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Design of Control Unit
To execute an instruction, the control unit of the CPU must generate the required control signal in the proper sequence. As for example, during the fetch phase, CPU has to generate PCout signal along with other required signal in the first clock pulse. In the second clock pulse CPU has to generate PCin signal along with other required signals. So, during fetch phase, the proper sequence for generating the signal to retrieve from and store to PC is PCout and PCin. To generate the control signal in proper sequence, a wide variety of techniques exist. Most of these techniques, howeve, fall into one of the two categories,
1. Hardwired Control
2. Microprogrammed Control.
Hardwired Control
In this hardwired control techniques, the control signals are generated by means of hardwired circuit. The main objective of control unit is to generate the control signal in proper sequence. Consider the sequence of control signal required to execute the ADD instruction that is explained in previous lecture. It is obvious that eight non-overlapping time slots are required for proper execution of the instruction represented by this sequence. Each time slot must be at least long enough for the function specified in the corresponding step to be completed. Since, the control unit is implemented by hardwire device and every device is having a propagation delay, due to which it requires some time to get the stable output signal at the output port after giving the input signal. So, to find out the time slot is a complicated design task. For the moment, for simplicity, let us assume that all slots are equal in time duration. Therefore the required controller may be implemented based upon the use of a counter driven by a clock. Each state, or count, of this counter corresponds to one of the steps of the control sequence of the instructions of the CPU. In the previous lecture, we have mentioned control sequence for execution of two instructions only (one is for add and other one is for branch). Like that we need
to design the control sequence of all the instructions.
Microprogrammed Control
In hardwired control, we saw how all the control signals required inside the CPU can be generated using a state counter and a PLA circuit. There is an alternative approach by which the control signals required inside the CPU can be generated . This alternative approach is known as microprogrammed control unit. In microprogrammed control unit, the logic of the control unit is specified by a microprogram. A microprogram consists of a sequence of instructions in a microprogramming language. These are instructions that specify microoperations. A microprogrammed control unit is a relatively simple logic circuit that is capable of (1) sequencing through microinstructions and (2) generating control signals to execute each microinstruction. The concept of microprogram is similar to computer program. In computer program the complete instructions of the program is stored in main memory and during execution it fetches the instructions from main memory one after another. The sequence of instruction fetch is controlled by program counter (PC) . Microprogram are stored in microprogram memory and the execution is controlled by microprogram counter ( PC).
To execute an instruction, the control unit of the CPU must generate the required control signal in the proper sequence. As for example, during the fetch phase, CPU has to generate PCout signal along with other required signal in the first clock pulse. In the second clock pulse CPU has to generate PCin signal along with other required signals. So, during fetch phase, the proper sequence for generating the signal to retrieve from and store to PC is PCout and PCin. To generate the control signal in proper sequence, a wide variety of techniques exist. Most of these techniques, howeve, fall into one of the two categories,
1. Hardwired Control
2. Microprogrammed Control.
Hardwired Control
In this hardwired control techniques, the control signals are generated by means of hardwired circuit. The main objective of control unit is to generate the control signal in proper sequence. Consider the sequence of control signal required to execute the ADD instruction that is explained in previous lecture. It is obvious that eight non-overlapping time slots are required for proper execution of the instruction represented by this sequence. Each time slot must be at least long enough for the function specified in the corresponding step to be completed. Since, the control unit is implemented by hardwire device and every device is having a propagation delay, due to which it requires some time to get the stable output signal at the output port after giving the input signal. So, to find out the time slot is a complicated design task. For the moment, for simplicity, let us assume that all slots are equal in time duration. Therefore the required controller may be implemented based upon the use of a counter driven by a clock. Each state, or count, of this counter corresponds to one of the steps of the control sequence of the instructions of the CPU. In the previous lecture, we have mentioned control sequence for execution of two instructions only (one is for add and other one is for branch). Like that we need
to design the control sequence of all the instructions.
Microprogrammed Control
In hardwired control, we saw how all the control signals required inside the CPU can be generated using a state counter and a PLA circuit. There is an alternative approach by which the control signals required inside the CPU can be generated . This alternative approach is known as microprogrammed control unit. In microprogrammed control unit, the logic of the control unit is specified by a microprogram. A microprogram consists of a sequence of instructions in a microprogramming language. These are instructions that specify microoperations. A microprogrammed control unit is a relatively simple logic circuit that is capable of (1) sequencing through microinstructions and (2) generating control signals to execute each microinstruction. The concept of microprogram is similar to computer program. In computer program the complete instructions of the program is stored in main memory and during execution it fetches the instructions from main memory one after another. The sequence of instruction fetch is controlled by program counter (PC) . Microprogram are stored in microprogram memory and the execution is controlled by microprogram counter ( PC).
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