Control unit design(1)

Control unit design(1)

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  Control Unit Design Mohammad Nazir
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  The Control Unit –The control unit is responsible for generating control signals as well as the timing signals. – Hence the control unit is responsible for the synchronization of internal as well as external events. – By means of the control signals, the control unit instructs the data path what to do in every clock cycle during the execution of instructions.
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  Control Unit Design –Since the control unit performs quite complex tasks, its design must be done very carefully. – Most errors in processor design are in the Control Unit design phase. – There are primarily two approaches to design a control unit. 1. Hardwired approach 2. Micro programming
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  Hardwired approach – Hardwiredapproach is relatively faster, however, the final circuit is quite complex. – The micro-programmed implementation is usually slow, but it is much more flexible.
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  Finite-state machine – “Finite-statemachine” concepts are usually used to represent the CU. – Every state corresponds to one “clock cycle” i.e., 1 state per clock. In other words each timing step – could be considered as just 1 state and therefore from one timing step to other timing – step, the state would change
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  – The complexityof the control is a function of the following • Number of states • Number of inputs to the CU • Number of the outputs generated by the CU The complexity of the control unit
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  Hardwired Implementation of theControl Unit – The accompanying block diagram shows the inputs to the control unit. The output control signals generated from control unit to the various parts of the processor are also shown in the figure
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  Microprogrammed Control – Thereis 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 very instructions that specify microoperations.
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  Microprogrammed Control – Theconcept 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)
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  Difference:
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  Hardwired control unit
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  Microprogammed
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  Advantages & Disadvantages
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  Advantages & Disadvantages