Understanding CPU Features

Reading PC specs may seem like a foreign language at times. In this article, we will attempt to demystify the specifications and put some context around them so you know what you really need for a responsive PC.

Speeds and Feeds. Processor performance is usually a number in either MHz (megahertz) or GHz (gigahertz.) That number represents how many times the internal clock inside the CPU ticks in cycles per second. The clock inside a 2.5GHz CPU ticks 2.5 billion times each second.

But clock frequency isn’t a complete measure of performance. Efficiency — how much work can be done by the CPU in each clock cycle — is also important. This is measured in terms of instructions per cycle, often abbreviated as IPC. A CPU with a very high clock frequency, but low IPC, may not perform as well as a CPU with a lower clock frequency and high IPC.

Cache. Fast access to memory is important to a PC processor, and that's what your cache is for. Waiting for data to come in from system memory can make your CPU inefficient. The memory hierarchy is a way to show where memory bottlenecks exist. The CPU registers hold the data actually being used in calculations and comprise the fastest memory. The next fastest are levels 1 and 2 caches.

These are small amounts of fast memory built right onto the CPU core. Level 1 (L1) cache is faster but smaller than Level 2 (L2). Some CPUs have a third level, which is often shared between different CPU cores. Even the L3 cache is much faster than your computer's main memory.

If an item of data is not in the cache, the CPU must wait until the data is retrieved from memory. In the slowest case, the data isn’t even in main memory, but must be fetched from the hard drive. However, while waiting for that data to be retrieved from system memory, the CPU can turn to a different task to help keep the PC running.

Cores. Modern PC processors are multicore. Each core is as fully functional as the others. Each has its own cache, but can communicate with other CPU cores as needed. Cores can also share information in a cache, such as the level 3 cache built into AMD Phenom II processors. AMD’s Accelerated Processing Unit (APU) technology adds a new concept — the GPU core — which is better at certain kinds of math than the traditional core. The technology, originally developed for AMD's DirectX 11-capable line of graphics cards, brings sophisticated graphics capability and parallel compute technology directly onto the CPU.

Bandwidth. CPUs are part of a larger platform. They need to communicate with the outside world. Data buses (circuits that carry the data) communicate with the chipsets built onto the motherboard. The chips act as traffic cops to route data to the outside world, for example to the PCI Express slots, which communicate with discrete graphics cards and other expansion cards, and USB ports, which communicate with external storage, keyboards, mice, etc.

The speed of this communication is called bandwidth. It's split into two types: memory and input/output (I/O). AMD processors have circuitry integrated into the CPU itself to handle all communication with system memory. This block of circuits is called the memory controller.

AMD processors offer a rich set of features that come together to make all the pieces work in concert. Offering multiple CPU cores, robust cache, fast communication with memory and input/output, these features work in parallel to create a responsive PC experience.

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