The processor

Computer systems generally consist of three main components: the internal components that are involved in processing data and or a set of instructions in the form of a computer program, memory which holds the programs and data being processed and input /output devices; used for communication with the outside world. We shall consider here, the function of the internal components of a computer system. 

The processor consists of the Control Unit (CU), a series of registers and the Arithmetic Logic Unit (ALU). 

Control Unit

The control unit is a digital circuit that coordinates the movement of data, the sequencing of instructions and directs the operation of the other components of a computer system by providing control and timing signals. Timing signals are provided by an internal clock which helps to synchronize the operation of the internal components and which helps determine how many computer instructions are carried out (executed) each second. The CPU needs a certain number of clock ticks to carry out each instruction. Clock speed is expressed in megahertz (MHz) or gigahertz (GHz). 

Arithmetic Logic Unit

The Arithmetic Logic Unit (ALU) is digital circuit which forms part of the processor. It carries out mathematical and logical operations on data associated with instructions being executed. 

Registers

Internal Memory Much as we need to remember steps and values to perform even the simplest calculations, so too must the processor have somewhere to store the data and instructions currently being operated upon. Main memory and cache memory are two types of memory used by the processor in the execution of instructions. 

Main Memory (RAM)

Main Memory Main memory is often referred to as RAM or Random Access Memory or Immediate Access Memory. Main memory at any time during the operation of the computer stores a portion of the operating system, application software currently running and other data items the processing unit needs when carrying out operations. Main memory is volatile, meaning when the power is removed from the system the data will be lost, it is referred to as random access memory because the processor can go directly to any location to access an item of data; it does not have to access the memory locations in a particular (sequential) order. 

Cache Memory

Cache Memory Cache memory is a form of RAM which can be accessed, by the processor, at a faster rate than main memory. It is high speed volatile memory that is often integrated into the processor chip (as shown in the diagram above) or it may sit between the processor and main memory in the form of a separate memory chip. Cache memory is used to store copies of the contents of the most frequently used memory locations. During the processing of data items, the processor will search cache first before searching the main memory. 

Buses

Buses Operating in isolation, each of the internal components of a typical computer system has a very specific function. Communication between each of these components is necessary during the execution of program instructions. Communication systems known as buses are used to support the transfer of signal between the internal components of computer system (internal buses) and to allow for communication with the various input and output (I/O) devices used for communication with the outside world (external buses). A bus is basically a set of physical connections in the form of set of physical cables or printed circuits used to connect the various hardware components used in the execution of a set of instructions. 

Address Bus

Will be used to identify a physical location in memory to be accessed. The location may be accessed to read data from or write data to memory. If an item of data is to be stored in memory the location to be used as a repository for the data item will be identified using the address bus. 

Data Bus

Allows for the transfer of data between one component of a processor or computer system and another. If an item of data is to be stored in memory, the data item will travel to the memory location via the data bus. 

Control Bus

Carries commands from the CPU to other devices in relation to the instructions being executed at a particular time. If an item of data is to be stored in memory the control bus will carry the write command to the memory location. 

Ports

While buses allow for the transfer of data between various components in a computer system it is important to also consider how the data can be transferred from the input/output devices to the internal components of the computer. To allow data to be transferred from peripheral devices they must be connected to the computer via an Input/Output (I/O) port. Types of computer ports include:– 

Serial port which transmits one data bit at a time across a single wire, used mainly for connections to mice and modems 

Parallel port which transmits multiple bits of data over multiple wires at one time and can be used to connect some printers. USB (Universal Serial Bus) ports now provide a high speed serial connection to most types of peripheral devices. 

Registers

Registers were mentioned briefly at the beginning of this fact sheet. A register is a high speed memory location forming part of the processing unit. The registers listed below have an important role to play in the processing of instructions by the control unit in what is known as the fetch-execute cycle. 

There are 4 different registers we need to be aware of.

Program Counter

Stores the address/location in memory of the next program instruction to be executed. As soon as an instruction has been fetched from memory the contents of the program counter are incremented to ensure it points to the next location to be accessed. 

Memory Address Register

Stores the address of a memory location to be accessed either for a data/ instruction read or write 

Memory Data Register

Stores data that has just been fetched from memory or data waiting to be stored / written to a location in memory 

Current Instruction Register

Stores the instruction currently being executed by the processor 

Fetch Execute Cycle

This is the basic principle on which modern CPUs operate. Data and instructions are fetched from memory, they are taken to the CPU where they are decoded (broken down) and executed (carried out). The next instruction is fetched... The cycle contines.

The fetch-execute cycle is the process used by a computer to retrieve a program instruction from memory, determine what that instruction is expected to do and then carry out the actions associated with that instruction. Let us consider how the registers and buses outlined previously can be used as part of the fetch-execute cycle. 

The address of the first instruction to be read is loaded into the Program Counter (PC). Before data/instructions can be accessed from main memory the contents of the Program Counter (PC) are placed in the Memory Address Register. (The MAR will use the address bus to access the location where the instruction is stored). 

The Program Counter (PC) is immediately updated to point to the location of the next instruction to be executed. A read signal will be sent via the control bus to memory and the instruction is transferred from memory to the Memory Data Register (MDR) via the data bus. 

The instruction is then copied into the Current Instruction Register (CIR) where it will be examined or decoded by the processor. The flowchart below shows how this process is repeated for each instruction being executed as part of a larger program. 

Processor Speed

By manipulating some of the components previously examined we can affect the speed of operation of the CPU. 

Clock speed 

Previously we looked at how microprocessors use an internal clock to control the number of instructions to determine how many instructions are executed per second by the CPU. If the CPU requires a specific number of clock ticks to execute an instruction then by increasing the clock rate (the number of clock ticks / cycles per second) we can increase the number of instructions executed per second; i.e. the faster the clock speed, the more instructions the CPU can execute per second. 

Cache size 

Cache memory provides high speed access to the most frequently used instructions and data items. By increasing the size of cache memory we can also increase the number of instructions and or data items being held in fast access memory in close proximity to the processor. Increasing the speed of access to a greater number of frequently accessed instructions and data items can help to increase the speed of operation of the processor. 

Processor Type

Most computing devices can use one of two main types of processor known as Reduced Instruction Set Computing (RISC) or Complex Instruction Set Computing (CISC). As the name suggests RISC has a smaller number of instructions at its disposal during processing. This means that in complex processing tasks RISC processing units have to combine many simple instructions to complete each task meaning an increased number of fetch-execute cycles need to be completed to carry out complex tasks; thus slowing down the overall speed of processing. 

The number of cores will also affect the speed of a processor. The more cores a processor has the more instructions which can be executed at the same time. A dual core is more efficient than a single core. A quad core is more efficient than a dual core.