"Intel has admitted that not all Linux operating systems support
its Pentium 4 chip. The company is blaming suppliers of some
'out-of-the-box' versions of the open source operating system which
cannot identify the microprocessor."
"But according to the Linux 2.2.18 kernel notes, this is
because Intel has broken the usual pattern in CPUID model numbering
without informing Linux developers."
"Intel chose to ignore all precedents in model numbering via
CPUID and report a family of '15'," stated the notes. "This sudden
jump broke assumptions in the kernel tree without any warning."