3: Components of a CPU
The Central Processing Unit (CPU)
ALU: Arithmetic and Logic Unit
does the arithmetic calculations and places the result in the accumulator
MAR: Memory Address Register
sorts out all the internal paths needed inside the microprocessor to make sure that details fetched from the right place are taken to the right place
instructs the ALU which logical operations to be performed
controls the Fetch-Decode-Execute sequence
PCR: Program Counter Register
holds the address of the next piece of data/instruction that needs to be fetched from the main memory
when the address of the current instruction has been dealt with, the program counter is incremented by 1, so the processor is ready to fetch the next instruction
CU: Control Unit
IR: Instruction Register
the binary digits of the most recently fetched instruction are held here so that the instruction decoder in the Control Unit can decode them
MDR: Memory Data Register
holds the address of the instruction (or data) currently being accessed
Address Bus
Data Bus
Main Memory
ROM and RAM
Contains programs and data currently in use.
How programs are executed by the CPU
The Fetch Execute Cycle
1: Fetch
2: Decode
3: Execute (and back to 1)
Stored Program Concept (aka Von Neumann architecture): a computer with this stores program data and instruction data in the same memory
Register
A special high-speed storage area within the CPU. All data must be represented in a register before it can be processed: MAR MDR IR PC A
Accumulator
Used to accumulate results. Where answers from operations are temporarily stored before being put out to the computer’s memory.