Why does the 80486 take longer to execute simple instructions than complex ones?
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The 80486 processor can execute many instructions in a single cycle, such as a register-to-register add instruction ( ADD EAX, EBX
, for example), which one would generally assume is fairly complex, requiring two registers to be read, a full 32 bit ALU operation, and both a register and flag writes.
However, other instructions that one would naturally assume to be relatively simple take multiple cycles, including the various flag manipulation instructions like STC
and CLC
, which are a strict subset of the operations required for an ADD
instruction (only loading a precalculated value into the flags, rather than having to actually determine what the value to load should be based on inputs, and not needing a register update at all), yet take 2 cycles.
What is the reason for this apparent discrepancy?
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