Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!apple!keith
From: keith@Apple.COM (Keith Rollin)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Bug in MPW Pascal 2.0.2 ???
Message-ID: <33096@apple.Apple.COM>
Date: 12 Jul 89 22:17:27 GMT
References: <1270@draken.nada.kth.se> <33064@apple.Apple.COM> <1275@draken.nada.kth.se>
Organization: Apple Computer Inc, Cupertino, CA
Lines: 60

In article <1275@draken.nada.kth.se> d88-bli@nada.kth.se (Bo Lindbergh) writes:
>In article <33064@apple.Apple.COM> cjp@Apple.COM (Chris Plummer) writes:
>>In article <1270@draken.nada.kth.se> d88-bli@nada.kth.se (Bo Lindbergh) writes:
>>>Consider the following Pascal program.
>>>
>>>   program bug;
>>>      var
>>>         u  : 0..65535;
>>>   begin
>>>      u:=60000;
>>>      writeln(u);
>>>   end.
>>>
>>
>>I'm not sure about MPW Pascal 2.0.2, but the 3.0 manual says that the
>>parameter must be of type char, integer, real, string, packed array of
>>char, or boolean.  Since an integer is -32768..32767, I'd imaging that
>>it's simply typecasting "u" to an integer which will cause numbers greater
>>that 32767 to be negative.  You could assign "u" to a real and then
>>pass the real to writeln.
>
>You mean printing longints isn't allowed? Since the type of u is a subrange
>of longint, the compiler should complain when I try to pass it
>as a parameter to writeln. And this program:
>
>   program bugTwo;
>      var
>         l  : longint;
>   begin
>      l:=60000;
>      writeln(l);
>   end.
>
>prints 60000 and nothing else. Finally, if I assign the value of u to a real,
>the result is -5536. I don't think the problem has anything to do with writeln.
>More suggestions, anyone?
>

It probably has to do with the backflips that the Pascal compiler goes through
to implement this specific range (0..65535). Although you'd think that it 
would be stored as a longint (because it overlaps into the longint range), it
is actually implemented as a short/word/2 bytes/whatever. When this value is
passed to writeln, it is passed as a word, and is hence treated as a 
integer value.

Why is (0..65535) treated special? Probably because of convenience. Some
Toolbox and OS routines require an unsigned integer passed to them. C can
handle this, but Pascal doesn't know about unsigned types. If the toolbox
wants a 2 byte number in the positive range, this hack is required.

Actually, through the magic of MultiFinder, I just tried out an experiment. It
seems that types (0..xxxxx) where xxxxx < 65536, then variables of that type
are treated as integers; this feature is not limited to just (0..65535).

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"Argue for your Apple, and sure enough, it's yours" - Keith Rollin, Contusions