The Cortex-A9 executes more instructions per clock because it can decode more than one instruction at a time (2 to 4, usually 4) and execute them out of order t

The Cortex-A9 executes more instructions per clock because it can decode more than one instruction at a time (2 to 4, usually 4) and execute them out of order to get more instruction-level parallelism. Atom can decode a maximum of two instructions at a time, and executes them in order. The Atom also uses precious pipeline stages on decoding the x86 instruction set -- all of it. There are a lot of instructions there, and all but the most common require the processor to read sequences of several micro-instructions from a ROM. Now, the Atom tries to compensate for this with Hyperthreading: they share the decode unit among two threads (hypothetically) and have those threads share the execution units on the chip. This gives Atom a boost in performance per watt if you can find two processes that need to run at the same time. For single-threaded workloads, Cortex-A9 looks like it will continue to trounce Atom pretty severely, and for multi-threaded workloads, it's not expensive to put down some more A9 cores. It's not like they have to decode x86 instructions or anything, so they can be pretty darn small.