As far as I'm concerned, the real gem of Intel's Centrino brand is the new Pentium M processor, which integrates all sorts of neat power saving features to preserve notebook battery life. The Pentium M is loaded with cache and other goodies that let it execute a much higher number of instructions per clock (IPC) than mobile versions of Intel's Pentium 4, too.
I was lucky enough to get my hands on a Pentium M 1.4GHz-powered notebook for a few days, and I ran it through a gauntlet of performance benchmarks against my own Mobile Pentium 4 1.8GHz laptop. What's the Pentium M all about? Can it perform enough work per clock cycle to make up for a 400MHz clock speed disadvantage? Read on to see.
Code-named Banias, the Pentium M actually owes a lot more to Intel's previous generation of Pentium III processors than it does the new Pentium 4 line. Here are the key ingredients of Intel's new mobile chip.
- A monster L2 cache - The Pentium M's 1MB cache is massive. Intel's current high-end mobile and desktop CPUs have only 512KB of L2, which makes the Pentium M look very different right off the bat. The Pentium M's L2 cache is 8-way associative, just like the Pentium 4's, but to conserve power, elements of the Pentium M's L2 cache are only activated when needed. Having to power up parts of the L2 cache before use will add latency that can degrade system performance, but it should reduce the Pentium M's power requirements, which is key for a mobile chip.
- Don't skimp on the L1 - The Pentium M's huge L2 cache is complemented by 64KB of L1 cache that's split evenly between instruction and data caches. Intel has yet to divulge the exact size of the Pentium 4's L1 instruction cache, but the Pentium 4 M's 8KB L1 data cache is a quarter the size the Pentium M's.
One more difference between the Pentium M and Pentium 4's L1 cache is that the former is a write-back cache, while the latter is a write-through cache. With a write-through cache, data is written to L1 and main memory simultaneously; write-back caching only writes L1 data to main memory when absolutely necessary. In theory, a write-back cache should be faster than a write-through cache because the write-back cache does fewer slow memory writes.
- A 400MHz system bus - Like early Pentium 4 desktop processors and Intel's current mobile Pentium 4 offerings, the Pentium M uses a quad-pumped 100MHz front-side bus, or effectively a 400MHz bus. The Pentium M's 400MHz front-side bus is actually identical to that of the Pentium 4.
- 0.13-micron tech - Intel's next Pentium M revision will apparently be spun using 0.09-micron manufacturing technology, but the current Banias core is built using 0.13-micron tech. With 77 million transistors, nearly half of which are used for L2 cache, the Pentium M has 23 million more transistors than the Pentium 4.
- SSE2 support - The Pentium M supports MMX, SSE, and SSE2 instructions, though it won't support the 13 new instructions that will reportedly be incorporated into Intel's upcoming Prescott-core Pentium 4 desktop chip.
- Somewhere between 10 and 20 - Taking a cue from NVIDIA, Intel isn't revealing exactly how deep the Pentium M's main pipeline is. They have said that it's deeper than the Pentium III's 10-stage pipeline, but shallower than the Pentium 4's 20-stage pipeline. A deeper pipeline does help enable higher clock speeds, but it also increases the penalty for branch mispredictions. The Pentium M aims to avoid branch mispredictions as much as possible using a swanky new branch prediction unit.
- Micro-ops fusion - With an eye towards overall efficiency, the Pentium M sends micro-ops down the pipeline to execution units in bundles rather than individually. This fusion of micro-ops gives the pipeline a little extra idle time, which saves power. However, the bundling process increases latency, since micro-ops aren't always executed immediately.
- Low power consumption - Many of the Pentium M's features aim to improve the chip's performance, but some are there solely to conserve power. Given that the Pentium M is primarily a mobile chip that can also be used in dense, clustered server environments, customers may first look at power consumption before they examine the chip's performance. The Pentium M 1.4GHz that we'll be looking at today requires between 0.96 and 1.48V, although ultra-low-power versions of the chip clocked between 600 and 900MHz need only between 0.84 and 1.0V.
To help conserve power, all Pentium M chips use third-generation SpeedStep technology to raise and lower their clock speeds and core voltages. SpeedStep lets the Pentium M deliver more performance when it's needed and consume less power when it's not. The chip is also able to shut down internal components such as unused segments of L2 cache to draw even less power. As if that weren't enough, the Pentium M actually boots in a sleep state and only activates internal components as they're needed; the Mobile Pentium 4 boots at full power and disables functional units as they become idle.
A word on the chipset
Intel designed the Pentium M to work with the 855 core-logic chipset, which has a 400MHz front-side bus, support for DDR333 memory and AGP 4X, and a few extra power saving features of its own. The 855 chipset can send power-down instructions to the Pentium M and is also capable of shutting down elements of the system bus to further conserve power. A series of ultra-deep sleep states rounds out the 855 chipset's power-saving features.
The 855 chipset is available with and without an integrated graphics core as the 855PM and 855GM, respectively. Both north bridge chips interface with a mobile version of Intel's ICH4 south bridge, the ICH4M. The desktop and mobile versions of the ICH4 are identical in terms of features, but the ICH4M supports deeper power saving states.
For notebooks, Intel is bundling the Pentium M with its 855 chipset and Pro/Wireless 2100 802.11b Wi-Fi networking technology under the Centrino name. Centrino is little more than a branding ploy that ties the Pentium M to Intel's own chipset and Wi-Fi card, but that's not necessarily a bad thing. At least when buying a Centrino-branded notebook, consumers know they're getting a Pentium M with all the power-saving features of the 855 chipset and 802.11b Wi-Fi connectivity.