[Coco] Convert floating point formats

[Joel Rees]

2019年8月20日(火) 17:02 Walter Zambotti <zambotti at iinet.net.au>:

> Not sure why it is PDP1 but that is how it is documented in the C manual!
>

Which C manual?


> Walter
>
> -----Original Message-----
> From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of James Jones
> Sent: Tuesday, 20 August 2019 12:13 PM
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Subject: Re: [Coco] Convert floating point formats
>
> I'm a little confused by the name PDP1data; the PDP-1 had an 18-bit word,
> did one's complement arithmetic, and the floating point library referenced
> in
> https://web.archive.org/web/20110514105011/http://www.dbit.com/~greeng3/pdp1/pdp1.html
> used
> two words for a value, with one word for mantissa (and presumably sign
> bit) and one for the exponent.
>
> On Tue, Aug 13, 2019 at 11:12 PM Walter Zambotti <zambotti at iinet.net.au>
> wrote:
>
> > Here is the PC side code that I ended up making to convert CoCo OS9 C
> > PDP1 floating point numbers to modern PC IEEE754 floating point and visa
> versa.
> >
> > union _Data
> > {
> >   unsigned long long llval;
> >   double dval;
> >   unsigned int lval;
> >   unsigned char bytes[8];
> >   unsigned short words[4];
> >   unsigned int dwords[2];
> > };
> >
> > typedef union _Data PDP1data;
> > typedef union _Data IEEE754data;
> >
> > double ConvertDBLPDP1toIEEE754(PDP1data PDP1data) {
> >         IEEE754data iee754;
> >         unsigned long long signbit, exp, mantissa;
> >
> >         if (PDP1data.llval == 0)
> >         {
> >                 return 0.0;
> >         }
> >
> >         signbit  =  PDP1data.llval & 0x8000000000000000;
> >         exp  = (PDP1data.llval & 0x00000000000000ff) + 0x37e;
> >         mantissa =  PDP1data.llval & 0x7fffffffffffff00;
> >
> >         iee754.llval = signbit | (exp<<52) | (mantissa>>11);
> >
> >         return iee754.dval;
> > }
> >
> > PDP1data ConvertDblIEEE754toPDP1(double dvalue) {
> >         PDP1data PDP1data;
> >         IEEE754data IEEE754data;
> >         unsigned long long signbit, exp, mantissa;
> >
> >         IEEE754data.dval = dvalue;
> >
> >         // IEEE floats can have a negative zero that PDP1 floats
> > cannot have
> >         // if the value is zero then we make it a good zero
> >
> >         if (IEEE754data.dval == 0.0)
> >         {
> >                 IEEE754data.llval = 0;
> >                 return IEEE754data;
> >         }
> >
> >         signbit  =   IEEE754data.llval & 0x8000000000000000;
> >         exp   = ((IEEE754data.llval & 0x7ff0000000000000)>>52) - 0x37e;
> >         mantissa =   IEEE754data.llval & 0x000fffffffffffff;
> >
> >         PDP1data.llval = signbit | (exp) | (mantissa<<11);
> >
> >         return PDP1data;
> > }
> >
> > -----Original Message-----
> > From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of James
> > Jones
> > Sent: Thursday, 4 July 2019 7:38 PM
> > To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> > Subject: Re: [Coco] Convert floating point formats
> >
> > On Fri, May 24, 2019 at 9:30 AM Alex Evans <varmfskii at gmail.com> wrote:
> >
> > > I don't have a line to a particular solution (though they should be
> > > easy to implement), but do you mean IEEE double precision as is used
> > > on current x86 CPUs, or are you saying that there was some other
> > > floating point format used by x86 FPUs such as the 8087 sometime in
> > > the past that you are interested in converting to/from. I also was
> > > under the impression that the Microware C Compiler used IEEE
> > > floating point in which case no conversion needs to be made, it is
> > > already in the right format.
> > >
> >
> > The switch to IEEE 754 floating point came with OS-9/68000.
> >
> > --
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> >
> >
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