Programming
Uniprocessor and SMP systems require different programming methods to achieve maximum performance. Programs running on SMP systems may experience a performance increase even when they have been written for uniprocessor systems. This is because hardware interrupts that usually suspend program execution while the kernel handles them can execute on an idle processor instead. The effect in most applications (e.g. games) is not so much a performance increase as the appearance that the program is running much more smoothly. In some applications, particularly compilers and some distributed computing projects, one will see an improvement by a factor of (nearly) the number of additional processors.
Systems programmers must build support for SMP into the operating system: otherwise, the additional processors remain idle and the system functions as a uniprocessor system.
SMP systems can also lead to more complexity regarding instruction sets. A homogenous processor system typically requires extra registers for "special instructions" such as SIMD(MMX, SSE etc.), while a heterogenous system can implement different types of hardware for different instructions/uses.
Read more about this topic: Symmetric Multiprocessing
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