CPU (Central Processing Unit)

CPU (Central Processing Unit)

1. A computer system takes a set of digital inputs, processes them and creates a set of outputs. This is done using a combination of hardware and software.

2. Parts of a CPU

   • The Control Unit (CU)
     • The CU regulates and integrates the operations of the computer. It selects and retrieves instructions from the main memory in proper sequence and interprets them so as to activate the other functional elements of the system at the appropriate moment
   • The Arithmetic Logic Unit (ALU)
     • The ALU handles the data processing inside the CPU. It performs standard arithmetic operations such as addition, subtraction etc. It deals with logic and comparisons such as ‘is this value greater than that value?’
   • The Registers
     • A register is a type of temporary memory located inside the CPU. It is faster to shift data to and from registers rather than RAM so it speeds up processing time. There are many registers inside the CPU.
   • The Cache
     • The Cache is another type of temporary memory. It is faster to access than RAM, but slower to access than the registers.

3. Fetch / Decode / Execute

   • In this step the CPU fetches some data and instructions from RAM and then stores them in its own temporary memory called ‘registers’. The address of the next item that the CPU wants is put onto the ‘address bus’. For this to happen, the CPU uses a piece of hardware path called the ‘address bus’. Data from this area then travels from the RAM to the CPU on another piece of hardware called the ‘Data Bus’.
   • The decode step is where the CPU understands / works out what the instruction it has just fetched actually means. The CPU ‘decodes’ the instruction and gets things ready for the next step.
   • The Execute stage is where data processing happens. Instructions are carried out on the data. Once a cycle has completed, another begins.

4. Von Neumann Architecture - John Von Neumann

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