1. Processor Organization and Architecture
Module III
Processor Organization and Architecture

2. Block Diagram of Control Unit

3. Inputs to Control Unit Clock Instruction Register Flags
One micro-operation (or set of parallel micro-operations) per clock cycle
Instruction Register
Op-code and addressing mode of current instruction to determine which micro-operations need to be performed
Flags
Status of CPU and outcome of previous ALU operations
Control Signals from control bus
Signals to control unit like Interrupts and Acknowledgements

4. Outputs from Control Unit
Control Signals within CPU
Cause data movement between registers
Activate specific ALU functions
Control Signals to control bus
To memory
To I/O modules

5. Control Signal Example

6. Control Signal Example
It has single accumulator(AC).
Data paths are shown
Termination of Control signals are shown Ci and indicated by a circle
With each clock cycle, CU reads all its input and emit a set of control signals

8. Control Unit Implementation
Control unit implementations fall into one of two categories:
• Hardwired implementation
• Microprogrammed implementation

9. Hardwired implementation
In this, the control unit is essentially a state machine circuit.
The input logic signals are transformed into a set of output logic signals, which are the control signals
It uses a fixed logic circuits to generate control signals

10. Hardwired Control Unit

11. Hardwired Control Unit
Instruction Register
Uses opcodes to perform different actions for different instruction
Decoder
There should be unique logic input for each opcode
It is done by decoder (n-bits  2n outputs ,each will activate a single unique output)
Decoder becomes complex for variable length opcodes

12. Hardwired Control Unit
Clock
It issues repetitive pulses to execute micro-operations.
Timing generator
At the end of each subcycle, the timing generator is reinitialized to generate new T1
Control Unit Logic
It defines how it produces output control signals as function of input signals
For each control signal , a boolean expression is derived

13. Control Logic Example
Let us consider a single control signal C5 (It causes data to be read into MBR)

14. Control Logic Example Let us define two control signals P and Q as
PQ = 00  Fetch
PQ = 01  Indirect
PQ = 10  Execute
PQ = 11  Interrupt
Then C5 may be defined as

15. Control Logic Example C5 is also needed during the execute cycle.
For example, let us assume that there are only 3 instructions that read from memory: LDA,ADD, and AND. Now C5 is defined as
This same process is to be done for every control signal.

16. Hardwired Control Unit
Modern processors result in large number of boolean expressions and implementing combinatorial circuits are extremely difficult.
Hence simpler microprogramming approach is used

17. Advantages of Hardwired Control Unit
Works faster as combinational circuits generates control signals based on input status.

18. Disadvantages of Hardwired Control Unit
Complex in design if it requires larger number of control points
No flexibility : difficult to make corrections or add a new feature
Uses too many logic gates.

19. Micro-programmed Control Unit

20. Micro-programmed Control
Microprogramming is a method of control unit design in which the control signal selection and the sequencing information is stored in a ROM or RAM called control memory.
A sequence of microinstructions designed to control a specific instruction is called a microprogram.

21. Micro-programmed Control
For each micro-operation, control unit have to generate a set of control signals
For any micro-operation any control line may be on/off which can be represented by a binary pattern.
Thus a control word is constructed for each micro-operation.

22. Microprogrammed Control
The control information is stored in a control memory, and the control memory is programmed to initiate the required sequence of microoperations
Adv: Any required change can be done by updating the microprogram in control memory
Disadv: Slow operation