Components of a Computer
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Arithmetic Logic Unit
Carries out calculations and logic (Comparisons of binary)
Control Unit
Control Unit is a register in the CPU that controls and co-ordinates the activity of the CPU. Control signals are sent along the control bus between the control unit and the other components of the computer.
Registers
Registers are small storage locations used to hold data temporarily. They have high read and write speeds.
Program Counter
stores the address of the next instruction
Memory Address register (MAR)
stores the address of instructions and data that need to be fetched from memory
Memory Data register (MDR)
stores the data which is to be sent or fetched from memory
Current instruction register
stores the actual instruction that is being decoded
Accumulator
stores the result of calculations made by the ALU
Interrupt register
generates and detects interrupts
Buses
the communication channels between the CPU and the memory and other components.
Address Bus
A one directional bus that transmits memory addresses that are used as operands in programming instructions, so that data can be retrieved from main memory
Data Bus
A bi-directional path for moving data and instructions between system components
Control Bus
A bi-directional bus to transmit command, timing and specific status information between system components
system bus
collective term for address, data and control bus
fetch decode execute cycle
PC MAR Address Bus Memory Data bus MDR CIR Decode Unit
Fetch
• The address of the next instruction is copied from the PC to the MAR • The fetch signal is sent across the control bus. The content of the MAR are transferred across the address bus. • The contents of the memory location stored in the MAR are sent across the data bus and stored in the CIR • the PC increments by 1
Decode
The contents of the CIR are sent to the CU and divided. The Opcode tells you the instruction to be carried out and the operand stores the address of any data which might be needed, the operand is sent to the MAR and the data is fetched from memory and stored in the MDR
Execute
The appropriate instruction/ opcode is carried out on the data/ operand
program branch reason
occurs due to an if statement, function, procedure call or loop.
program branch result
the next instruction held in the PC is not carried out
Clock speed
Indicates the number of instructions the CPU can process per second.
Clock speed Unit
Hertz (usually gigahertz)
Positives of increasing clock speed
you can carry out more instructions in a given time; improved performance
Negatives of increasing clock speed
more heat generated meaning computers must be cooled (either with heat sync/fan or water/oil cooling)