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 multi core. 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 Direct X 11-capable line of graphics cards, brings sophisticated graphics capability and parallel compute technology directly onto the CPU.

Current time you can see the cores like the mention below:-

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 chip sets 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.