Computer Architecture

Three characteristics of a CPU set how quickly it can process work: clock speed, cache size, and number of cores.

Clock speed

Clock speed is the number of fetch-decode-execute cycles the CPU can complete in one second, measured in hertz (Hz).

• 1 Hz = 1 cycle per second.
• 1 GHz (gigahertz) = 1 billion cycles per second.
• A typical modern CPU runs at 2 to 5 GHz.

A higher clock speed means the CPU completes more cycles per second, so it processes more instructions per second. Doubling the clock speed roughly doubles the rate of instructions (assuming nothing else becomes a bottleneck).

Why not just push the clock speed to 100 GHz? Higher clock speeds:

• Use more electrical power
• Generate more heat
• Require more advanced cooling
• Hit physical limits in the silicon

So modern CPUs balance clock speed against power and heat, often around 3 to 5 GHz.

Cache size

Cache is a small, high-speed memory built into (or sitting beside) the CPU. It holds copies of frequently-used instructions and data so the CPU can grab them without making a slow trip out to main memory (RAM).

• The CPU can access cache much faster than RAM.
• A larger cache holds more frequently-used data, so more requests can be served by the cache and fewer slow trips to RAM are needed.
• Typical sizes: from a few kilobytes (L1 cache, the fastest level) up to tens of megabytes (L3 cache, the slowest of the cache levels but still faster than RAM).

A larger cache size means better CPU performance, because the CPU spends less time waiting for data to arrive from main memory.

Number of cores

A core is a complete processing unit within a CPU chip, with its own Control Unit, ALU, registers and accumulator. A modern chip usually has multiple cores that can each fetch, decode and execute instructions at the same time.

• A single-core CPU handles one instruction stream at a time.
• A dual-core CPU has 2 cores and can handle 2 instruction streams in parallel.
• A quad-core CPU has 4 cores; an 8-core CPU has 8; modern server CPUs may have 64 or more cores.

For tasks that can be split across cores (video encoding, scientific computation, running many apps at once), more cores means more work done per second.

Side-by-side summary

Trade-offs and the headline number

Manufacturers usually advertise the clock speed first, but cache and cores often matter just as much. A 4 GHz quad-core with a large cache will usually outperform a 5 GHz single-core with a tiny cache on real-world workloads. When comparing two CPUs, you need to look at all three characteristics.