P6 (microarchitecture) - P6 Variant Pentium M (microarchitecture)

P6 Variant Pentium M (microarchitecture)

Upon release of the Pentium 4-M and Mobile Pentium 4, it was quickly realized that the new mobile NetBurst processors were not ideal for mobile computing. The Netburst-based processors were simply not as efficient per clock or per watt compared to their P6 predecessors. Mobile Pentium 4 processors ran much hotter than Pentium III-M processors and didn't offer significant performance advantages. Its inefficiency affected not only the cooling system complexity, but also the all-important battery life.

Realizing their new microarchitecture wasn't the best choice for the mobile space, Intel went back to the drawing board for a design that would be optimally suited for this market segment. The result was a hybrid, and at the time, modernized P6 design called the Pentium M:

Design Overview

• Quad-pumped Front Side Bus. With the initial Banias core, Intel adopted the 400 MT/s FSB first used in the Pentium 4. The Dothan core moved to the 533 MT/s FSB, following the Pentium 4's evolution.
• Larger L2 cache. Initially 1 MB in the Banias core, then 2 MB in the Dothan core. Dynamic cache activation by quadrant selector from sleep states.
• SSE2 Streaming SIMD (Single Instruction, Multiple Data) Extensions 2 support.
• A 12-14-stage instruction pipeline to achieve higher clock speeds than the Pentium III-M.
• Dedicated register stack management.
• Addition of global history, indirect prediction, and loop prediction to branch prediction table. Removal of local prediction.
• Micro-ops Fusion of certain sub-instructions mediated by decoding units. x86 commands can be combined into fewer RISC micro operations.

The Pentium M was the most power efficient x86 processor for notebooks for several years, consuming a maximum of 27 watts at maximum load and 4-5 watts while idle. The processing efficiency gains brought about by its modernization allowed it to rival the Mobile Pentium 4 clocked over 1 GHz higher (the fastest-clocked Mobile Pentium 4 compared to the fastest-clocked Pentium M) and equipped with much more memory and bus bandwidth.