Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!bbn!uwmcsd1!ig!agate!ucbvax!UOGUELPH.BITNET!BOTCHAIR
From: BOTCHAIR@UOGUELPH.BITNET (Alex Bewley)
Newsgroups: comp.lang.modula2
Subject: Re: Recursion & Types
Message-ID: <INFO-M2%88021200183659@UCF1VM>
Date: 11 Feb 88 22:50:03 GMT
References: <markh@csd4.milw.wisc.edu>
Sender: daemon@ucbvax.BERKELEY.EDU
Reply-To: Info-Modula2 Distribution List <INFO-M2%UCF1VM.bitnet@jade.berkeley.edu>
Organization: The Internet
Lines: 36


  Pointers are extremely useful.  They can be used for type conversion (but
we, being good programmers :-), don't do this).  They can also be used to
'fix' data structures to certain locations in memory.  For example consider the
video screen of a PC.  It is composed of 4000 bytes.  The colour screen memory
is located at B800H:0000H, and the monochrome screen memory is located at
B000H:0000H.  A small procedure could determine which type of video card is
being used and set an array to point to the appropriate memory location.  Thus,
every time you use a data structure (usually a two dimensional array, in this
case) it will change the screen display.
  Some code is included below.
  For this screen exercise you could use the absolute variable declaration, but
that would mean defining an array for each screen.

      Alex

-----
TYPE
  ScreenType = POINTER TO ARRAY [0..25-1],[0..80-1] OF RECORD
                                                         Char, Attr : CHAR;
                                                       END;

VAR
  Screen : ScreenType;

BEGIN
  (* determine video card type *)
  ...
  IF ColourMonitor THEN
    Screen := 0B800H:0000H;
  ELSE
    Screen := 0B000H:0000H;
  END;
  ...
  Screen[0][0].Char := "A"; (* put an 'A' in the top left corner of the *)
END;                        (* screen *)