Intel IA-32 - Manual (Page 71)

Vol. 3A 2-23
SYSTEM ARCHITECTURE OVERVIEW
When enabling the global page feature, paging must be enabled (by setting the PG flag
in control register CR0) before the PGE flag is set. Reversing this sequence may affect
program correctness, and processor performance will be impacted.
See also: Section 3.12, “Translation Lookaside Buffers (TLBs).”
PCE Performance-Monitoring Counter Enable (bit 8 of CR4) — Enables execution of
the RDPMC instruction for programs or procedures running at any protection level
when set; RDPMC instruction can be executed only at protection level 0 when clear.
OSFXSR
Operating System Support for FXSAVE and FXRSTOR instructions (bit 9 of
CR4) — When set, this flag: (1) indicates to software that the operating system
supports the use of the FXSAVE and FXRSTOR instructions, (2) enables the FXSAVE
and FXRSTOR instructions to save and restore the contents of the XMM and MXCSR
registers along with the contents of the x87 FPU and MMX registers, and (3) enables
the processor to execute SSE/SSE2/SSE3 instructions, with the exception of the
PAUSE, PREFETCHh, SFENCE, LFENCE, MFENCE, MOVNTI, and CLFLUSH.
If this flag is clear, the FXSAVE and FXRSTOR instructions will save and restore the
contents of the x87 FPU and MMX instructions, but they may not save and restore the
contents of the XMM and MXCSR registers. Also, the processor will generate an
invalid opcode exception (#UD) if it attempts to execute any SSE/SSE2/SSE3 instruc-
tion, with the exception of PAUSE, PREFETCHh, SFENCE, LFENCE, MFENCE,
MOVNTI, and CLFLUSH. The operating system or executive must explicitly set this
flag.
NOTE
CPUID feature flags FXSR, SSE, SSE2, and SSE3 indicate availability
of the FXSAVE/FXRESTOR instructions, SSE extensions, SSE2
extensions, and SSE3 extensions respectively. The OSFXSR bit
provides operating system software with a means of enabling these
features and indicating that the operating system supports the features.
OSXMMEXCPT
Operating System Support for Unmasked SIMD Floating-Point Exceptions (bit 10
of CR4) — When set, indicates that the operating system supports the handling of
unmasked SIMD floating-point exceptions through an exception handler that is invoked
when a SIMD floating-point exception (#XF) is generated. SIMD floating-point excep-
tions are only generated by SSE/SSE2/SSE3 SIMD floating-point instructions.
The operating system or executive must explicitly set this flag. If this flag is not set, the
processor will generate an invalid opcode exception (#UD) whenever it detects an
unmasked SIMD floating-point exception.
TPL Task Priority Level (bit 3:0 of CR8) — This sets the threshold value corresponding
to the highest-priority interrupt to be blocked. A value of 0 means all interrupts are
enabled. This field is available in 64-bit mode. A value of 15 means all interrupts will
be disabled.