Components of a CPU AS Computing - F451

Microprocessor

“640kb ought to be enough for anybody” Bill Gates, 1981

Control Unit ALU Memory Unit (IAS) Registers PC MAR MDR CIR

Interrupt A signal telling a piece of software to “wait” whilst another, more important command is executed. What are some examples?

Buffer A region of memory that is temporarily used to hold data while it is being moved form one place to another Used when 2 devices transmitting to each other run at different speeds Buffers can be implement in either hardware or software

ALU – Arithmetic Logic Unit Where calculations are carried out Mathematical & Logical operations Instruction Set Acts as a gateway to and from the processor

Control Unit Controls the data flow in the processor Controls the fetch execute cycle Synchronises actions (using inbuilt clock) Sends control signals to other parts of the processor

Memory Unit (IAS) Purpose of RAM Store part of currently in use operating system program data currently in use

Non Volatile! Purpose of ROM Purpose of ROM Start up program Bootstrap Program. Non Volatile!

communication to send data Buses Channel of communication to send data

Buses Data Bus This is used to carry the data that needs to be transferred from one piece of hardware to another

Address Bus Carries identification about where the data is being sent/coming from

Control Bus This carries the command signals of the control unit to other components

Registers “Locations in memory used for specific purposes” What is a Register? “Locations in memory used for specific purposes”

IAS Program Counter Current Instruction Register Memory Address Data Register Stores where in IAS the next instruction is Stores the current instruction being executed Stores the address of where the data used in the current instruction Acts as a buffer between the IAS and CPU

Register Definitions Program Counter Current Instruction Register stores the address of the next instruction Current Instruction Register stores the instruction currently being operated on Memory Address Register stores the address in memory currently being accessed Memory Data Register stores the data being transferred to or from memory

External components The term external components refers to the parts that you physically interact with. These are also known as peripherals or input/output devices and include the: keyboard monitor printer

Internal components The term internal components refers to the components contained within the casing of the computer. They include the: processor memory (RAM and ROM) hard disk.

Types of Memory Random Access Memory (RAM) Immediate access store (IAS) It temporary stores data used by applications that are running. The contents are lost when the computer is switched off. Volatile Read Only Memory (ROM) ROM is a permanent storage area which cannot be changed by the user. ROM contains the start up software (this makes the PC boot up) Non-Volatile

Read Only Memory Non-Volatile – Contents of memory is not erased when hardware is powered off. Contains Start up program Start up program is required immediately when power is switched on

Random Access Memory Volatile – Contents of memory is erased when hardware is powered off Contains applications software currently being used, OS files Allows changes to be made to saved contents Fast access to data

Transferring Data – Primary Memory to Secondary Storage

This exact question has come up in 2 previous past papers, and can be worth 6+ marks. “Describe the use of interrupts and buffers when data is transferred from primary memory to a storage device.”

Define Terms Primary Memory – RAM, IAS Secondary Storage – USB Memory Stick Both transfer data at different speeds Therefore need to use Buffer Area of temporary memory used to transfer data between 2 devices Interrupt Method of stopping the CPU so it can perform a task and return to the previous task.

Key Points Data sent to buffer from primary memory. This allows processor can continue with other tasks Interrupt sent when buffer is full. Buffer is emptied to storage device. Buffer used to allow RAM and secondary storage device to transfer data as they do so at different speeds. When buffer is empty Interrupt is sent to processor To request buffer refill Process cycles until all data is transferred

Additional Points Error checking takes place once data is transferred. Interrupt given priority to CPU when buffer is full, required refilling Plan your answer. Use clear English. Read it first before writing on question paper. EXAM Technique!