A machine-check exception is a type of computer hardwareerror that occurs when a computer's central processing unit detects an unrecoverable hardware error in the processor itself, the memory, the I/O devices, or on the system bus. It is not caused by software. The error usually occurs due to component failure or the overheating or overclocking of hardware components. Most machine check exceptions halt the operating system and require a restart before users can continue normal operation. Diagnosing the failure can be often difficult because so little information about what caused the problem is captured during the error. Modern versions of Microsoft Windows on IA-32 and x86-64 processors handle machine check exceptions through the Windows Hardware Error Architecture. When WHEA detects a machine check exception, it displays the error in a Blue Screen of Death, with the following parameters : *** STOP: 0x00000124 Older versions of Windows handle similar exceptions through the Machine Check Architecture. In this case, the BlueScreen of Death will show an error similar to the following: STOP: 0x0000009C On Linux, a process writes a message to the kernel log and/or the console screen : CPU 0: Machine Check Exception: 0000000000000004 Bank 2: f200200000000863 Kernel panic: CPU context corrupt
Problem types
Most of these errors relate specifically to the Pentium processor family. Similar errors may occur on other processors and will cause similar problems. Some of the main hardware problems that cause MCEs include:
Machine checks are a hardware problem, not a software problem. They are often the result of overclocking or overheating. In some cases, the CPU will shut itself off once passing a thermal limit to avoid permanent damage. But they can also be caused by bus errors introduced by other failing components, like memory or I/O devices. Possible causes include:
Poor general case cooling due to inadequate or clogged case fans or filters.
Cooling problems are usually obvious upon inspection. A failing motherboard or processor can be identified by swapping them with functioning parts. Memory can be checked by booting to a diagnostic tool, like memtest86. Non-essential failing I/O devices and controllers can be identified by unplugging them if possible or disabling the devices to see if the problem disappears. If the failures typically only occur fairly soon after the OS is booted or not at all or not for days, it may be suggestive of a power supply issue. With a power supply problem, the failure often occurs when power demand peaks as the OS starts up any external devices for use.
Decoding MCEs
As noted previously, decoding MCE errors can prove difficult. Normally the processor manufacturer will be able to provide information about specific codes. For IA-32 and Intel 64 processors, consult the Intel 64 and IA-32 Architectures Software Developer's Manual Chapter 15, or the Microsoft KB Article on Windows Exceptions.
mcelog A Linux daemon by Andi Kleen to handle MCEs for modern x86 processors. mcelog can also decode machine checks.
parsemce a Linux program by Dave Jones to decode MCEs from AMD K7 processors.
mced a Linux program by Tim Hockin to gather MCEs from the kernel and alert interested applications. Note that it does not try to interpret the MCE data, it simply alerts other programs.
