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Altera_Forum's avatar
Altera_Forum
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14 years ago

Determining the index of the first 1 bit in a vector

Let's say I have a 64b wire, and I want to efficiently determine the index of the first 1 bit. For example: wire [63:0] sum; wire [5:0] index; assign sum = 64'b0000000111010110....1010...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

You can use a function, like the one below and hope that Quartus synthesizes it efficiently.

It will produce a rather long combinational path, but I don't think that can be avoided.

function [5:0] getIndex(input [63:0] data);

integer i;

for(i = 63; i >= 0; i = i - 1) begin

if (data[63-i] == 1'b1)

getIndex = i;

end

endfunction

assign index = getIndex(sum);