CODEWITHSUNDEEP
bit-manipulationsystem-verilogbitvector
shaymin shaymin
shaymin shaymin

Reputation: 197

Convert one hot bit vector to an integer in SystemVerilog without logarithms

I have a bit vector (one hot encoded, aka, one and only one bit will be 1) that represents a bitmap over another vector. For example, lets say these are my variables:

logic [3:0] bitmap;
logic [7:0] data_vector[3:0];
assign bitmap = 4'b0010;

I would like to access the index of data_vector that corresponds with the asserted bit in bitmap (data_vector[1] for this example). An obvious way would be the following:

logic [7:0] selection;
always_comb begin
   for (integer i = 0; i < 4; i++) begin 
      if (bitmap[i]) begin
         selection = data_vector[i];
      end
   end
end

I was wondering if there was a more efficient way to do this. The following is not a possibility

   selection = {8{bitmap[0]}} & data_vector[0] | ... 
              |{8{bitmap[3]}} & data_vector[3];

as this is just a small example to explain the question. Thanks in advance :)

Upvotes: -1

Views: 2551

Answers (2)

Biswajit Khandai
Biswajit Khandai

Reputation: 1

Say your bitmap is 256 bits and you want to produce an index of 8 bits.

logic [255:0] bitmap;
logic   [7:0] idx;

always_comb begin
  idx = '0;
  for (int i = 0; i < 256; i++) begin 
    if (bitmap[i]) begin
      idx |= 8'(i);
    end
  end
end

Upvotes: -1

dave_59
dave_59

Reputation: 42788

You can do

selection = data_vector[$clog2(bitmap));

If this needs to be synthesizable AND your synthesis tool does not support $clog2, then you could write the function yourself, or stick with what you originally wrote.

Upvotes: 0

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