Design Control Unit

Hard wired control unit

Delay Element method

Sequence counter method

Microprogrammed control unit

Hardwired and microprogrammed control unit

= 15th and 16th video, both combined ( less than 1.5hrs )

CONTROL UNIT DESIGN:

• Every instruction needs a collection of quick tasks, or micro-operations.

• The Control Unit produces control signals to carry out these Micro-Operations.

• Control signals are created on each T-state since each Micro-Operation needs a separate T-state.
• Several control signals are generated on various T-states for various instructions.

• The status flags determine which control signals should be generated in some conditional instructions.
• As a result, the instruction, the clock (T-state), and flags are the inputs to the control unit.

• Control signals are produced by the control unit.

• A Hardwired Control Unit is referred to if the control signals are generated by hardware.

• They are referred to as a Microprogrammed Control Unit if they are created by software, through a set of micro-instructions.

HARDWIRED CONTROL UNIT:

•  Here control signals are produced by hardware.
•  There are three types of Hardwired Control Units

1.STATE TABLE METHOD

1) This kind of hardwired control unit is the most fundamental.

2) A table called the state table is used to depict the activity of the control unit in this instance.

3) The columns show the commands, and the rows show the T-states.

4) In the associated T-state for each instruction, each intersection shows the control signal that will be generated.

5) For each instruction, a circuit is then built using each column of this table.

ADVANTAGE: 

It is the most straightforward way and works best with extremely small instruction sets.

DRAWBACK:

The circuit gets bigger and more complex as the number of instructions rises.

Due to the usage of a tabular technique rather than a logical approach (flowchart), numerous circuit parts appear multiple times in different instructions.

2.DELAY ELEMENT METHOD 

1) A flowchart is used to illustrate how the control unit will behave in this case.

2) The flowchart shows a control signal to be produced at each stage.

3) After completing each phase of a certain instruction, the entire instruction is carried out.

4) Control signals carry out Micro-Operations, which each call for a single T-state.

So, there needs to be a delay element in the flowchart between every pair of stages.

6) The delay must precisely correspond to one T-state. The D Flip-Flops are used to create this delay.

7) There are two D Flip-Flops placed between each pair of successive control signals.

8) Only one D Flip-Flop will be operational at any given moment. As a result, the technique is also known as “One Hot Technique”.

9) We utilise an OR gate in a multiple entry point system to combine two or more pathways.

10)A pair of two complementary AND gates are used in place of a decision box.

ADVANTAGE:

The method might lessen the complexity of the circuit because of its logical approach.

By reusing shared components across distinct instructions, this is accomplished.

DRAWBACK: 

When there are more instructions, there are more D Flip-Flops, which raises the price.

Furthermore, only one D Flip-Flop is truly in use at any given time.

3.SEQUENCE COUNTER METHOD

1) The most common type of hardwired control unit is this one.

2) It employs the same flowchart-based reasoning as the Delay element method, but without all the extra D Flip-Flops.

3) A flowchart illustrating the actions of a control unit is first created.

4) Using the same idea of AND and OR gates, it is then turned into a circuit.

5) Every time there are two consecutive control signals, there must be a one T-state (or one clock cycle) delay.

6) The aforementioned circuit accomplishes that.

7) We use a “mod k” and “k” output decoder if there are “k” separate stages creating the control signals.

8) At the commencement of the instruction, the counter will begin to run.

9) Each count will be created after one T-state thanks to the “clock” input provided by an AND gate.

10) The decoder receives the count, which causes the creation of “k” control signals, each with a 1 T-state delay.

11) The counter is reset at the end of the instruction so that the next time it runs, it starts at zero.

ADVANTAGE: 

Prevents the overuse of D Flip-Flops.

DRAWBACKS OF A HARDWIRED CONTROL UNIT IN GENERAL

1) Because they are dependent on hardware, the circuit gets increasingly complicated as the instruction set grows. Hardwired Control Unit creation is essentially difficult for current processors with hundreds of instructions.

2) It is quite tough to debug such big circuits.

3)Due to the rigidity of hardware design, the Control Unit must be completely redone whenever the processor is changed.

MICROPROGRAMMED CONTROL UNIT

WILKE’S DESIGN FOR A MICROPROGRAMMED CONTROL UNIT:

1) Using software and micro-instructions, a microprogrammed control unit generates control signals

2) A set of instructions makes up a programme.

3) A series of Micro-Operations are necessary for an instruction.

4) Control signals carry out Micro-Operations.

5) Micro-instructions are used in place of hardware to provide these control signals.

This implies that each instruction must be followed by a sequence of micro-instructions.

This is referred to as its micro-program.

8) In a tiny memory called the “Control Memory,” all instructions are microprogrammed.The processor contains the Control memory.

10) Take into account an Instruction that is loaded into the Instruction Register from the main memory (IR).

11) The processor utilises its distinctive “opcode” to determine the address of the initial microinstruction.

12) CMAR has that address loaded (Control Memory Address Register).

13) CMAR gives the decoder the address.

14) The decoder locates the appropriate micro-instruction in the Control Memory.

15) There are two fields in a micro-instruction: a control field and an address field.

16)Control field: This section specifies the control signals that will be generated.

Address field in ie: Displays the location of the following micro-instruction.

17) The next micro-instruction is fetched using this address, which is loaded into CMAR.

18) There are two address fields for a micro-instruction that is conditional.

19) This is so because the condition determines the address of the subsequent micro-instruction.

20) The proper control flag determines whether the condition is true or false.

21) Because FLASH ROM is readable but non-volatile, it is typically used to implement the control memory.

ADVANTAGES:

1) Flexibility is the main benefit.

2) The control unit can be changed in any way by just altering the micro-instruction.

This makes upgrading and changing the Control Unit relatively simple.

4) In addition, software is much easier to debug than a big Hardwired Control Unit.

DRAWBACKS:

1) The processor must have internal control memory, expanding its size.

2) This pushes up the processor’s price as well.

3) Every micro-address instruction’s field enlarges the control memory’s storage capacity. By properly sequencing the micro-instructions, this can be readily prevented.