When is assembly faster than C?

Question

One of the stated reasons for knowing assembler is that, on occasion, it can be employed to write code that will be more performant than writing that code in a higher-level

Answer 1

A use case which might not apply anymore but for your nerd pleasure: On the Amiga, the CPU and the graphics/audio chips would fight for accessing a certain area of RAM (the first 2MB of RAM to be specific). So when you had only 2MB RAM (or less), displaying complex graphics plus playing sound would kill the performance of the CPU.

In assembler, you could interleave your code in such a clever way that the CPU would only try to access the RAM when the graphics/audio chips were busy internally (i.e. when the bus was free). So by reordering your instructions, clever use of the CPU cache, the bus timing, you could achieve some effects which were simply not possible using any higher level language because you had to time every command, even insert NOPs here and there to keep the various chips out of each others radar.

Which is another reason why the NOP (No Operation - do nothing) instruction of the CPU can actually make your whole application run faster.

[EDIT] Of course, the technique depends on a specific hardware setup. Which was the main reason why many Amiga games couldn't cope with faster CPUs: The timing of the instructions was off.