Hardware Reference
In-Depth Information
can also be caused by buggy or faulty programs.
•
Fatal exception errors
—Error codes returned by a program when an illegal instruction has
been encountered, invalid data or code has been accessed, or the privilege level of an operation
is invalid.
•
Divide error
—A general-purpose error indicating that a division by 0 was attempted or the
result of an operation does not fit in the destination register.
If you are encountering these errors, they could be caused by defective or improperly configured
memory, but they can also be caused by software bugs (especially drivers), bad power supplies,
static discharges, close proximity radio transmitters, timing problems, and more.
If you suspect the problems are caused by memory, there are ways to test the memory to determine
whether that is the problem. Most of this testing involves running one or more memory test programs.
Another problem with software-based diagnostics is running memory tests with the system caches
enabled. This effectively invalidates memory testing because most systems have what is called a
write-back cache
. This means that data written to main memory is first written to the cache. Because
a memory test program first writes data and then immediately reads it back, the data is read back from
the cache, not the main memory. It makes the memory test program run quickly, but all you tested was
the cache. The bottom line is that if you test memory with the cache enabled, you aren't really writing
to the modules, but only to the cache. Before you run any memory test programs, be sure your
processor/memory caches are disabled. Many older systems have options in the BIOS Setup to turn
off the caches. Current software-based memory test software such as the Windows Memory
Diagnostic and Memtest86 automatically turn off the caches on newer systems.
The following steps enable you to effectively test and troubleshoot your system RAM.
Figure 6.20
provides a boiled-down procedure to help you step through the process quickly.
