Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-144 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
The PREFETCHh instruction is merely a hint and does not affect program behavior. If
executed, this instruction moves data closer to the processor in anticipation of future
use.
The implementation of prefetch locality hints is implementation-dependent, and can
be overloaded or ignored by a processor implementation. The amount of data
prefetched is also processor implementation-dependent. It will, however, be a
minimum of 32 bytes.
It should be noted that processors are free to speculatively fetch and cache data from
system memory regions that are assigned a memory-type that permits speculative
reads (that is, the WB, WC, and WT memory types). A PREFETCHh instruction is
considered a hint to this speculative behavior. Because this speculative fetching can
occur at any time and is not tied to instruction execution, a PREFETCHh instruction is
not ordered with respect to the fence instructions (MFENCE, SFENCE, and LFENCE) or
locked memory references. A PREFETCHh instruction is also unordered with respect
to CLFLUSH instructions, other PREFETCHh instructions, or any other general instruc-
tion. It is ordered with respect to serializing instructions such as CPUID, WRMSR,
OUT, and MOV CR.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
FETCH (m8);
Intel C/C++ Compiler Intrinsic Equivalent
void_mm_prefetch(char *p, int i)
The argument “*p” gives the address of the byte (and corresponding cache line) to
be prefetched. The value “i” gives a constant (_MM_HINT_T0, _MM_HINT_T1,
_MM_HINT_T2, or _MM_HINT_NTA) that specifies the type of prefetch operation to
be performed.
Numeric Exceptions
None.
Exceptions (All Operating Modes)
None.