Exactly how “fast” are modern CPUs?

Question

When I used to program embedded systems and early 8/16-bit PCs (6502, 68K, 8086) I had a pretty good handle on exacly how long (in nanoseconds or microseconds) each instruct

Answer 1

All you need is in the appropriate CPU manuals. Both AMD and Intel have PDF's available on their website describing the latencies of every instruction.

Just keep in mind the complexity of modern CPU's. They don't execute one instruction at a time, they can load 3-4 instructions per cycle, and almost all instructions are pipelined so when the next instructions are loaded, the current ones are nowhere near finished. It also reorders instructions to allow for a more efficient scheduling. A modern CPU can easily have 50 instructions in progress at a time.

So you're asking the wrong question. The time taken for a single instruction varies wildly depending on how and when you measure. It depends on how busy the instruction decoder is, on branch predictor, on scheduling and on which other instructions are being scheduled, in addition to the simple issues like caching.