[fpc-pascal] Freepascal Floating Point Issue

[Bernd Oppolzer]

Am 24.08.2017 um 14:00 schrieb James Richters:
>
> Thanks everyone for the explanation and solutions.  I can work around 
> it well enough now that I understand what’s happening, but I’m still 
> curious if there is a way to define fixed point variables?  In a C++ 
> Project I was helping with recently, they had a way of defining a 
> variable as having 2 places before the decimal and 30 places after? 
> Since the decimal could never move (float) It ended up being much 
> better defined.   It’s like always moving the decimal over 30 places 
> to the right, using integer math, then moving it back.  I’m not sure 
> if this was something the compiler did, or if maybe they had their own 
> functions to do this… I had a difficult time following how things 
> worked, but I do remember the fixed point variables.  Anyway I thought 
> I would just ask the question in case there is a way to define this in 
> FreePascal
>
> It seems to me that if you know you never need more than a fixed 
> number of places before and after the decimal, then moving the decimal 
> over the maximum number of places, performing integer math, which 
> would round to the nearest integer(and cut off the useless garbage), 
> then moving the decimal back would mean a fixed amount of precision 
> with the benefit that one could do things like exact conditionals and 
> get the expected result
>
> James
>
>

IBM mainframes have decimal data types involving 31 decimal digits
(maximum); the numbers of digits left and right of the decimal point
may be chosen freely. The types are all compatible and promoted to
floating point, when necessary. The mainframe languages, like PL/1
and COBOL, support them natively, and mainframe C, too:

    int x, y;
    decimal (15,2) amt;
    ...

These decimal data types are supported by the hardware (by
machine instructions). This was an optional feature in the 1960s,
but the modern machines of today all support it.

The decimal data is implemented as BCD data; hex digits are
decimal digits, and a sign half byte is added, that makes 16 bytes
for 31 digits. If shorter numbers are needed, less bytes are allocated
(all lengths from 1 to 16 are supported). All decimal fields have
odd numbers of digits.

I am thinking about adding support for decimal data in one of the
next versions of my Stanford Pascal compiler, which targets this
platform, as well as Windows, Linux, macOS etc.; of course, this
decimal format will need to be emulated on the non-mainframe
platforms.

For some years now, the mainframe also supports floating point
data based on a decimal representation. But I don't have experience
with this so far. Some compilers support this (PL/1).

Kind regards

Bernd

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