Atom is a low-power CPU from Intel with very low power dissipation (less than 3 W), targeted to laptops or handheld devices with internet access – dubbed MIDs, Mobile Internet Devices. In this tutorial we will explore the architecture used on this CPU.

It is important to know that there are two flavors of Atom CPUs. Atom series 2xx and N2xx (at this publishing only 230 and N270 models were available) – codenamed “Diamondville” – are targeted to 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 are physically smaller than other Atom CPUs (14 x 13 mm against 22 x 22 mm).

click to enlarge
Figure 1: Atom 230 CPU.

You may also hear references to the Centrino Atom plataform (codenamed “Menlow”). This platform consists of an Atom CPU, the new US15W chipset (codenamed “Poulsbo”) and radio capability (WiFi, Bluetooth, etc).

Speaking of codenames, we also have “Moorestown”, which will be the next version of Centrino Atom, scheduled to reach the market in 2009 or 2010 and will feature a “Lincroft” Atom CPU, a “Langwell” chipset and an “Evans Peak” radio chip.

The main specs from 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): 4 W for the 230 model, 2.5 W for the N270 and between 2 W and 2.64 W for the Z5xx models.
• HyperThreading technology.
• Virtualization technology.
• Execute disable bit (NX bit).
• SSE3 instruction set.
• 400 MHz or 533 MHz external clock (100 MHz or 133 MHz transferring four data per clock cycle).
• 128-bit internal datapath (“Digital Media Boost”).
• Enhanced Speed Step (except on Atom 2xx models).
• 32 KB L1 instruction cache and 24 KB L1 data cache
• 512 KB L2 cache
• 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).
• 16-stage pipeline
• Manufactured under 45-nm process
• Can be paired with a mobile Intel 945-class chipset (Atom 2xx and Nxxx models) or with an Intel US15W (“Poulsbo”) chipset (Atom Z5xx models). Models 2xx and Nxxx are targeted to laptops, while Z5xx models are targeted to handhelds with internet capability.
• 437 pins (“Diamondville” models, i.e. 2xx and Nxxx) or 441 pins (“Silverthorne” models, i.e. Z5xx).

 Let’s now see in more details the main features found on Atom processor.