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Control Unit Design
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Control Unit Design Approaches
Hardwired: - The control logic is wired into logic circuits Microprogrammed - Here the control logic is coded in to a low level program
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Hardwired Control Unit
CLK Control Step Counter Reset Clock Step Decoder T0 T1 T2 Tn IR Instruction Decoder INS1 Encoder Interrupts INS2 Condition Codes INS3 EOI Control Signals
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Hardwired Control Unit
The Control Signals issued by the Control Unit are generated by an Encoder module which is a combinational circuit. The instructions, addressing modes, clock cycle no. during the instruction cycle etc. given as inputs to the Encoder module. Each control signal is modeled as a Boolean function of the inputs to the encoder unit. Each of the Boolean functions representing the control signals is implemented using a combinational logic circuit.
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Control Signal Generation
Example Boolean Functions for Control Signals: Zin = T0 + add. (T3 + T6 + T9) + mov. T MDRout = T2 + add. ( T4 + T8 + T9) + mov. T add T0: PCout, MARin, Read, Select1, Add, Zin T1: Zout, PCin, WMFC T2: MDRout, IRin T3: R0out, MARin, Read, Select1, Add, Zin T4: Zout, R0in, WMFC T5: MDRout, Yin T6: R1out, MARin, Read, Select1, Add, Zin T7: Zout, R1in, WMFC T8: MDRout, MARin, Read, WMFC T9: MDRout, Add, Zin T10: Zout, MDRin, Write, WMFC Example Control Signal word Sequences: mov T0: PCout, MARin, Read, Select1, Add, Zin T1: Zout, PCin, WMFC T2: MDRout, IRin T3: R1out, MARin, Read, Select1, Add, Zin T4: Zout, R1in, WMFC T5: MDRout, MARin T6: R0out, MDRin, Write, WMFC
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Microprogramming Micro-order: A control signal issued by the control unit e.g. PCin, PCout, WMFC etc. Micro-operation: The micro level operations carried out through issuing of one or more micro-orders. e.g. transfer of data between a pair of registers, a memory read operation etc. Micro-instruction: Set of one or more micro-operations carried out within one clock step in a processor. Micro-routine: A sequence of micro-instructions to carryout a task in a processor. Micro-program: A collection of micro-routines to achieve an Instruction set architecture.
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Microprogram Control Word
Each control word represents a micro-instruction Consists of a bit string with one bit for each micro-order in a micro-programmed processor. The bit value for a micro-order in a micro-instruction is set to ‘1’ if the corresponding micro-order is required to be issued in that micro-instruction. Otherwise it is set to ‘0’. 1 1 1 1
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Microprogrammed Control Unit
IR Starting and Branch Address Generator Interrupts Condition Codes EOF, EOS, EOD Micro-program Counter Clock EOI Control Store Control Word
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Reducing Microinstruction Length
IR Starting and Branch Address Generator Interrupts Condition Codes EOF, EOS, EOD Micro-program Counter Clock EOI Decoder Group Mutually exclusive Micro-Orders and encode them into fields in the Microinstruction. e.g. ALU micro-orders, EOX, Rnout etc. Decode them while issuing. Decoder Control Store Decoder Decoder
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