ALU, CU, Registers, and Buses

For OCR A-Level Computer Science, SLR1

What is the role of the Control Unit

• Coordinates activities of the CPU

• Directs flow of data between CPU and other devices

• Accepts next instruction, decodes it, handles its execution.

• Sends read/write requests to main memory on control bus

What is the role of the Program Counter?

• Hold address of the next instruction to be executed

• At the start of every FDE cycle, address stored in PC is copied to the MAR

What is the role if the Memory Address Register?

• Holds address of the memory location from which data / instruction fetched or data written.

• Addresses get sent to memory down the address bus

What is the role of the Memory Data Register?

• Temporarily store data which is read from / written to memory

• All data to and from the MDR travels down the data bus and through MDR

What is the role of the Current Instruction Register?

• Holds current instruction being executed

• Contents of MDR are copied to CIR if it’s an instruction

• Contains opcode and operand(s) of current instruction

What is the role of the Accumulator?

• Temporarily stores results of calculations carried out by the ALU

What is the role of the Arithmetic Logic Unit?

• Performs arithmetic and logical operations on data

• Bitwise shift operations left & right

What does the Address Bus carry?

• Memory address that identify where the data is being red from / written to

• Unidirectional

What does the Data Bus carry?

• Binary 1s / 0s that make up the actual information being transmitted around the CPU

• Bidirectional

What does the Control Bus carry?

• Command and control signals to and from every component of the CPU

• Bidirectional

Why are registers used instead of always accessing RAM?

Registers are much faster to access than main memory

How do more registers improve CPU performance?

Reduce the need to access slower main memory