HANDS ON GUIDE

7 - RISC vs. CISC

The biggest conceptual battle played out during the history of instruction sets was the battle between RISC and CISC. Needless to say, it’s a battle that Intel has mostly won, given the dominance of x86. It started when processors became fast enough and memory was cheap, so you could afford to have more, unlike the very early days of x86 when every byte counted. This is when close-to-the-metal assembly language started falling out of favour and programmers moved on to easier, higher-level languages. The language called C was the favourite, but brought a new problem to computer science: compiler design. The C code – which is simply a text file written by the programmer – has to be converted into machine code, low-level instructions for whatever processor platform the program was running on.

One school of thought considered the huge list of x86 instructions – well over 100 even at that point – good for making machine code programming as easy as possible, but less efficient when it came to producing the best compiled code. As a result, RISC (reduced instruction set computer) processors were born. Unlike the dozens of instructions supported by x86, a RISC processor only has a few basic primitive instructions, but these can be executed at very high speed, typically taking only one or two clock cycles. It’s then down to the compiler to use these native instructions and build up the functions of the more complex ones in machine code.

However, an x86 PC is now known as CISC - a complex instruction set computer. It’s continued to dominate, due to clever design and other, more prosaic factors. For instance, the 486 introduced instruction pipelining, splitting instructions into RISC-like pieces for simultaneous decoding and execution. Add to that the fact that Intel has a far larger R&D budget than any other CPU manufacturer, so it can compensate for any architectural deficiencies with better materials, engineering and shrinking fabrication processes.

Finally, we come back to the unavoidable fact that x86 compatibility is needed to run the millions of existing PC applications available. RISC never had much of a chance.The most popular example of a RISC architecture is the IBM PowerPC CPUs, but these are harder than ever to find in consumer PCs now that the x86 steamroller has replaced PowerPC in Apple Macs. The RISC philosophy has endured pretty well in areas other than the desktop though. The PlayStation 3, Nintendo Wii and the iPhone (with its ARM processor) are all RISC-based.

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