Designing an ALU

Taken from various sourcesPrimary 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

– Even the simplest microprocessors contain one for purposes such as maintaining timers

– A combinational logic circuit

Complex ALU

Simple ALU

The S input is controlled by the processor based on the op code

5

full adder from a previous lecture

Adder

Adder/Subtractor

Textbook ALU

Our Target ALU, N=32

The F input is controlled by the processor based on the op code

Possible Implementationmodule 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

An Implementation

From Harris Text: a simple ALUA cover a cover a cover a cover a cover a cover a cover a cover

Test Cases

Test the Implementation

A simple testbenchA cover a cover a cover a cover a cover a cover a cover a cover

ALU Enhancements

1. Add a Zero output to the ALU. The output is TRUE when Y==0, otherwise it is FALSE.

1. Add and Overflow output to the ALU. The output is TRUE when the result of the adder overflows, otherwise it is FALSE.

An Implementation

From Harris Text: an enhanced ALUA cover a cover a cover a cover a cover a cover a cover a cover

A Better Testbench

• Improved Testbench• Data file