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What does the Verilog localparam X code below mean? From my understanding it is as follows:

if(y==4)      X = +64;
else if(Y==3) X = +32;
else if(Y==2) X = +16;
else if(Y==1) X = +8;
else if(Y==0) X = +4;
else          X = +2;

Is this correct? If so, another thing that doesn't make sense to me is the '+' after the '=' sign, e.g., (X = +2)? Is this something unique to localparam variables?

localparam  X = (Y==4) ? +64 :
                (Y==3) ? +32 :
                (Y==2) ? +16 :
                (Y==1) ? +8  :
                (Y==0) ? +4  :
                         +2  ;
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  • \$\begingroup\$ Your understanding of the conditional operator is correct. The unary + can be omitted. It is not unique to localparam, it can be anywhere. \$\endgroup\$
    – Eugene Sh.
    Jan 13, 2023 at 16:12
  • \$\begingroup\$ Is there any reason why designers would use the '+' ? \$\endgroup\$
    – user367640
    Jan 13, 2023 at 16:14
  • \$\begingroup\$ Could be multiple reasons or no reason at all :) For example it could be a remnant of some longer code involving negative numbers as well, so the +s were added for visual consistency. \$\endgroup\$
    – Eugene Sh.
    Jan 13, 2023 at 16:17

2 Answers 2

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Yes, the localparam statement is logically equivalent to the if/else pseudocode you showed. The statement uses the conditional operator which has precedence like your if/else.

The + signs are unnecessary in the code. The code behaves the same as if they were not there. Perhaps someone copy-and-pasted more complicated code (with expressions like A+2) and forgot to simplify it. A simpler way to write it is:

localparam  X = (Y==4) ? 64 :
                (Y==3) ? 32 :
                (Y==2) ? 16 :
                (Y==1) ? 8  :
                (Y==0) ? 4  :
                         2  ;
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The conditional operator ?: is an expression that behaves similar to an if/else conditional branching procedural statement as you observed. It can be used in places where is is not practical to use procedural statements. A parameter/localparam must be assigned using an expression. You could wrap the if/else in a constant function, but the conditional operator is a convenience. If you were making assignments to a variable, a procedural case statement would be much clearer.

The unary + operator has no purpose other than symmetry with the unary - operator. In can make the intent of the code more clear if you have a mix of positive and negative numbers.

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