Hardware and Virtual Machines

//RISC vs CISC

RISC has few, simple, fixed-length instructions that mostly take one cycle — which makes pipelining easy and power use low, so it dominates phones. CISC has many complex instructions that can take several cycles; programs are shorter but the hardware is complicated and harder to pipeline.

//Pipelining

Instead of finishing one instruction before starting the next, the stages overlap — while instruction 1 is being executed, instruction 2 is being decoded and instruction 3 fetched. Throughput rises sharply. A branch can force the pipeline to be flushed, which is the cost.

        cycle1  cycle2  cycle3  cycle4
instr 1 fetch   decode  execute
instr 2         fetch   decode  execute
instr 3                 fetch   decode

//Flynn's four classifications

SISD: one instruction, one data stream — a simple single-core machine. SIMD: one instruction applied to many data items at once — graphics and vector work. MISD: several instructions on one data stream — rare, used where results must be cross-checked. MIMD: independent instructions on independent data — multi-core computers.

//Virtual machines

Software that behaves like a separate computer. Benefits: run another operating system, test software safely in isolation, run legacy software, and use one physical server for several logical ones. Drawbacks: it runs more slowly than real hardware and uses extra memory.

CHECK YOURSELF