons. As a result, handcrafting patterns from the output in address printing mode may, in exceptional cases, yield better results.
Please be aware that not all errors reported by Memtest86+ are due to bad memory. The test implicitly tests the CPU, caches, and motherboard. It is impossible for the test to determine what causes the failure to occur. Most failures will be due to a problem with memory. 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 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 some techniques that you may wish to use:
Removing modules
Rotating modules
Replacing modules
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. The components are not necessarily bad but certain combinations may need to be avoided.
In the vast majority of cases errors reported by Memtest86+ 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 always valid.
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.
Memtest86+ can not diagnose many types of PC failures. For example a faulty CPU that causes your OS to crash will most likely just cause Memtest86+ to crash in the same way.
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.
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 strategy.
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 tests work 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 ensure 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 it less effective. It is possible to turn off caching but the memory buffering in new high performance chips cannot be disabled. To address this limitation a new algorithm called Modulo-20 was created. This algorithm is not affected by caching 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 (1.1, 1.2) and the read pass (1.4) 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.
Memtest86+ executes a series of numbered tests to check for errors. These tests consist of a combination of test algorithm, data pattern and caching. The execution order for these tests were arranged so that errors will be detected as rapidly as possible. A description of each test follows.
To allow testing of more than 4GB of memory on 32-bit CPUs, the physical address range is split into 1GB windows which are be mapped one at a time into a virtual memory window. Each 1GB window may contain one or more contiguous memory regions. For most tests, the test is performed on each memory region in turn. Caching is enabled for all but the first test.
In each memory region in turn, tests all address bits by using a walking ones address pattern. Errors from this test are not used to calculate BadRAM patterns.
In each memory region in turn, each address is written with its own address and then each address is checked for consistency. This test is performed sequentially with each available CPU, regardless of the CPU sequencing mode selected by the user.
Across all memory regions, each address is written with its own virtual address plus the window number (for 32-bit images) or own physical address (for 64-bit images) and then each address is checked for consistency. This catches any errors in the high order address bits that would be missed when testing each window in turn. This test is performed sequentially with each available CPU, regardless of the CPU sequencing mode selected by the user.
In each memory region in turn, and for each pattern in turn, uses the moving inversions algorithm with patterns of all ones and all zeros.
In each memory region in turn, and for each pattern in turn, uses the moving inversions algorithm with patterns of 8-bit wide walking ones and walking zeros.
In each memory region in turn, and for each pattern in turn, uses the moving inversions algorithm with patterns of a random number and its complement. The random number is different on each test pass so multiple passes increase effectiveness.
In each memory region in turn, and for each pattern in turn, uses the moving inversions algorithm with patterns of 32-bit wide (on 32-bit builds) or 64-bit wide (on 64-bit builds) walking ones and walking zeros. Unlike previous tests, the pattern is rotated 1 bit on each successive address.
This test stresses memory by using block move (movs) instructions and is based on Robert Redelmeier's burnBX test.
In each memory region in turn, memory is initialized with shifting patterns that are inverted every 8 bytes. Then blocks of memory are moved around using the movs 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. In consequence, errors from this test are not used to calculate BadRAM patterns.
In each memory region in turn, each address is written with a random number, then each address is checked for consistency and written with the complement of the original data, then each address is again checked for consistency.
In each memory region in turn, and for each pattern in turn, uses the Modulo-20 algorithm with patterns of a random number and its complement. The random number is different on each test pass so multiple passes increase effectiveness.
Across all memory regions, and for each pattern in turn, initialises each memory location with a pattern, sleeps for a period of time, then checks each memory location for consistency. The test is performed with patterns of all zeros and all ones.
Please see the list of open issues and enhancement requests on GitHub.
Feel free to submit bug reports!
Code contributions are welcomed, either to fix bugs or to make enhancements. See the README_DEVEL.md in the doc directory for some basic guidelines.
Memtest86+ v6.0 was based on PCMemTest, developed by Martin Whitaker, which was based on Memtest86+ v5.01, developed by Samuel Demeulemeester, which in turn was based on Memtest86, developed by Chris Brady with the resources and assistance listed below:
The initial versions of the source files bootsect.S, setup.S, head.S and build.c are from the Linux 1.2.1 kernel and have been heavily modified.
Doug Sisk provided code to support a console connected via a serial port. (not currently used)
Code to create BadRAM patterns was provided by Rick van Rein.
The block move test is based on Robert Redelmeier's burnBX test.
Screen buffer code was provided by Jani Averbach. (not used by Memtest86+ v6.0)
Eric Biederman provided all of the feature content for version 3.0 plus many bugfixes and significant code cleanup.
Major enhancements to hardware detection and reporting in version 3.2, 3.3 and 3.4 provided by Samuel Demeulemeester (from Memtest86+ v1.11, v1.60 and v1.70).
In addition, several bug fixes for Memtest86+ were imported from anphsw/memtest86.
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