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Computing Academy GCSE Computer Science

Fetch Get location of

Fetch Get location of next instruction from Program Counter (PC), Get instruction and data by going to specified location in register, Update (+1) to Program Counter (so the CPU knows the address of the next instruction it needs to fetch) Decode The CPU now identifies the operation code (op-code), part of the instruction that has been fetched so it knows which operation to perform. The second part of the instruction will contain the data to be operated upon. Execute The instructions and the data are fed to the Arithmetic and Logic Unit (ALU) which then performs the calculation. The result is saved back to a register and then back to RAM to be output to the user. Executing Instructions Whether it’s playing the latest computer game, editing an image or simply browsing the Internet, all of those complicated tasks are broken down into a series of simple mathematical calculations to be processed by the CPU. The CPU runs incredibly quickly but it doesn’t store data, it simply crunches numbers. We know that data is stored using secondary storage such as a HDD or a SSD, but these devices are not quick enough to keep supplying data to the CPU at the rate at which it can 37

process this data. Computer Systems therefore make use of RAM, and an even quicker type of memory called cache. This constant stream of data from the HDD (or SSD) into RAM and then into Cache means that there will always be data ready to pass to the CPU for executing. As cache is so fast and so expensive, computer systems do not have very much cache. There are multiple levels of cache (depending on the brand and type of CPU). • Level 1 (L1) cache is always built into the CPU. • Level 2 (L2) cache is usually either built into the CPU or on the motherboard between the CPU and main memory (RAM) • Level 3 (L3) cache is usually built into the motherboard Each level of cache is increasingly quicker (L1 being the quickest) but with a smaller capacity 38

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