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Computing Academy GCSE Computer Science

Registers Each CPU core

Registers Each CPU core executes just one instruction at a time. A typical instruction for a CPU to execute might be: ADD 8, 4 Right before these values are passed to the ALU, they are moved from L1 cache into the CPU registers. Registers are very fast, very small segments of memory. Registers are used to temporarily store the very next instruction and data to be processed. Once the ALU has performed a calculation, the result of that calculation is then stored back to a register, before moving back through memory into cache, then into RAM. Hit or Miss If everything goes to plan, the CPU will always have enough data to process. When the CPU can find the next instructions in cache and load these into the registers ready to execute this is known as a cache hit; the data and instructions are ready and the CPU does not have to wait around. If the instructions are not ready as they are still being loaded into cache from RAM (or even from the HDD or SSD) the CPU will have to wait, this is known as a cache miss. Clock Speed In the human body, the beating of a heart is controlled by the Sinoatrial Node (SAN); some individuals with heart conditions need to have a pacemaker fitted in order to replicate the functionality of the SAN. The SAN (or pacemaker) causes the heart to beat and at that moment oxygenated blood is pumped out of the heart and around the body, and deoxygenated blood is pumped out of the heart into the capillaries surrounding the lungs 39

in order to collect more oxygen. In between beats the heart and the blood vessels are at rest. The operation of a CPU within a computer system has some remarkable similarities to this system. Rather than a Sinoatrial node, a CPU has an electronic clock chip which keeps time and regulates how frequently the CPU executes instructions. With each signal from the clock chip data flows through the CPUs buses, calculations are performed by the ALU, data is exchanged with RAM and data is read and written to the HDD or SSD. If a computer system had a clock speed of 500Hz, this would mean 500 cycles per second. Modern single core CPUs running at 3GHz execute around 3 billion instructions per second. Most computer systems today have multi-core CPUs which feature 2, 4 or even 8 cores. This means that the CPU might have up to 8 ALUs each executing several billion instructions per second. While a CPU might be clocked at 3GHz, the fastest RAM available today runs at 1066MHz; RAM is operating much slower than the CPU. This is why cache is so important. L1 cache operates the same clock speed as the CPU and L2 cache operates at a clock speed between RAM and L1 cache. The whole operation of the computer system is timed to the clock chip and within the CPU itself the clock chip controls the fetch-decode-execute cycle. 40

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