[Coco] Convert floating point formats

[Walter Zambotti]

Not sure why it is PDP1 but that is how it is documented in the 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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