The stored program concept and processor components

• Every time a program is run, the processor runs through the fetch-execute cycle.
• FETCH- the processor fetches the program's next instruction from memory.
• DECODE- the processor works out what the binary code of the instruction means.
• EXECUTE- the processor carries out the instruction which may involve reading an item of data from memory, performing a calculation or writing data back into memory.
• The Control Unit- part of the processor, supervises the fetch-execute cycle. Ensures all the data being processed is routed correctly (to the correct register or RAM).
• ALU- carries out arithmetic and logic operations. It performs normal maths functions
• ALU is sent the opcode and the operand
• The Clock- ALL computers have an internal clock. It generates a signal used to synchronise the operation of the processor and the movement of data around the other components of the computer.

• Register- a small section of temporary storage that is part of / inside the processor. Stores data or controls instructions during the fetch-decode-execute cycle.
• They play a vital role in the operation of the computer.
• A register must be large enough to hold an instruction
• There are general purpose registers that can be used by a programmer when creating a program.

There are a number of special purpose registers that you have to learn:

• Status Register- keeps track of the various functions of the computer such as if the result of the last calculation was positive or negative
• Program Counter (PC)- Stores the memory address of the next instruction that will be needed by the processor
• Memory Address Register (MAR)- Stores the memory location of the address that data is either written to or copied from by the processor.
• Memory Buffer Register (MBR)- Also known as the memory data register (MDR). Holds data that has just been read from or is about to be written to main memory.

• Current Instruction Register (CIR)- Holds the instruction that the CPU is currently decoding/executing.

Interrupt Register

• Stores details of incoming interrupts
• These have been recieved by the processor from other components attatched to it
• This will be looked at more for A2.

• The PC holds the address of the next instruction. The processor sends this along the address bus to the main memory.
• The contents of the memory location at the address are sent via the data bus to the CIR and at the same time the PC is incremented
• The details of the addressess are initailly loaded into the MIR and the data initially goes into the MBR.

([X] = Contents of X)

MAR <- [PC]

PC <- [PC] + 1; MBR <- [Memory] Addressed

CIR <- [MBR]

[CIR] decoded and executed

• The processor then takes the instruction from the CIR and decides what to do with it
• It does this by referring to the instruction set
• This is a library of all the things a processor can be asked to do
• This set is described as either reduced or complex incstruction set (RISC or CISC)
• Once the instruction has been decoded the processor can carry out the instruction
• This is the execute bit and once it finished this goes to the fetch
• A simple instruction might need one clock cycle, a complex instruction more
• The results of these calculations are written either to a register or to a memory location

• Clock Speed- Measure of performance of the computer
• It indicates how fast each instruction will be executed
• In therory, increasing the clock speed increases the speed at which ther processor executes instructions
• But if other components within the processor are limited it will have little effect on the performance
• Bus Width- Increasing the width of the data bus means that more bits can be passed down with each clock pulse
• Address Bus- Increasing the width of the address bus increases the amount of memory that can be addressed and therefore allows more memory to be installed on a computer
• Word Length- Related to the data bus width. This is a collection of bits that can be addressed and manipulated as a single unit. Commonly 32 or 64 bits.
• Multiple Cores- Multiple Processors incorporated onto a single chip. Dual core and quad core are common. Core defines the components that enable instructions to be fetched and executed
• Cache Memory- Instructions and data that are needed frequently are placed in a temporary area of memory that is seperate from main memory. Cache can be accessed much quicker than main memory so programs run faster