1. Types of Micro-operation  Transfer data between registers  Transfer data from register to external  Transfer data from external to register  Perform arithmetic or logical operations, using registers for input and outputs

2. Functions of Control Unit  Sequencing Causing the CPU to step through a series of micro-operations  Execution Causing the performance of each micro- operation This is done using Control Signals

3. Model of Control Unit

4. Control Unit - input  Clock This is how the control unit “keeps time” One micro-operation (or set of parallel micro- operations) per clock cycle (referred to as the processor cycle time)  Instruction register Op-code and addressing mode of current instruction determine which micro-operations to be performed  Flags State of CPU -Results of previous operations  From control bus Interrupts -Acknowledgements

5. Control Unit - output  Within CPU Cause data movement Activate specific ALU functions  To control bus To memory To I/O modules

6. Example Control Signal Sequence - Fetch  MAR ← (PC) Control signal to open gates between PC and MAR  MBR ← (memory) Open gates between MAR and address bus Memory read control signal Open gates between data bus and MBR  Control signal to logic that add 1 to the contents of the PC  IR ← (MBR) opens gates between the MBR and the IR

7. Data Paths and Control Signals

8. Example

9. CPU with Internal Bus

10. Control unit implementation  Hardwired implementation the control unit is essentially a state machine circuit. Its input logic signals are transformed into a set of output logic signals, which are the control signals  Microprogrammed implementation the logic of the control unit is specified by a microprogram. A microprogram consists of a sequence of instructions in a microprogramming language

11. Control Unit with Decoded Inputs

12. Hardwired Implementation 1  Flags and control bus Each bit means something  Instruction register Op-code causes different control signals for each different instruction Unique logic for each op-code Decoder takes encoded input and produces single output

13. Hardwired Implementation 2  Clock Repetitive sequence of pulses Useful for measuring duration of micro-ops Must be long enough to allow signal propagation Different control signals at different times within instruction cycle Need a counter with different control signals for t1, t2 etc.

14. Example  Let us consider a single control signal, C5. This signal causes data to be read from the external data bus into the MBR.  Then the following Boolean expression defines C5: C5 = P’Q’ t2 + P’Q t2

15. Problems With Hardwired Designs  Complex sequencing & micro-operation logic  Difficult to design and test  Inflexible design due to the difficulty of adding new instructions

16. Micro-programmed Control  A sequence of instructions is known as a microprogram or firmware  A set of micro-operations occurring at one time is known as a micro- instruction  Use sequences of micro-instructions to control complex operations of control unit

17. Implementation  All the control unit does is generate a set of control signals  Each control signal is on or off  Represent each control signal by a bit  Have a control word for each micro-op  Have a sequence of control words for each machine code instruction  Add an address to specify the next micro- instruction, depending on conditions

18. Typical Microinstruction Formats

19. Microinstruction operation  To execute this microinstruction, turn on all the control lines indicated by a 1 bit; leave off all control lines indicated by a 0 bit.  The resulting control signals will cause one or more micro-operations to be performed.  If the condition indicated by the condition bits is false, execute the next microinstruction in sequence.  Else the next microinstruction to be executed is indicated in the address field.

20. Organization of Control Memory

21. Control Unit Microarchitecture

22. Control Unit Organization

23. Control Unit Function  Sequence logic unit issues read command  Word specified in control address register is read into control buffer register  Control buffer register contents generates control signals and next address information  Sequence logic loads new address into control buffer register based on next address information from control buffer register and ALU flags

24. Advantages and Disadvantages of Microprogramming  Simplifies design of control unit Cheaper Less error-prone  Slower

25. RQ: 15.1, 15.2, 15.5, 15.6, 15.7, 15.8 P: 15.2, 15.3, 15.4 RQ: 16.1, 16.2, 16.3, 16.4, 16.6