Intel's Atom: Changing Intel from the Inside

For years at Intel the rule of thumb for power vs. performance was this: a designer could add a feature to a microprocessor design if you get a 1% increase in performance for at most a 2% increase in power. Unfortunately, it's thinking like that which gave us the NetBurst architecture used in the Pentium 4 and its derivatives.

The Intel Atom was the first Intel CPU to redefine the rule of thumb and now the requirement is that a designer may add a feature if it yields a 1% increase in performance for at most a 1% increase in power consumption. It's a pretty revolutionary change and it's one that will be seen in other Intel architectures as well (Nehalem comes to mind), but Atom was the first.

While Atom started as a single-issue, in-order microprocessor the Austin team quickly widened it to be a dual-issue core. The in-order decision stuck however.

Modern day x86 processors can operate on instructions out of program order. It's like if you had to tie your shoe and turn on the TV, you may choose to tie your shoe first and then walk over to the remote control to turn on the TV. You'd complete the quicker task first before moving onto the one that took more time since you didn't have the remote on hand. Processors that are capable of OoOE (Out of Order Execution) work in the same way; when data isn't available in their caches instead of idly waiting on the data, they can execute other instructions that are ready while waiting for the required data to be fetched from memory.

The problem with these out of order processors is that all of this instruction reordering takes up additional die space and increases power consumption. Performances goes up as well but remember, Intel's goal here wasn't to be the fastest, but to be fast enough. Thus the Atom remained an in-order CPU, incapable of executing instructions out of program order and Intel's first in-order x86 core since the original Pentium processor.

The decision to go in-order eliminated the need for much complex, power hungry circuitry. While you get good performance from out-of-order execution, the corresponding increase in scheduling complexity was simply too great for Atom at 45nm. Keep in mind that just as out-of-order execution wasn't feasible on Intel CPUs until the Pentium Pro, there may come a time where transistor size is small enough that it makes sense to implement an OoOE engine on Atom. I don't expect that you'll see such a change in the next 5 years however.

Atom at a Glance 2-Issue and In-Order: Intel's Version of the Cell's PPE
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