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    output[0:3] result;
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    output[0:3] result;
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    output cout;
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    output cout;
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    wire bit3_cout, bit2_cout, bit1_cout;
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    wire bit3_cout, bit2_cout, bit1_cout;
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    /* Link 4 1-bit ALU's together to make a 4-bit ALU.
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     * Connect from LSB to MSB so the ripple-carry works correctly. */
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    ALU1 bit3 (a[3], b[3], cin, op, bit3_cout, result[3]);
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    ALU1 bit3 (a[3], b[3], cin, op, bit3_cout, result[3]);
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    ALU1 bit2 (a[2], b[2], bit3_cout, op, bit2_cout, result[2]);
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    ALU1 bit2 (a[2], b[2], bit3_cout, op, bit2_cout, result[2]);
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    ALU1 bit1 (a[1], b[1], bit2_cout, op, bit1_cout, result[1]);
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    ALU1 bit1 (a[1], b[1], bit2_cout, op, bit1_cout, result[1]);
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    ALU1 bit0 (a[0], b[0], bit1_cout, op, cout, result[0]);
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    ALU1 bit0 (a[0], b[0], bit1_cout, op, cout, result[0]);
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endmodule
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endmodule
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                $time, x, y, op, cin, cout, result);
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                $time, x, y, op, cin, cout, result);
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    end
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    end
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    initial begin
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    initial begin
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           x='b0000; y='b0000; op='b00; cin=0; $display ("\nAND");
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        #1 x='b0001; y='b0000; op='b00; cin=0;
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           x='b0101; y='b1011; op='b00; cin=0;
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        #1 x='b0010; y='b0000; op='b00; cin=0;
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        #1 x='b0101; y='b1011; op='b01; cin=0;
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        #1 x='b0011; y='b0000; op='b00; cin=0;
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        #1 x='b0101; y='b1011; op='b10; cin=0;
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        #1 x='b0011; y='b1100; op='b00; cin=0;
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        #1 x='b1101; y='b0011; op='b00; cin=0;
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        #1 x='b1010; y='b0101; op='b00; cin=0;
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        #1 x='b1010; y='b0011; op='b01; cin=0;
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        #1 x='b1010; y='b1010; op='b00; cin=0;
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        #1 x='b0101; y='b0011; op='b10; cin=0;
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        #1 x='b0101; y='b0101; op='b00; cin=0;
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        #1 x='b1010; y='b1001; op='b10; cin=0;
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        #1 x='b0001; y='b0000; op='b01; cin=0; $display ("\nOR");
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        #1 x='b1001; y='b0110; op='b01; cin=0;
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        #1 x='b0001; y='b0001; op='b10; cin=0; $display ("\nADD");
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        #1 x='b0011; y='b0011; op='b10; cin=0;
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        #1 x='b0011; y='b0001; op='b11; cin=0; $display ("\nSUB");
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    end
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    end
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    ALU4 alu4 (x, y, cin, op, cout, result);
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    ALU4 alu4 (x, y, cin, op, cout, result);
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endmodule
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endmodule