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
– 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
Adder
full adder from a previous lecture
5
Adder/Subtractor
Textbook ALU
Our Target ALU, N=32
The F input is controlled by the processor based on the op code
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
An Implementation
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From Harris Text: a simple ALU
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Test Cases
Test the Implementation
AA
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simple
testbench
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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
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From Harris Text: an enhanced ALU
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A Better Testbench
• Improved Testbench
• Data file