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COMP541 Datapaths II

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COMP541 Datapaths II

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    1. 1 COMP541 Datapaths II Montek Singh Mar 22, 2007

    2. 2 Topics Continue datapath Inside ALU Addition and Subtraction Implementing add/sub in ALU Shifting

    3. 3 Datapath Fairly complete datapath Arithmetic Logic Shifter One bit, R or L

    4. 4 ALU This one has 2 control lines for arithmetic S2 selects logical ops

    5. 5 Addition and Subtraction Lets take side trip into how these are done at low level Then come back to look at detailed design of ALU

    6. 6 Looking Inside Cin also an input

    7. 7 Design of B Select Logic Just straightforward truth table

    8. 8 4-Bit Circuit Weve seen similar before

    9. 9 Now Add Logic Section Mux to choose which

    10. 10 Logic Section Design Generous number of operations

    11. 11 Resulting Control

    12. 12 Shifter: First Thought Make a parallel-load, bidirectional shift register Anything wrong with that?

    13. 13 Takes Three Clocks First clock loads shift reg Second makes the shift Third loads to destination reg Any alternatives?

    14. 14 Mux as Shift Register Combinational no clock

    15. 15 Is This Better? Three short clocks may be faster than one limited by gate delays But complex control We have long ALU delay, so maybe this doesnt matter Our one-clock shifter becomes multi instruction to shift n (>1) bits

    16. 16 Shifting More Than 1 Bit Barrel shifter

    17. 17 Impractical As-Is When n gets larger, this becomes huge Can use levels of muxes Similar to carry-lookahead adder Can design with transistors

    18. 18 Datapath Higher-level view for hierarchical design Can replace modules with same interface but different implementation

    19. 19 Register File Can be fairly large (32 registers) Memory with three addresses D for write A and B for read What are Inputs Outputs

    20. 20 Function Unit Arithmetic and shifter Can be bypassed at output

    21. 21 Control Lines Control inputs have been blended together into one control word (see next slide)

    22. 22 Operations of Func. Unit

    23. 23 Particular 8-bit Datapath For examples to follow Note that all control lines now have sizes and names Look at each in next slide

    24. 24 Control Word Collect all lines in a control word for convenience Has 7 fields

    25. 25 Table of Fields (for reference)

    26. 26 Microoperation Microoperation Maps to

    27. 27 Other Examples

    28. 28 Using Numeric Notation

    29. 29 Have Simple Machine With part of an instruction set No branches, for example

    30. 30 Timing Note that regs are latched on next clock

    31. 31 Next Time Either look at control or specifically at how to add memory to your MIPS design, and implement load and store

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