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From: winfave@dutrun.UUCP (Alexander Verbraeck)
Newsgroups: comp.lang.pascal
Subject: Strange... The Turbo Pascal NEW command
Message-ID: <845@dutrun.UUCP>
Date: 31 Jul 89 23:35:14 GMT
Reply-To: winfave@dutrun.UUCP (A.Verbraeck)
Organization: Delft University of Technology, The Netherlands
Lines: 76

There's one thing about Turbo Pascal that I don't understand,
and that is the how the New procedure knows the size of the
block on the heap to be allocated. At this  moment some of my 
students are working on a heap manager on top of the standard
TP heap manager that allows 4 Gbyte of heap space using paging 
and virtual memory swapping pages to (Ram)disk, where it is
*not* neccesary to change any existing code. You can still use
the standard ^ types, New and Dispose. If there is any interest 
in this manager when it is finished, I can post it to the net
(if-and-only-if we succeed of course).

The problem is that we do not understand the way the New 
procedure works. When you call New(P) where P points to a certain
type, the New procedure allocates the correct number of bytes on
the heap. We think that the compiler inserts some extra parameters 
when calling New, for the following program works _NOT_ ok:

program TestNew(input,output);

const
  AS = 500;

type
  TT = array[1..AS] of integer;
  PT = ^TT;

var
  IP,JP : PT;
  i,j   : integer;


procedure New(var P);

begin
  System.New(pointer(P));
end;

begin
  writeln(MemAvail);
  New(IP);
  writeln(MemAvail);
  System.New(JP);
  writeln(MemAvail);
  for i:=1 to AS do IP^[i]:=i;
  for i:=1 to AS do JP^[i]:=0;
  for j:=1 to 10 do write(IP^[j]:5);
  writeln;
  write('Press Enter');
  readln;
end.

Output of this program is for instance:

------------------------------------------------------------
502314
502314    --> No memory allocated at all!
501314    --> 1000 bytes allocated
    0    0    0    0    0    0    0    0    0    0
Press Enter
------------------------------------------------------------

Which means that no memory is allocated during the New call,
the correct amount of memory is allocated during the System.New
call, and IP and JP share Heap-memory.

Does anyone know what parameters are passed extra to the New
procedure? How can I use these parameters? Is it possible to
make a customized version of the New procedure? How do I get
it to pass the right parameters on to the System.New procedure?

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Alexander Verbraeck                            e-mail:
Delft University of Technology                 winfave@hdetud1.bitnet
Department of Information Systems              winfave@dutrun.uucp
PO Box 356, 2600 AJ  The Netherlands
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