The central processing unit, also known as the processor or CPU, is the component that carries out all the instructions or calculations in the computer.
Components of the CPU
The ALU (Arithmetic and Logic Unit) – this performs all the arithmetic and logic calculations including simple maths and if statements.
The Registers – these are storage spaces that store instructions and data currently being used by the processor. The accumulator stores the results of calculations.
The Internal Memory – also known as cache level 1, this is a storage area for commonly used commands that are not currently being used, but may be required imminently. Here they can be accessed quickly, speeding up the processing of information. It has a significantly faster read write speed than RAM but is very expensive so is much smaller (eg 2Mb)
The Controller – the controller tells the other components what to do, sending instructions to the input and output devices. It also sends instructions to the ALU and the internal memory.
Busses – these are the cables that allow data to flow from one component to another.
Quality of the CPU
The Clock Speed – This refers to the number of fetch-execute cycles that the processor can carry out every second. For example, a 2GHz processor can carry out 2 billion cycles per second.
The Number of Cores – standard processors carry out only one instruction at a time. A multicore process has the capacity to carry out multiple processes at a time. For example, a quad core processor can carry out 4 processes at one time. This helps when multitasking with multiple programs. Common Mistake – a dual core is not exactly twice as fast as a single core processor as the cores need to communicate with one another.
Cache Size – this is a small temporary storage space for instructions that the processor may need to use regularly. The larger the cache, the more instructions that can be stored, potentially speeding up the running of programs.
Word Length – this is the number of bits that the processor can handle per cycle. Common word lengths are 16bit, 32 bit and 64bit.
TIP – Remember CCCW – Clock, Cores, Cache, Word
Types of CPU
CISC (Complex Instruction Set Computer) – these can process complex instructions but as a result need more circuitry and are therefore larger in size. This means they consume more energy which makes them get hotter. A heat sink and fan are required to keep them cool. This makes them more suitable for larger computing devices like desktop computers and game consoles.
RISC (Reduced Instruction Set Computer) – these need to break down complex instructions into smaller chunks which means they are slower. However, with less circuitry the chips can be smaller and they produce less heat which negates the need for a heat sink and fan. This makes them suitable for smaller devices such as phones, tablets and smart watches.
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