The Processor
Central Processing Unit
Similar to a human brain, (although it cannot think or make decisions for itself) a computer processor controls, calculates and executes instructions.
A processor has a number of different components, each with their own role to perform:
Arithmetic-Logic Unit (ALU)
Control Unit
Clock
General purpose registers
Dedicated registers
Arithmetic-Logic Unit
This part of the processor is responsible for the problem solving. It can perform arithmetic, logical and shift operations on data.
Add, Subtract, Multiply, Divide
Control Unit
This part coordinates the activity of all other components. It also synchronises all steps of the fetch, decode and execute cycle with a regular pulse from the system clock.
The System Clock
A series of regular ON/OFF signals are used to synchronise the operations of the processor's components.
Actions are usually carried out on the rising edge of the clock. Each action takes a fixed number of cycles to complete.
General Purpose Registers
The results produced by the ALU need to be stored somewhere, rather than writing data back to the slow memory (RAM), the processor use inbuilt fast memory locations called registers to temporarily store data. This allows the processor to quickly access and reuse the data for subsequent calculations (e.g. 2+3+4). Some registers have a single general purpose register called the accumulator.
Dedicated Registers
Program Counter: Holds address of the next instruction to be executed.
Current Instruction Register: Holds the current instruction
Memory Address Register: Holds address of instruction or data which is to be fetched or stored.
Memory Buffer Register: Holds data and instructions moving between the processor and main memory.
Status Register: Holds info about current state of operations. It is used to set flags (e.g. overflow) and detect error conditions.
The Processor
Fetch Decode Execute Cycle
Processors operate in defined stages that are used to carry out program instructions. The process is repeated over and over again for each instruction in the program.
Fetch (steps 1 – 4)
Decode (steps 5 – 7)
Execute (Steps 8 – 10)
Factors affecting performance
The Fetch-Execute cycle is triggered from the clock pulses of the system clock. The faster the clock speed, the faster a computer can fetch, decode and execute instructions. A 4GHz processor would tick 4 billion times per second. However, increasing the clock speed to a very large number may make the processor emit a lot of heat, and it may also get damaged.
Cache Memory
Cache is a small amount of superfast memory that stores data that is frequently used by the processor. It is larger and slower than a register, faster and smaller than RAM. Having larger amounts of cache memory can improve processing speed.
Other Factors
Number of Cores: More processors can be linked on a single chip, thus allowing more instructions to be executed simultaneously
Word Length: Allowing large word length would allow the processor to process simultaneously.
Address and Data Bus width: This will allow more data to be transferred at once and access to more memory locations.
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