
Machine-check exception

A machine-check exception (MCE) is a type of computer hardware error that occurs when a computer's central processing unit detects a hardware error in the processor itself, the memory, the I/O devices, or on the system bus; in some architectures, an MCE only occurs for an unrecoverable error. On x86 architectures, a machine-check exception is not caused by software. However, on other architectures such as PowerPC, certain software bugs such as invalid memory accesses can cause machine-check exceptions. 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 an uncorrectable machine check exception, it displays the error in a Blue Screen of Death, with the following parameters (which vary, but the first parameter is always 0x0 for a machine check exception):
 *** STOP: 0x00000124 (0x0000000000000000, 0x0000000000000000, 0x0000000000000000, 0x0000000000000000) 
Older versions of Windows handle similar exceptions through the Machine Check Architecture. In this case, the Blue Screen of Death will show an error similar to the following:
 STOP: 0x0000009C (0x00000030, 0x00000002, 0x00000001, 0x80003CBA) 
On Linux, a process (such as codice_1) writes a message to the kernel log and/or the console screen (usually only to the console when the error is non-recoverable and the machine crashes as a result):
 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:
Possible causes.
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:
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.
For IA-32 and Intel 64 processors, consult the Intel 64 and IA-32 Architectures Software Developer's Manual Chapter 15 (Machine-Check Architecture), or the Microsoft KB Article on Windows Exceptions.