Chapter 10: Computer systems (1) The CPU The central processing unit (CPU) executes the instructions in a computer system.  It consists of: an arithmetic logic unit a control unit registers buses.

Arithmetic logic unit (ALU) The arithmetic logic unit (ALU) carries out the calculations and logical decisions. The results of its calculations are stored in the accumulator.

Control unit (CU) The control unit sends out signals to co-ordinate how the processor works. It controls how data moves around parts of the CPU and how it moves between the CPU and memory. Instructions are decoded in the control unit.

Registers Registers are memory locations within the processor itself. They work at extremely fast speeds. Program counter (PC): Keeps track of the memory location of the line of machine code being executed. Memory data register (MDR): Stores the data that has been fetched from or stored in memory. Memory address register (MAR): Stores the address of the data or instructions that are to be fetched from or sent to. 

Registers (cont.) Current instruction register (CIR): Stores the most recently fetched instruction, waiting to be decoded and executed. Accumulator (ACC): Stores the results of calculations made by the ALU. General purpose registers: Processors may also have general purpose registers. These can be used temporarily to store data being used rather than sending data to and from the comparatively much slower memory.

Buses Buses are the communications channels through which data can be sent around the computer. There are three buses inside the computer we need to consider: the data bus, control bus and address bus. The data bus carries data between the processor and memory. The address bus carries the address of the memory location being read from or written to. The control bus sends control signals from the control unit.

Processor The processor carries out instructions through the fetch–decode–execute cycle. Fetch: An instruction is fetched from memory into the processor. The MAR provides the location, which is sent down the address bus, and the fetch signal is sent down the control bus. The contents of the memory location are then sent from memory to the processor down the data bus.

Processor (cont.) Decode: The instruction is decoded in the control unit. Execute: The processor carries out the instruction. This might be, for example, adding two numbers together, loading a number from memory into the accumulator or storing the contents of the accumulator into memory.

Processor (cont.) Processor performance can be improved in several ways.  Cache memory: This is small memory that works much faster than the main memory. By anticipating the data and instructions that are likely to be regularly accessed and keeping these in cache memory, the overall speed at which the processor operates can be increased.

Processor (cont.) Cores: Each core is a distinct processing unit on the CPU. When multi-tasking, different cores can run different applications. It is also possible for multiple cores to work on the same problem. Pipelining: This means while one instruction is being executed, the next instruction is being decoded and the one after that is being fetched.

Graphics processing unit Graphics processing units (GPUs) have instruction sets specifically designed for the sorts of calculations required in graphics processing. GPUs have the ability to perform the same instruction on multiple pieces of data at the same time. GPUs have uses outside of graphics including: modelling physical systems, audio processing, breaking passwords and machine learning.