Digital Systems - Control Unit Flashcards

Flashcards covering the control unit in digital systems.

What is the main function of the control unit in a processor?

To execute the fetch-decode-execute cycle endlessly.

What drives the control unit?

A clock that gives regular, timed electrical pulses or ‘ticks’.

Name three control lines used in the ALU and registers.

Latch, output enable, function select, shift L/R, and carry in.

What are the two special registers used by the control unit to control memory?

Memory Address Register (MAR) and Memory Data Register (MDR).

What is the function of the Instruction Register (IR)?

Holds the instruction currently being executed.

Briefly outline the steps in tracing an ADDA C instruction (adding constant C to Reg. A).

1. Read byte from memory pointed to by PC into IR. 2. Decode the instruction in IR. 3. Advance PC to point to operand byte. 4. Read operand byte from memory. 5. Add value to Register A. 6. Advance PC ready for next instruction. 7. Repeat.

What are the two ways to implement the control unit’s fetch-decode-execute loop?

As a finite state machine (FSM) or as microcode.

What is pipelining?

A way to exploit inherent parallelism inside the control unit to speed up the fetch-decode-execute cycle by splitting the cycle into sub-stages.

Name the three categories of pipeline hazards.

Control hazards, data hazards, and structural hazards.

What is the impact of pipeline hazards?

Such hazards may cause the pipeline to stall, reducing the speedup.

What is the role of Memory Address Register (MAR)?

Holds the memory address from which the processor will read or write data.

What is the role of Memory Data Register (MDR)?

Holds data going to or coming from the memory address in the MAR.