Inside Atom Architecture

Power-Saving Modes

Not all Atom CPUs are equal, and the most important difference between them is on the supported power-saving modes. For a detailed explanation of all power-saving modes, please read or Everything You Need to Know About the CPU C-States Power Saving Modes tutorial.

Atom 2xx (at this writing only Atom 230 was available) supports only C1 (Halt) power-saving mode, introducing a new C1 sub-mode, called MWAIT, which is entered upon the execution of an instructional called “MWAIT,” thus being similar to the traditional C1 mode, which is entered upon the execution of the “HALT” instruction. The difference, however, is that on regular Halt state the CPU can only exit when an interruption is given, while on the MWAIT state other events can put the CPU back on fully operational state.

Atom Nxxx (at this writing only Atom N270 was available) supports modes C1, C1E, C2, C2E, C3, C4 and C4E.

And Atom Z5xx supports all above plus C6 mode.

On all these CPUs C1, C2 and C4 states can be assigned to each virtual CPU (individual thread). Since Atom features Hyper-Threading technology, each CPU core is recognized as two CPUs by the operating system; so a single-core CPU is recognized as two CPUs or “two threads” using the industry’s jargon. Atom can put any thread into any C1, C2 or C4 states.

Atom can disable a p
ortion of its L2 cache size when entering C4 or C4E states, feature known as “Dynamic Cache Sizing” (Atom 2xx does not support C4 mode and thus doesn’t have this feature). Usually when the CPU enters the C4 state the memory cache is fully turned on, and under C4E state the memory cache is fully turned off.

Another traditional power-saving feature available on Atom CPUs (only in N270 model) is SpeedStep technology. This technology makes the CPU to run at lower clock rates and lower voltages when you are running a program that isn’t demanding the CPU full processing power.