3.1.4 Hardware a. describe the function and purpose of the control unit, memory unit and ALU (arithmetic logic unit) as individual parts of a computer; b. explain the need for, and use of, registers in the functioning of the processor (Program Counter, Memory Address Register, Memory Data Register, Current Instruction Register and Accumulator);

The processor External Bus Memory Arithmetic Logic Unit Program Counter Control Unit Current Instruction Register Memory Address Register Memory Data Register Accumulator

Registers Registers are storage locations within the processor that are used for specific tasks. They are used to store data & instructions whilst they are being manipulated or executed.

Processor components Describe the function and purpose of the a.Control unit, b.Memory unit c.ALU (arithmetic logic unit) as individual parts of a computer.

Control Unit Coordinates the activities taking place in the CPU, memory and peripherals by sending control signals to various devices The control unit has three jobs: 1.Decide which instruction to execute next then fetch it. 2.Decipher that instruction, work out what needs to be done to carry out the instruction 3.It has to tell the other parts of the processor what they should do to execute the instruction

Memory unit The memory unit stores everything that the processor is going to use. This includes the program instructions and the data needed to carry out the instructions.

Arithmetic Logic Unit The ALU is where things are actually done in the processor. 1.The ALU contains the circuitry that allows it to carry out arithmetic operations. 2.It allows logical decisions to be made (if statements etc) 3.The ALU also acts as a gateway between the processor and other parts of the computer system. All input and output to and from the processor goes into the ALU and waits there while the control unit decides what to do.

Registers Explain the need for, and use of, registers in the functioning of the processor. ● Program Counter ● Memory Address Register ● Memory Data Register ● Current Instruction Register ● Accumulator

Program Counter The program counter keeps track of where in the computer's memory the next instruction can be found so the next instruction can be placed in the CIR.

Memory Address Register The Memory Address register is used to hold the address (in memory) that contains the next instruction, or piece of data to be used.

Memory Data Register The Memory Data Register is used to temporarily store data read from or written to memory. It acts like a buffer and holds anything that is copied from memory ready for the processor to use it.

Current Instruction Register Holds the operand and operator of the current instruction. LOAD #12893iy4 OperatorOperand

Accumulator Is used for performing arithmetic functions. It acts as a working area. For example the instruction to add together the contents of memory locations 128s8u and 987hj6v7 might be broken down in to: Load contents of 128s8u in to the accumulator Add the contents of 987hj6v7 to the accumulator Store the contents of the Accumulator in 124gj89h

How registers are used 1.The address of the next instruction is copied from the PC to the MAR 2.The instruction held at that address is copied to the MDR At the same time the content of the PC is incremented so it holds the address of the next instruction. 3.The contents of the MDR are copied to the CIR 4.The instruction held in the CIR is decoded 5.The instruction is executed