1 The characteristics of contemporary processors, input, output and storage devices Structure and function of a processor The central processing unit (CPU) carries out the instructions in computer programs. Essentially it’s what makes a computer a computer! Inside a processor there are billions of transistors (effectively electronic switches). This section looks at how CPUs work and different developments in CPU technology. The arithmetic logic unit, control unit and registers The main components of the CPU are the arithmetic logic unit, control unit and registers. These are connected to the system’s main memory using pathways called buses. Arithmetic logic unit The arithmetic logic unit (ALU) carries out the calculations and logical decisions. The results of its calculations are stored in the accumulator. Control unit The control unit (CU) 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 areas of memory within the processor itself. They can be accessed at extremely fast speeds so can be used by the processor without causing a bottleneck. Registers in the processor that have a specific use are called special purpose registers. The special purpose registers you need to know are listed below. Register Purpose Program counter (PC) Keeps track of the memory location of the line of machine code being executed. With each iteration of the fetch–decode–execute cycle, it gets incremented to point to the next instruction, allowing the program to be executed in sequence, instruction by instruction. The program counter can also be changed by instructions that alter the flow of control (for example, branch like BRA, BRP and BRZ) Memory data register (MDR) Stores the data or instructions that are to be fetched from or sent to memory Memory address register (MAR) 8 Current instruction register (CIR) Accumulator (ACC) Stores the results of calculations made by the ALU Check your understanding and progress at www.hoddereducation.co.uk/myrevisionnotesdownloads Stores the address of the data or instructions that are to be fetched from or sent to memory Stores the most recently fetched instruction, which will be decoded and executed Central processing unit (CPU) The central processing unit ‘runs programs’ by continually fetching, decoding and executing instructions. Register A discrete piece of memory built onto the CPU that holds a single piece of data.