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# Lecture 20 - Th The University of Texas at Dallas Erik...

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Erik Jonsson School of Engineering and Th U i it f T t D ll Computer Science The University of Texas at Dallas The CPU Control Unit We now have a fairly good picture of the logic circuits in the CPU. Having “designed” the ALU or datapath so that it can perform the necessary instructions, we now have to do the same thing for the control unit which decodes the same thing for the control unit, which decodes instructions and provides direction to the CPU. The MIPS control unit decodes the six bits on either d f th 32 bit i t ti d th t i th d end of the 32-bit instruction word, that is, the op code and function code * fields, to determine each instruction sequence. 1 © N. B. Dodge 09/09 Lecture # 20: Control Unit Design and Multicycle Implementation * On R-R instructions.

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Erik Jonsson School of Engineering and Th U i it f T t D ll Computer Science The University of Texas at Dallas The Central Processor Unit (CPU) ALU Control Unit Registers In lecture 19, we covered processing elements (blue), i l di th i t ALU d d t b Instruction Fetch/Decode including the registers, ALU, and data buses. We now address the control unit circuitry (red). Instruction decoding. 2 © N. B. Dodge 09/09 Lecture # 20: Control Unit Design and Multicycle Implementation Control signals to ALU and other control elements.
Erik Jonsson School of Engineering and Th U i it f T t D ll Computer Science The University of Texas at Dallas Functionality of Control Unit 00 0000 10 0000 0 1000 0 1001 0 1010 0 0000 Op. code Shift amt. \$t0 \$t1 \$t2 Fn. code The control unit determines ALU functions in each instruction and selects operands for the ALU. Bit 0 Bit 31 The operation code (the left six bits of the instruction) determines the type of operation and in some cases (such j i t ti ) th t l i t ti it lf as jump instructions) the actual instruction itself. In the case of register-register instructions, the function code determines the instruction (for example, in the R/R 3 © N. B. Dodge 09/09 Lecture # 20: Control Unit Design and Multicycle Implementation determines the instruction (for example, in the R/R instruction above, the function code 0x 20 means “add”).

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Erik Jonsson School of Engineering and Th U i it f T t D ll Computer Science The University of Texas at Dallas Functionality of Control Unit (2) As mentioned before, the control unit is a 10 0000 Bit 0 To function code decoder the control unit is a collection of decoders and multiplexers . 1010 0 0000 To shift amount decoder To dest. reg. decoder The decoded instruction fields tell (1) the ALU what 000 0 1001 01 To source 2 reg. decoder To source 1 reg decoder function to perform, (2) what operands to use 00 0000 0 10 Bit 31 To operation code decoder To source 1 reg. decoder 4 © N. B. Dodge 09/09 Lecture # 20: Control Unit Design and Multicycle Implementation use.
Erik Jonsson School of Engineering and Th U i it f T t D ll Computer Science The University of Texas at Dallas Current Architecture The ALU control uses instruction bits 0-5 to obtain information about the ALU operation in register-to- i i i register instructions.

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