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Designing an ALU

Designing an ALU. Taken from various sources Primary source: Digital Design and Computer Architecture by Harris &Harris. ALU. An arithmetic logic unit ( ALU ) Performs arithmetic and logic operations A fundamental building block of the Central Processing Unit (CPU) of a computer

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Designing an ALU

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  1. Designing an ALU Taken from various sources Primary source: Digital Design and Computer Architecture by Harris &Harris

  2. ALU • An arithmetic logic unit (ALU) • Performs arithmetic and logic operations • A fundamental building block of the Central Processing Unit (CPU) of a computer • Even the simplest microprocessors contain one for purposes such as maintaining timers • A combinational logic circuit

  3. Complex ALU

  4. Simple ALU The S input is controlled by the processor based on the op code

  5. Adder full adder from a previous lecture

  6. Adder/Subtractor

  7. Textbook ALU

  8. Our Target ALU, N=32 The F input is controlled by the processor based on the op code

  9. Possible Implementation module alu (input [31:0] A, B, input [2:0] F, output reg [31:0] Y); // implement everything using a case statement??? always @ (*) case (..) … endcase endmodule

  10. An Implementation From Harris Text: a simple ALU A cover a cover a cover a cover a cover a cover a cover a cover

  11. Test Cases

  12. Test the Implementation A simple testbench A cover a cover a cover a cover a cover a cover a cover a cover

  13. ALU Enhancements • Add a Zero output to the ALU. The output is TRUE when Y==0, otherwise it is FALSE. • Add and Overflow output to the ALU. The output is TRUE when the result of the adder overflows, otherwise it is FALSE.

  14. An Implementation From Harris Text: an enhanced ALU A cover a cover a cover a cover a cover a cover a cover a cover

  15. A Better Testbench • Improved Testbench • Data file

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