Modern processors are complicated digital circuits run by internal state machine controlled by
microcode. It is impossible to extensively debug the processor until it is released as a silicon chip. E.g. list of currently discovered problems with 5300 series Xeons is a
54 page document (known knowns). By the time bugs are found it is impossible to change the silicon masks. This is where microcode update comes into play - it's a way to patch some of internal CPU operation by uploading a reversible update.
Intel never admits to what problems each new update fixes so I assume they don't make it into the above document (known unknowns?)
Typically BIOS or EFI uploads microcode into the CPU when system boots. Otherwise it can be updated by the OS. There was a scare in 2007 when serious bug was found affecting
most Intel CPUs and even Microsoft released a
system patch that would update microcode if BIOS fails to do so. They obviously did not want to be blamed for random system hangs. Apple
updated EFI almost simultaneously on many Intel based Macs fixing "stability issues" which makes me think that it has something to do with Intel...
Now MacPro1,1 and MacPro2,1
do not have microcode data for many Xeons including X5365 and X5355. Running them unpatched is like ignoring all errata that has been found and fixed for the last 2-3 years.
Frequently asked questions
Q: What issues are fixed by the microcode reliability update?
A: The microcode reliability update fixes the following issues:
- possible Intel processor marginality
- potential source of unpredictable system behavior
- "stop 0x7E" error that may occur during startup on some systems that are running an older Pentium 5 CPU (This issue occurs only in the 32-bit version of Windows XP and in the x86-based version of Windows Server 2003.)
Q: How do you know if the system needs the microcode reliability update?
A: Although these issues are uncommon, you may receive a Stop error, or you may experience unpredictable system behavior.
Q: What errors occur?
A: You may receive a Stop error, or you may experience unpredictable system behavior.
Q: Which Intel processors are affected?
A: Although these issues are uncommon, the following Intel processors may be affected:
Mobile: Intel Core 2 Duo mobile processor.
Desktop: Intel Core 2 Duo desktop processor, Intel Core 2 Quad desktop processor, and Intel Core 2 Extreme processor.
Server: Intel Xeon processors 3000, 3200, 5100, and 5300 series.
Q: If you install the microcode reliability update, do you still require the latest BIOS update?
A: If you install the microcode reliability update, you do not require the latest BIOS update.
Q: Does the microcode reliability update have to be proactively installed?
A: Affected Intel processor users must use the microcode reliability update.