An intrinsic is a function known by the compiler that directly maps to a sequence of one or more assembly language instructions. Intrinsic functions are inherently more efficient than called functions because no calling linkage is required.
Intrinsics make the use of processor-specific enhancements easier because they provide a C/C++ language interface to assembly instructions. In doing so, the compiler manages things that the user would normally have to be concerned with, such as register names, register allocations, and memory locations of data.
For information on how to detect the capabilities of a CPU, see CPUID Sample: Determines CPU Capabilities.
All the MMX, SSE and SSE2 intrinsics are only available as intrinsics, thus, they are not affected by the setting of /Oi, and #pragma function may not be used on them.