Atom is a low-power CPU from Intel targeted to laptops or handheld devices with internet access. In this tutorial we will explore this CPU's technical specifications and will list all models released so far.

It is important to know that there are two flavors of Atom CPUs. Atom series 2xx, 3xx and N2xx – codenamed “Diamondville” – are targeted to low-power desktops and laptops (because they use chipsets from Intel 945 series, which are big and use two chips) while Atom series Z5xx – codenamed “Silverthorne” – are targeted to handheld devices with internet access, not only because they use a new chipset called US15W, which is very small and uses only one chip, but also because Atom Z5xx CPUs are physically smaller than other Atom CPUs (14 x 13 mm against 22 x 22 mm). They also have a different pin-out: Z5xx series uses a 441-pin grid while other models use a 437-pin grid.

Atom is based on a different microarchitecture from other Intel processors (Core microarchitecture), but has the same instruction set of CPUs based on Core microarchitecture, like Core 2 Duo. The main difference from the Atom microarchitecture is that it processes instructions in order, which is the way CPUs up to the first Pentium used to work. CPUs starting with Pentium Pro and Pentium II use an out-of-order engine. This change was made in order to save energy, since the components in charge of issuing and controlling microinstructions execution could be removed. Atom can decode two instructions per clock cycle. If you want to know more about Atom architecture, read our Inside Atom Architecture tutorial.

Atom CPUs have a 32 KB L1 instruction cache, a 24 KB L1 data cache and a 512 KB L2 cache (1 MB on dual-core model). They don’t have an integrated memory controller, so memory types and maximum sizes are defined by the memory controller inside the chipset, not by the CPU.

Most Atom also features Hyper-Threading technology, which is the ability of using unused CPU units to form a second virtual CPU, making the operating system to see each CPU core as having two CPUs (two threads, on the industry’s jargon) even though only one core is physically present. Of course this technique is less efficient than having two real CPU cores, but this extra core – and thus extra performance – you are gaining for free.

Another characteristic is that most Atom processors do not support Enhanced Speedstep technology, nor any other technology that would allow it to slow down when idle. So, they always work at full clock speed. Only N2xx models brings this tecnology.

The main specs of Atom CPU include:

• Full compatible with x86 instruction set, meaning it can run directly PC software and operating systems. Several other CPUs targeted to handheld devices have proprietary instruction set.
• Very low Thermal Design Power (TDP): from 0,65 W to 4 W for the single-core models.
• Hyper-Threading technology (except Z510 model).
• Virtualization technology.
• Execute Disable Bit (NX bit).
• Intel EM64T Technology (on 230 and 330 models).
• SSE3 instruction set.
• 400 MHz, 533 MHz or 667 MHz external clock (100 MHz, 133 MHz or 166 MHz transferring four data per clock cycle).
• 128-bit internal datapath (“Digital Media Boost”).
• 32 KB L1 instruction cache and 24 KB L1 data cache.
• 512 KB L2 cache (1 MB on model 330, which is dual core).
• Dynamic cache sizing: ability to turn off portions of the memory cache when CPU enters C4 or C4E power-saving modes (not available on Atom 2xx models).
• Manufactured under 45-nm process.
• Can be paired with a mobile Intel 945-class chipset (Atom 2xx and N2xx models) or with an Intel US15W (“Poulsbo”) chipset (Atom Z5xx models). Models 2xx and N2xx are targeted to laptops, while Z5xx models are targeted to handhelds with internet capability.
• 437 pins (“Diamondville” models, i.e. 2xx and N2xx) or 441 pins (“Silverthorne” models, i.e. Z5xx).

Let’s now see Atom models released so far.