Lab

Processors

If your PC is struggling with high-definition video, the latest games or even Windows Vista, you should consider buying a new processor. Whether you're upgrading an old PC or building a new one, many of the latest processors provide incredible amounts of computing power for little money.

All modern processors have at least two processing cores, allowing them to undertake multiple tasks with ease, while many have four cores for speeding through demanding tasks. Even the slowest processor in this group test scored a respectable 124 in our Shopper benchmarks.

This month we've tested a total of 38 processors to give you a definitive guide. We've covered the full extent of both Intel's and AMD's current processor ranges, with processors for both LGA775 and AM2 motherboards. The cheapest processor on test costs only £36 including VAT, proving that a fast PC doesn't have to cost a fortune.

If you're planning to upgrade your PC, you'll need a processor that matches your current motherboard. The Intel processors here use socket LGA775; the AMD processors use socket AM2. It's not quite that simple, though, and as newer processors are released, older motherboards won't support them. Check your motherboard's manual before you buy. If you're building a new system, you can choose any processor, as long as you buy a motherboard that supports it.

Cores and effect

Every processor here has either two or four cores. In effect, these can do the work of two or four older single-core processors running at the same speed. This makes them better at handling multiple tasks such as watching a video while encoding to a CD or MP3.

Many new applications take advantage of multiple cores. These multi-threaded applications use all the cores to reduce the time it takes to perform a task. This explains the very high scores achieved by the quad-core processors in our multi-threaded Video test.

On the buses

Two factors determine a processor's clock speed: the external bus speed multiplied by the multiplier. Most of the processors here have a 200MHz external bus speed, but Intel's new processors have faster 266MHz or 333MHz bus speeds.

The external bus speed also determines other factors. In Intel processors, the external bus speed is quad-pumped to get the frontside bus (FSB) speed (a 266MHz external bus makes for a 1,066MHz FSB). The FSB talks to other components including the PCI Express bus and system memory, which also has its speed determined as a ratio of the external bus.

AMD processors use HyperTransport instead. This is a direct-connect technology and its speed is based on a multiplier of the external bus. HyperTransport 2.0 runs at a maximum of 1.4GHz, which is a x7 multiplier on the 200MHz external bus. HyperTransport gives dedicated connections to system components, such as the PCI Express bus and system memory. System memory is still linked to the external bus speed, though.

You can increase a processor's speed by increasing the multiplier or external bus speed. Increasing the multiplier is the easiest option, as it affects only the processor's speed; increasing the external bus affects everything that's related to it, such as system memory. This can cause instabilities. Unfortunately, most processors are multiplier locked, so external bus overclocking is your only option. Our reviews tell you which processors are not multiplier locked.

Cache rich

Processor speeds alone don't tell you how fast a processor is. Other factors, such as the number of instructions each can perform in a clock cycle (known as its efficiency), play a big part. Accessing system memory can also slow a processor down, so caches are important. A cache stores recently accessed instructions and data in super-fast memory on the processor to cut the number of memory accesses needed. L1 cache is the smallest and fastest type, and each processor core has its own. L2 cache is a little slower and can be shared or dedicated to one core. Only AMD's quad-core Phenom processors use L3 cache, which is shared across all cores. Our reviews tell you how much of each type of cache each processor has, as well as its advantages and disadvantages.