Intel - Sandy Bridge Core i3/i5/i7 review
Second Generation Core i3/i5/i7
Review date: 05 January, 2011. Review by: IT Reviews Staff
While the new chips are based on their predecessors, code-named Lynnfield and Clarkdale, masses of changes have been made under the bonnet. For starters, Sandy Bridge uses a new type of LGA1155 socket that is matched to Intel's new P67 and H67 chipsets - so you'll need a new motherboard for your Sandy Bridge processor. You can't simply upgrade your current LGA1156 P55/H55 motherboard.
The H67 chipset supports graphics ports on the motherboard itself, as well as a single PCI Express 2.0 x16 graphics slot, while the P67 supports dual graphics slots. These P67/H67 chipsets are similar to the previous generation P55/H55 chipsets, but with two of the six SATA ports upgraded from 3Gbps to 6Gbps. It seems that we'll have to wait a little while longer for native USB 3.0 support, though.
The current crop of LGA1366 Core i7 900 'Bloomfield' processors remain the highest clocked chips in Intel's desktop range, matched with the X58 motherboard chipset and supporting triple-channel DDR3 memory.
Lower down the existing range, Intel's quad-core 'Lynnfield' CPUs don't include a separate graphics core, as there's no space inside the processor, which is manufactured using a 45nm process. Lower-specified dual-core Core i3/Core i5 'Clarkdale' models employ a hybrid design that uses 32nm technology for the CPU, and a 45nm process for the graphics core. The two component parts are then assembled in a single package that looks like a regular CPU.
It's all change for Sandy Bridge, though, with Intel included a graphics core in each of the new models and manufacturing the new processors using a 32nm fabrication process throughout. The new design now combines the CPU, GPU and memory controller into a single unified piece of silicon. This means that it can share the chip's 8MB of L3 cache between the processor and graphics cores, and power supply and heat output can be managed more efficiently.
There has also been a significant increase in the density of transistors squeezed into Sandy Bridge chips, compared to their predecessors. Lynnfield Core i5/i7 processors (without graphics) have a die size of 296mm^2 and a transistor count of 774 million. The transistor count in the Core i7 Sandy Bridge has increased to 995 million, but due to the use of the 32nm process, the die area has been reduced to 216mm^2. And where the previous integrated Intel HD 2000 graphics had a single block of six shaders while HD 3000 graphics boasted two blocks, for a total of 12 shaders.
Intel has worked on its previously feeble on-chip graphics, improving its spec from DirectX 10.0 compatibility to 10.1, and increasing its maximum core speed from 900MHz to 1350MHz. Also included is an interesting new hardware feature called Quick Sync, which handles video encoding. Most graphics chips include a Universal Video Decoder that handles the playback of HD video but this is the first time we've seen hardware that handles video encoding.
This change to the graphics is relatively minor compared to two more significant features that Intel has included in Sandy Bridge. The first is the introduction of AVX (Advanced Vector Extensions), which are designed to speed up the heavy-duty maths involved in so-called 'floating point calculations'. These sit alongside the SSE4.2 (Streaming SIMD Extensions) instructions that were introduced with Core i7.
The traditional approach has been to decode instructions into micro-ops as they head into the CPU pipeline. As the SSE instruction set has increased in size, the Out-of-Order (OoO) execution hardware has required an ever-larger amount of CPU die area, along with a fair amount of electrical power.
AVX is more efficient, but requires a physical register file where the data can be parked. Luckily, the die shrinkage from 45nm to 32nm has created the necessary free space inside the processor. This means Sandy Bridge can store the operands in the physical register file, so that the micro-ops passing through the OoO hardware only need pointers to the data. Our tests indicate that AVX gives Sandy Bridge an impressive increase in performance compared to Lynnfield.
The second major feature in Sandy Bridge is Turbo Boost 2.0. Lynnfield introduced the first version of this dynamic method of overclocking, in which all four CPU cores can run two multiples of the 133MHz base clock above the reference speed of the CPU. If the workload only requires two CPU cores, they can go even faster - and if you get lucky and only need a single core, that core can run faster still.
