1.1 System Architecture
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Basic computer system model
Includes input, processing, output and storage
CPU - name and function
Central Processing Unit - Decodes and executes instructions and manages the rest of the hardware.
Input device
A device used to input information into the computer
Output device
A device that presents data from the computer
Storage device
A device on which information can be stored.
Von Neumann Architecture
Instructions are fetched, decoded and executed one at a time. Instructions and data are held together in the same memory space.
Bus
Paths/lines on the motherboard that data, electrical power and instructions move on.
Address Bus
Carries addresses from the processor to memory and input/output devices.
Data Bus
Sends data between the processor, memory and input/output devices.
Control Bus
Carries signals to co-ordinate computer activities.
CU
Control Unit - Co-ordinates activities taking place inside the CPU.
Functions of the Control Unit:
• Controls the execution of instructions in the right order. • Decodes the instructions. • Regulates and controls processor timing using regular pulses from the system clock. • Sends and receives control signals to and from other devices within the computer.
ALU
Arithmetic Log Unit. Carries out mathematical and logical operations.
ALU functions:
• Logical Operations: AND, OR, NOT. • Shift Operations: The bits in a binary number can be shifted to the left or the right a number of times. • Arithmetic Operations: +, -, x, /.
What are special Purpose Registers?
Special very fast memory locations within the CPU with specific purposes in the process of executing instructions.
MAR
Memory Address Register - Holds the address of data ready for use by the MDR, or the address of an instruction passed from the PC.
MDR
Memory Data Register - Holds data fetched from or to be written to memory.
PC
Program Counter - The register that contains the address of the next instruction to be executed. It is incremented for each instruction of the program being executed.
ACC
Accumulator - Holds the result of calculations.
FDE Cycle
Fetch-Decode-Execute. The CPU uses this cycle to carry out program instructions.
Fetch
• The contents of the Program Counter are put into the MAR. • The address is transferred along the address bus to main memory.
Decode
The CU decodes the instruction in the CIR to see what is to be done next.
Execute
The instruction is then executed.
Factors affecting the speed of the CPU.
• Clock Speed • Cache size • Number of cores