4 - DesignofCLU Hardwireddesign TheopcodefieldofIR....

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Design of CLU - Hard wired design - Microprogrammed design
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HARDWIRED CONTROL UNIT
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 Hard-wired Control Unit - The opcode field of IR. This field is decoded to   provide the encoder information about   instruction being decoded - Signals from status and condition - Control step information   ( Step generator for T1, T2, ….) - External signals such as start, MFC,   interrupts etc.
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Control signal generator generates Individual control signals. Ex: Zin = T1 + T6.ADD  + T5. BR + …. IMPLEMENTED AS A COMBINATIONAL CIRCUIT .
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Combinational design could be using PLDs. Hard wired logic difficult to implement changes Provides faster execution. - Another approach  Microprogrammed control unit
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Microprogrammed Unit - Sequence of microinstructions corresponding   to each instruction is stored in ROM called   Control Memory - Called Microprogram - Provides flexibility of implementation - Less hardware
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  Micro instruction word  is the word whose  bits represent control signals Ex:  Y           R1       ; R1 out , Y in ,  Wait for MFC  Z           Y + MDR ; MDR out , ADD, Z in    R1        Z ; Z out , R1 in ,  END
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Step :R1in R1out Yin Zin  Zout  MDRout ADD WMFC   END 1        0          1       1      0       0          0         0          1         0 1        0          0        0      1       0          1         1         0         0 7         1          0        0       0      1          0         0         0         1
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Microprogramming Types Horizontal Microprogramming Vertical Microprogramming
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Horizontal Approach - 1 bit per control signal - Many control signals generated concurrently - Permitting very fast operation - Requires large control memory area
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Vertical Approach -  Most Micro instructions are mutually exclusive    and  never invoked simultaneously -  Possible to divide into groups and use few bits   to represent each group -   A Decoder then used to select a particular    Microoperation to be invoked
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Decoder
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4 - DesignofCLU Hardwireddesign TheopcodefieldofIR....

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