What RISC stands for. A way of structuring microprocessors which lead to higher-performance designs than CISC (ComplexInstructionSetComputer) was capable of. Back in the 80's and early 90's, a great RISC-vs-CISC war was waged in the industry, with numerous RISC architectures (early ARM, SPARC, MIPS, PowerPC) pitted against the two predominant CISC architectures (x86 ExEightySix, MotorolaSixtyEightKay).

In RISC (in theory), you have a set of simple instructions with simple addressing modes, large general-purpose register sets, and memory transfers are separated from arithmetic instructions. Originally, each instruction was a fixed length (aligned to a machine word) and executed in a fixed number of cycles. This a) allows you to minimize the CriticalPath, increasing the CPU frequency you can get; b) makes pipelining and superscalar design tractable, and c) greatly simplifies instruction decoding.

Who won the RiscVsCisc battle? RISC, sort of:

* The Intel x86, for all its warts, remains the dominant processor family for desktop computing, and serves as the basis for many servers as well these days.
** However, much of this had to do with the fact that it's the processor (along with its clones) that runs MicrosoftWindows.  
** The x86 these days is fundamentally a RISC core with a layer on top that translates the horrid CISC x86 instructions into the RISC core. High-end x86 chips are as fast, or nearly so, as their RISC counterparts.
*** Of course, Intel has access to better IC processes than anyone save IBM.
*** That speed comes at a price in power consumption - Intel CPUs need huge heatsinks to run. You could probably kill someone with a Pentium 4 with its heatsink attached.
*** High-end Intel CPUs require a power supply capable of supplying '''125 AMPS'''. That ain't a typo. If the voltage were higher (12V vs 1.8V, or whatever voltage these things run off), you could jump-start a car with it.
**** 125 amps at 1.8v is 225 watts; does a P4 use all of this, or is that just peak capacity?
**** The VW Polo I watched crank (in 1999?) was 330 amps.  Voltage will be around 12 volts, likely to dip sharply at that current.  This is 3300 - 3960 watts.
* Most modern RISC-ish processors aren't anywhere near as pure as early versions. Many have variable-length instructions, optimized instructions/addressing modes for manipulating TheStack, doing tight loops, etc. Some even have string instructions. RISC processors designed for low-memory environments (like PDAs) have compressed instruction sets (i.e. ThumbCompression)
* The 68k is dead for most applications; though it lives on in EmbeddedSystem''''''s. Early Palm Pilots were based on a 68k derivative (the Motorola "DragonBall" family); however Motorola has migrated that product line to ARM.
* Modern CPUs now have enhanced DigitalSignalProcessor functionality as well, often through vector processing units like AltiVec, MMX/SSE, etc.

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CategoryHardware