Instruction mixes have several disadvantages. Today’s computers provide many more complex classes of instructions that are not reflected in the mixes. In modern computer systems, instruction time is highly variable depending upon addressing modes, cache hit rates, pipeline efficiency, and interference from other devices during processor-memory access cycles. The instruction times also vary according to parameter values such as frequency of zeros as a parameter, the distribution of zero digits in a multiplier, the average number of positions of preshift in floating-point add, and the number of times a conditional branch is taken. The mixes do not reflect the virtual addressing facilities (for example, page translation tables) that are provided by some processors.
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