Re: 1364 issue 140 and negative exponents

From: Michael McNamara (mac@verisity.com)
Date: Mon May 05 2003 - 15:02:52 PDT

  • Next message: Steven Sharp: "Re: errata/344: Case Statements with Real Expressions"

    Precedence: bulk

    Indeed, you've succeeded in reminding me what I remember learning when
    implementing reals in VCS: Verilog rounds reals 'away from zero',
    which one can perhaps justify by the notion that it is better to make
    someting a bit larger, rather than a bit smaller; but it is biased
    none the less.

    Kurt Baty writes:
    > Precedence: bulk
    >
    >
    > > I am not sure what you are trying to convey with that
    > > first $display statement before the main loop in your
    > > test program.
    >
    > Yes, I to noticed the
    >
    > > 2 ** -1 = 1
    > > -2 ** -1 = -1
    >
    > rounding cases so the extra display statement shows that
    > verilog-XL does in fact round the real value 0.5
    > to the integer value 1
    >
    > >
    > > Are you trying to show that reals are being rounded
    > > to nearest as opposed to being rounded towards zero?
    > >
    > > I had figured the results
    > >
    > > 2 ** -1 = 1
    > > -2 ** -1 = -1
    > >
    > > were a result of rounding to nearest.
    > >
    >
    > Actually, this rounding is not "round nearest"
    > this is "round up" (I say "round away-from-zero"
    > so as to not be confused in negative numbers).
    > 0.5 would round to 0 in "round nearest".
    >
    > > But, my specific question is, are these the results you are
    > > advocating for these particular operands?
    >
    > yes, to took x ** y where x,y are integers and converted
    > x (integer -> real -> bits(ieee 64 floating point))
    >
    > did a x_bits ** y using the pow function
    >
    > function [63:0] pow; // l**r, where l is fp64 and r is integer
    >
    > converted answer_bits (bits(ieee 64 floating point) -> real -> integer)
    >
    > and I make the claim that this is the most "math correct answer".
    >
    > >
    > > >From James' email today and from the discussion this morning, I had
    > > thought the desired results would be:
    > >
    > > 2 ** -1 = 0
    > > -2 ** -1 = 0
    > >
    >
    > I too was taken in!! (fished-in?!)
    >
    > kurt
    >
    >
    > > Kurt Baty wrote:
    > > >
    > > > Precedence: bulk
    > > >
    > > > > From drm@xilinx.com Mon May 5 13:41:55 2003
    > > > >
    > > > > > 2 ** -1 = 1 = 0.5
    > > > > > -2 ** -1 = -1 = -0.5
    > > > >
    > > >
    > > > please note that:
    > > >
    > > > initial
    > > > begin
    > > > x_real = 0.5;
    > > > x = x_real;
    > > > $display("x = %0d = x_real =",x,x_real);
    > > > $display;
    > > > for(y=limit;y>=-limit;y=y-1) begin
    > > >
    > > > displays:
    > > >
    > > > Highest level modules:
    > > > ieee_etf_140_test
    > > >

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