General purpose computers consist of ............... and software. ...................... is the physical components of the computer, such as the central processing unit (CPU), hard disk, monitor, keyboard and mouse. Software is the programs that run on a computer.

After upgrading the RAM, Quinn could make further changes to improve his computer’s performance. Make a list of three "things" Quinn could use to improve the performance?

ROM can be read from, but not written to, hence the term 'read only'. This makes ROM ideal for storing instructions and data that are needed for the computer to run. These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten.

The BIOS (Basic Input Output System) is an example of a program that is stored in ROM. The BIOS runs as soon as the computer is switched on. It checks that the hardware is functioning correctly, then runs a second program known as the bootup or bootstrap program that loads the computer's operating system from the hard drive into the RAM. The BIOS is always needed so it is stored in ROM.

The fetch-decode-execute cycle

The fetch-decode-execute cycle is followed by a processor to process an instruction. The cycle consists of several stages.

The memory address held in the program counter is copied into the MAR.

The address in the program counter is then incremented (increased) by one. The program counter now holds the address of the next instruction to be fetched.

The processor sends a signal along the address bus

to the memory address held in the MAR.

The instruction/data held in that memory address is sent along the data bus to the MDR.

The instruction/data held in the MDR is copied into the CIR.

The instruction/data held in the CIR is decoded and then executed. Results of processing are stored in the ACC.

The cycle then returns to step one.

Depending on the type of instruction, additional steps may be taken:

If the instruction is to transfer data held in the ACC back to primary memory, the intended memory address is copied into the MAR. The data to be transferred is copied into the MDR and then transferred to the specified address using the address bus and data bus.

The executed instruction may require the program to jump to a different place in the program. In this case, the memory address of the new next instruction to be fetched is copied into the program counter. The process then restarts at step one.

Control unit

The CU provides several functions:

• it fetches, decodes and executes instructions

• it issues control signals that control hardware

• it moves data around the system

Arithmetic logic unit

The ALU has two main functions:

• It performs arithmetic and logical operations (decisions). The ALU is where calculations are done and where decisions are made.

• It acts as a gateway between primary memory and secondary storage . Data transferred between them passes through the ALU.

Key fact

The ALU performs calculations and makes logical decisions.

Cache

Cache is a small amount of high-speed random access memory (RAM) built directly within the processor. It is used to temporarily hold data and instructions that the processor is likely to reuse. This allows for faster processing as the processor does not have to wait for the data and instructions to be fetched from the RAM.

Registers

Registers are small amounts of high-speed memory contained within the CPU. They are used by the processor to store small amounts of data that are needed during processing, such as:

• the address of the next instruction to be executed

• the current instruction being decoded

• the results of calculations

Different processors have different numbers of registers for different purposes, but most have some, or all, of the following:

• program counter

• memory address register (MAR)

• memory data register (MDR)

• current instruction register (CIR)

• accumulator (ACC)

ROM can be read from, but not written to, hence the term 'read only'. This makes ROM ideal for storing instructions and data that are needed for the computer to run. These instructions and data are usually programmed by the computer's manufacturer and cannot be overwritten.

The BIOS (Basic Input Output System) is an example of a program that is stored in ROM. The BIOS runs as soon as the computer is switched on. It checks that the hardware is functioning correctly, then runs a second program known as the bootup or bootstrap program that loads the computer's operating system from the hard drive into the RAM. The BIOS is always needed so it is stored in ROM.

Random access memory (RAM) is volatile primary memory. Once the computer is switched off, the data and instructions held in RAM are lost. RAM is given the term ‘random access’ because data can be stored and accessed from any location within the memory.

RAM is used to hold data and instructions that are currently in use. In a modern PC, RAM is used to hold the operating system and any open documents and programs that are running.

There are times when the amount of RAM needed to hold all running programs and data is greater than the amount of RAM available to the computer. When this happens, part of the computer's secondary storage, such as the hard disk, can be used to store data temporarily.

Virtual memory enables data that is in RAM and not currently being used to be transferred to the hard disk. This frees up room in RAM for other programs and data. When the data on the hard disk is needed again, any other unused data is transferred to the hard disk before the original data is transferred back to RAM. This process is known as swapping.

Using virtual memory makes a computer run slower, as the processor has to wait while data is swapped between hard disk and RAM. As secondary storage devices have slower access times than RAM, the computer's processing performance can be severely impaired. Processing performance can be improved by increasing the size of the RAM, thereby reducing the need for virtual memory.

The CPU has a number of components. The control unit makes sure that all these components do their operations at the correct time and in the correct sequence. The registers store small amounts of information. Buses transfer information from one part of the CPU to another. In a 64 bit processor the data bus would have 64 lines.

Bus Information travels along a bus from one part of the CPU or computer to another

Address Bus This is a one way bus which requests data from an address in memory

Data Bus This bus moves data in both directions as data is both read and written to memory

Control Unit (CU) This is responsible for the timing on the CPU. It makes use of the clock to make sure all operations in the processor happen in the correct sequence

Arithmetic Logic Unit (ALU)This does all arithmetic calculations and logical operations. Results go in the Accumulator

Registers Small areas where binary digits are read and written on the CPU

Accumulator (ACC) Results from the ALU are temporarily stored here until they are written to memory

Program Counter (PC) This stores the memory address of the next instruction. At each fetch / execute cycle it will be incremented (1 is added to it)

Current Instruction Register (CIR) The current instruction is stored in this register in binary

Memory Address Register (MAR) This stores the memory address of the current instruction being executed

Memory Data Register (MDR)This stores the data that was read or written to memory