I don't understand why "a [2]" to "a [7]" are not "No output dependent on input pin"

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The following Verilog source code was logically synthesized with Quartus II 13.0 sp1.

module dmem (input clk, we,
            input [31: 0] a, wd,
            output [31: 0] rd);

  reg [31: 0] RAM [63: 0];

  assign rd = RAM [a [31: 2]]; // word aligned

  always @ (posedge clk)
    if (we)
      RAM [a [31: 2]] [25] <= wd;
endmodule

Then, the following warning was issued. In addition, logic synthesis was successful.

Warning (21074): Design contains 26 input pin (s) that do not drive logic
Warning (15610): No output dependent on input pin "a [0]"
Warning (15610): No output dependent on input pin "a [1]"
Warning (15610): No output dependent on input pin "a [8]"
Warning (15610): No output dependent on input pin "a [9]"
Warning (15610): No output dependent on input pin "a [10]"
Warning (15610): No output dependent on input pin "a [11]"
Warning (15610): No output dependent on input pin "a [12]"
Warning (15610): No output dependent on input pin "a [13]"
Warning (15610): No output dependent on input pin "a [14]"
Warning (15610): No output dependent on input pin "a [15]"
Warning (15610): No output dependent on input pin "a [16]"
Warning (15610): No output dependent on input pin "a [17]"
Warning (15610): No output dependent on input pin "a [18]"
Warning (15610): No output dependent on input pin "a [19]"
Warning (15610): No output dependent on input pin "a [20]"
Warning (15610): No output dependent on input pin "a [21]"
Warning (15610): No output dependent on input pin "a [22]"
Warning (15610): No output dependent on input pin "a [23]"
Warning (15610): No output dependent on input pin "a [24]"
Warning (15610): No output dependent on input pin "a [25]"
Warning (15610): No output dependent on input pin "a [26]"
Warning (15610): No output dependent on input pin "a [27]"
Warning (15610): No output dependent on input pin "a [28]"
Warning (15610): No output dependent on input pin "a [29]"
Warning (15610): No output dependent on input pin "a [30]"
Warning (15610): No output dependent on input pin "a [31]"

I don't understand why "a [2]" to "a [7]" are not No output dependent on input pin. Please tell me the reason.

verilog
1 Answers

The rd output is driven by a[2] through a[7]. You declared the RAM with 64 addresses. This requires 6 bits to access, and you chose to address the RAM with the 6 bits [7:2].

The other 26 a bits are not used to drive the rd output (or any other output).

For example, to make this more obvious:

  assign rd = RAM [a [31: 2]]; // word aligned

could be written as:

  assign rd = RAM[ a[7:2] ]; // word aligned
user_1977580
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