display a real in verilog but bitstoreal returning only 0.000000

Question

I am trying to display a real number during the simulation of my verilog code in modelsim. But I only get 0 as output. I am trying to use the bitstoreal system function. I'm not so good at verilog so it could be a stupid beginner's mistake.

Following is my code:

reg [31:0] y[1:0];
integer    file;
localparam [31:0] test = 32'h3fb0d05d;

task read_data_from_fifo();
   begin
     file = $fopen("/tmp/data.fifo", "r");
     $fread(y, file);
     $display("y0 = %d, %f, %h", $bitstoreal(y[0]), $bitstoreal(test), $bitstoreal(y[0]));
     $display("y1 = %f, %f, %h", y[1], $bitstoreal(32'h5dd0_b03f), y[1]);    
   end
endtask

(the task is called from an initial begin block) Outputs:

# y0 =          0, 0.000000, 00000000
# y1 = 3742779199.000000, 0.000000, df16473f

All help appreciated.

Update

Looks like bitstoreal only supports double precision floats (64-bit). Because

localparam [63:0] test = 64'h_3FF61A0BE5109071;
$display("%f", $bitstoreal(test));

results in

1.381359

Answer 1

use $bitstoshortreal: ... $shortrealtobits converts values from a shortreal type to the 32-bit vector representation of the real number. $bitstoshortreal converts a bit pattern created by $shortrealtobits to a value of the shortreal type. ...

Answer 2

$bitstoreal takes a 64bit input double precision float.

Solution: Do a bit conversion from single to double precision float. Like this:

reg [31:0] z;      // single precision float
reg [63:0] double; // double precision float

double = {z[31], z[30], {3{~z[30]}}, z[29:23], z[22:0], {29{1'b0}}};

$display("%f", $bitstoreal(double));

Disclaimer: I'm not sure if this single to double conversion is safe/correct.

Update

This is to be read in the IEEE document mentioned by toolic:

$realtobits converts values from a real type to a 64-bit vector representation of the real number. $bitstoreal converts a bit pattern created by $realtobits to a value of the real type.