Explain the function of wilkis control with an example
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There are primarily two different approaches for designing a control unit:
a. Hardwired control unit
b. Microprogrammed control unit
In hardwired control unit, the control units use fixed logic circuits to interpret instructions and generate control signals from them. A controller that uses this approach can operate at high speed; however, it has little flexibility, and the complexity of the instruction set it can implement is limited. The hardwired approach has become less popular as computers have evolved.Figure 5 below shows a detailed block diagram of a hardwired control unit:
Here, the fixed combinational circuits namely the encoder and decoder generate the necessary control signals as shown above.
The decoder decodes the instructions that are loaded in the IR (Instruction Register).Accordingly the number of output lines of the decoder depends on the size of the IR. For example if the IR is 10 bit then the number of output lines of the decoder will have 210 or 1024 lines one for each possible instruction.According to the code in the IR only one of the possible 1024 lines will be set to 1, all the others will be 0 only.The counter is used to keep track of the control steps. Each state or count of this counter corresponds to one control step. The step decoder provides a separate signal line for each step, or time slot in a control sequence.
The encoder takes inputs from instruction decoder, step decoder, external inputs and condition codes as shown in the diagram. The required control signals are thus determined by the following:
a. Contents of the control step counter
b. Contents of the instruction register
c. Contents of the condition code flags
External input signals can be from an external interrupting source or a device requesting access to an external bus. Conditions codes are as per condition and status flags set in earlier instructions.The input signals to the encoder block in Figure are combined to generate the individual control signals Y in, PC OUT Add, End, and so on. The output of the control unit is responsible for indicating the function that needs to be performed (like addition), selection of the storage units and for selection of data path/data route.If the logic function below is responsible for the generation of the END control signal End = T7 . Add + T6.BR + (T5.N + T4. N¯). BRN + ...End = T7 . Add + T6.BR + (T5.N + T4. N¯). BRN + ...
a. Hardwired control unit
b. Microprogrammed control unit
In hardwired control unit, the control units use fixed logic circuits to interpret instructions and generate control signals from them. A controller that uses this approach can operate at high speed; however, it has little flexibility, and the complexity of the instruction set it can implement is limited. The hardwired approach has become less popular as computers have evolved.Figure 5 below shows a detailed block diagram of a hardwired control unit:
Here, the fixed combinational circuits namely the encoder and decoder generate the necessary control signals as shown above.
The decoder decodes the instructions that are loaded in the IR (Instruction Register).Accordingly the number of output lines of the decoder depends on the size of the IR. For example if the IR is 10 bit then the number of output lines of the decoder will have 210 or 1024 lines one for each possible instruction.According to the code in the IR only one of the possible 1024 lines will be set to 1, all the others will be 0 only.The counter is used to keep track of the control steps. Each state or count of this counter corresponds to one control step. The step decoder provides a separate signal line for each step, or time slot in a control sequence.
The encoder takes inputs from instruction decoder, step decoder, external inputs and condition codes as shown in the diagram. The required control signals are thus determined by the following:
a. Contents of the control step counter
b. Contents of the instruction register
c. Contents of the condition code flags
External input signals can be from an external interrupting source or a device requesting access to an external bus. Conditions codes are as per condition and status flags set in earlier instructions.The input signals to the encoder block in Figure are combined to generate the individual control signals Y in, PC OUT Add, End, and so on. The output of the control unit is responsible for indicating the function that needs to be performed (like addition), selection of the storage units and for selection of data path/data route.If the logic function below is responsible for the generation of the END control signal End = T7 . Add + T6.BR + (T5.N + T4. N¯). BRN + ...End = T7 . Add + T6.BR + (T5.N + T4. N¯). BRN + ...
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