[Oberon] Static variable overflow (bug?)

Hans Klaver hklaver at dds.nl
Mon Feb 24 01:15:13 CET 2020

Recently I tried to implement XYplane.Mod from the book of Reiser & Wirth 'Programming in Oberon' for Project Oberon. Within Peter de Wachter's RISC emulator I ran into an overflow problem (leading to the remaining empty of the menu line of a viewer) that appears to be dependent on which disk image is used.

Otherwise my port works fine (produces the IFS fractal fern as expected). 
For those interested I will post it to my github repository.

A minimal module that can reproduce this menu problem can be found below.

When starting the emulator in macOS as follows this problem occurs:
$ ./risc --fullscreen DiskImage/Oberon-2019-01-21.dsk

But the problem does NOT occur when starting thus: 
$ ./risc --fullscreen DiskImage/Oberon-2016-08-02.dsk

I do understand that there is a limit to the use of static variables within the PO version of Oberon, but I would not expect this limit to be reached this easily, and I would not expect it to appear in newer versions of the software and not in older.

Does anyone have a clue where to look for the cause of this happening?

Hans Klaver


MODULE MyViewer;  
(* Test of the menu line of a viewer *)

IMPORT Display, Viewers, Oberon, MenuViewers, TextFrames;

   max = 16377;   (* If max >= 16367 the frame name disappears; If max >= 16377 the whole menu line disappears! *)
   menu = "System.Close System.Copy System.Grow"; 

   XYframe = POINTER TO XYframeDesc;
   XYframeDesc = RECORD (Display.FrameDesc) END;

VAR bitmap: ARRAY max OF SET;  (* with this variable present the menu may remain empty! *)

PROCEDURE Restore(F: XYframe);
   Oberon.RemoveMarks(F.X, F.Y, F.W, F.H);
   Display.ReplConst(Display.black, F.X, F.Y, F.W, F.H, Display.replace);  (* clear *)
END Restore;

PROCEDURE Handle(F: Display.Frame; VAR M: Display.FrameMsg);
   VAR F1: XYframe;
   CASE F OF XYframe:
      CASE M OF 
            IF M.id = Oberon.track THEN Oberon.DrawMouseArrow(M.X, M.Y) END |
               Oberon.RemoveMarks(F.X, F.Y, F.W, F.H); NEW(F1); F1^ := F^; M.F := F1 |
            IF (M.Y # F.Y) OR (M.H # F.H) THEN F.Y := M.Y; F.H := M.H; Restore(F) END        
END Handle;

   VAR F: XYframe;    V: Viewers.Viewer;   x, y: INTEGER;
   NEW(F);   F.handle := Handle; 
   Oberon.AllocateUserViewer(Oberon.Par.vwr.X, x, y); 
   V := MenuViewers.New(TextFrames.NewMenu("MyViewer", menu), F, TextFrames.menuH, x, y)
END Open;

END MyViewer.
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