[fpc-pascal] subtract longwords with rollover

[Klaus Hartnegg]

Sorry, I forgot to mention there is {$Q-} in the source.

The trick was indeed to use dec(longint(a),b). However I had missed one 
place in the code, so I got another runtime error from there.

But why does it only work with longint, and not with longword?

This looks like a missing declaration of one variant of the dec function.


var
   a,b : longword;
   i,j : byte;

begin
   {$Q-}
   for i := 0 to 4 do begin
      for j := 0 to 4 do begin
         write (i,' ',j,'  ');
         a := longword(j) * 1000000000;
         b := longword(i) * 1000000000;
         write (a:10,' ',b:10,' ');
       { a := a-b;  }         { fail at 1 0 }
       { dec (a,b); }         { fail at 1 0 }
         dec (longint(a),b);  { ok }
         writeln (a:11);
      end;
   end;
end.