[fpc-pascal] Strange swap function, what does it do?

[Sven Barth]

Am 13.08.2020 um 22:37 schrieb Bo Berglund via fpc-pascal:
> I am working on the conversion of a service application from
> Windows/Delphi to Linux/FPC.
>
> When going through the code I found this procedure, which I do not
> quite understand the workings of, can someone please explain how it
> operates?
>
> procedure Swap2(var Source);
> var
>    H, L: word;
> begin
>    H := Hi(word(Source));
>    L := Lo(word(Source));
>    word(Source) := (L * 256) + H;
> end;
>
> It is used in the code like this where Hd is a record and numSteps is
> a word sized field:
>
> Swap2(Hd.numSteps);
>
> In other places when word sized variables need to be swapped
> (Little/Big-endian conversions) it is done like this instead where
> CmdProgNum is a smallint value and the built-in swap function is used:
>
> FR.CmdProgNum := Swap(FR.CmdProgNum);
>
> So the code uses a mix of these and I do not understand what the
> strange looking function does...

It also does an endian swap, but in place (it's a *procedure* with a var 
parameter) instead of as a function that returns the swapped value, but 
leaves the input untouched.

Regards,
Sven