Turbo Boost keeps the CPU safely inside the limits of its TDP (Thermal Design Power) safety limit, but if you choose to overclock your Core i5 or Core i7 by 500MHz or more, you'll be beyond the bounds of Turbo Boost and the feature it has no effect.
Turbo Boost 2.0 improves matters by adding a Burst Mode that's faster than the Sustained Mode, so when you start to run a task the CPU cooler will be able to handle an extra dose of heat before it warms up and the CPU drops back to the regular Turbo speed.
The Intel motherboards we used in our testing offer two settings in the BIOS for Turbo Boost, with default figures of 95 Watts for the Sustained mode and 118 Watts for the Burst limit. This power limit includes the graphics core and memory controller, so Intel is able to take account of how hard each part of the CPU is working in order to maximise the efficiency and performance of your Sandy Bridge CPU.
The existing crop of Core i3, i5 and i7 processors used a three digit model code to roughly indicate the CPU's clock speed, and whether or not the processor has a graphics core.
With Sandy Bridge, Intel has moved to a four-digit model number, in which the first digit, number 2, denotes that the chip uses this second-generation technology. It would be an impossible task to try to indicate clock speed, the amount of Turbo, the type of graphics core and the graphics clock speed using only the three remaining digita, so Intel has attempted to clarify the matter by adding a suffix to the model code for each CPU that gives an indication of the TDP. A model code with no suffix such as Core i5-2500 has a TDP of 95W. An S model of Core i5/i7 has a lower TDP of 65W, and a T is rated at an even lower 45W.
Under the new nomenclature, the Sandy Bridge Core i5-2500 has a clock speed of 3.3GHz, with a Maximum Turbo speed of 3.7GHz and a TDP of 95W. The Core i5-2500S has a clock speed of 2.7GHz and a TDP of 65W, while the low-power Core i5-2500T runs at a mere 2.3GHz, with a TDP of 45W.
To 'unlocked' models are included in the range, with the suffix 'K'. These can be overclocked manually. The chips are the Core i5-2500K and Core i5-2600K. Unlike earlier Intel CPUs, you cannot adjust the clock multiplier and neither can you raise the front-side bus or base clock speed. Intel has locked down these settings by combining the various clock figures including the PCI Express and memory clock speeds into a single host clock. While you can adjust this setting from its default speed of 100MHz, you'll rapidly find your system becoming unstable - so you can only expect a safe speed increase of around five per cent, if you're lucky.
When we tested a Core i5-2300 using an Intel DP67BG motherboard, we found the processor speed was effectively locked to its stock speed of 2.8GHz. Once we'd fiddled in the BIOS, we had a maximum Turbo speed of 3.3GHz.
This sounds relatively uninspiring, but the performance of the Core i5-2300 is formidable. In our tests it matched the performance of an overclocked Core i7-750 running at 4.2GHz. That in itself is impressive - but the bigger story is that the Sandy Bridge CPU drew a phenomenal 130 Watts less power at the mains socket when both systems were under load.
When we raised the Turbo Boost limit for the Core i7-2500K and i7-2600K, we found that both CPUs would run at 4.2GHz with the voltage settings left on Auto. When the system was idling or under light load the processors would run at their stock speed of 3.3GHz or 3.4GHz, but as the load increased the speed ramped up to 4.2GHz. This gives you the best of both worlds, as the system draws very little power while delivering plenty of performance. Then, when you need extra performance, Turbo Boost leaps into action with the minimum of fuss and the maximum impact.
Verdict
Intel's new Sandy Bridge family of processors still carry the name Core i3, i5 and i7 - but inside, the hardware has been given an extensive makeover. The 32nm fabrication process has given Intel plenty of scope to revise the layout of the core, and the result is impressive. In particular we feel that the AVX instruction set delivers superb performance and the revised Turbo Boost 2.0 is a work of genius.
The only fly in the ointment is that, with a unified system-wide base clock speed, Sandy Bridge appears to mark the end of cheap overclocking.
Company: Intel
Contact: 01793 403000
