Topic : The VGA Training Program
Author : Grant Smith
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          "It's a computer," he replies.
           "Yes, dear, but what does it do?"
           "It computes! It's a computer."
           "What does it compute?"
           "What? Er? Um. Numbers! Yes, numbers!" He smiles
           "Why? Well why?" He starts to sweat.
           "I mean, is it just something to dust around, or does
              it actually do something useful?"
           " can call other computers with it!" Hope lights
              up his eyes. "So you can get programs from other computers!"
           "I see. Tell me, what do these programs do?"
           "Do? I don't think I fol..."
           "I see. They compute. Numbers. For no particular reason." He
              withers under her gaze.
           "Yes, but..."
           She smiles, and he trails off, defeated. She takes another look
               at the thing. "Although," she says, with a strange look in
               her eyes. He looks up, an insane look of hope on his
               face. "Does it come in pink?" she asks.
                                                     - Grant Smith
                                                        Tue 27 July, 1993
                                                         9:35 pm.

See you next time,
    - Denthor

--==[ PART 4 ]==--

[Note: things in brackets have been added by Snowman.  The original text
has remained mostly unaltered except for the inclusion of C++ material]


Howdy all! Welcome to the fourth part of this trainer series! It's a little
late, but I am sure you will find that the wait was worth it, becase today I
am going to show you how to use a very powerful tool : Virtual Screens.

If you would like to contact me, or the team, there are many ways you
can do it : 1) Write a message to Grant Smith in private mail here on
                  the Mailbox BBS.
            2) Write a message here in the Programming conference here
                  on the Mailbox (Preferred if you have a general
                  programming query or problem others would benefit from)
            3) Write to ASPHYXIA on the ASPHYXIA BBS.
            4) Write to Denthor, Eze or Livewire on Connectix.
            5) Write to :  Grant Smith
                           P.O.Box 270 Kloof
            6) Call me (Grant Smith) at 73 2129 (leave a message if you
                  call during varsity)

NB : If you are a representative of a company or BBS, and want ASPHYXIA
       to do you a demo, leave mail to me; we can discuss it.
NNB : If you have done/attempted a demo, SEND IT TO ME! We are feeling
        quite lonely and want to meet/help out/exchange code with other demo
        groups. What do you have to lose? Leave a message here and we can work
        out how to transfer it. We really want to hear from you!

  What is a Virtual Screen and why do we need it?

Let us say you are generating a complex screen numerous times on the fly
(for example scrolling up the screen then redrawing all the sprites for
each frame of a game you are writing.) Do you have any idea how awful it
would look if the user could actually see you erasing and redrawing each
sprite for each frame? Can you visualise the flicker effect this would
give off? Do you realise that there would be a "sprite doubling" effect
(where you see two copies of the same sprite next to each other)? In the
sample program I have included a part where I do not use virtual screens
to demonstrate these problems. Virtual screens are not the only way to
solve these problems, but they are definately the easiest to code in.

A virtual screen is this : a section of memory set aside that is exactly
like the VGA screen on which you do all your working, then "flip" it
on to your true screen. In EGA 640x350x16 you automatically have a
virtual page, and it is possible to have up to four on the MCGA using a
particular tweaked mode, but for our puposes we will set one up using base

  Setting up a virtual screen

As you will have seen in the first part of this trainer series, the MCGA
screen is 64000 bytes big (320x200=64000). You may also have noticed that
in TP 6.0 [and C++] you arn't allowed too much space for normal variables.
For example, saying :


  VAR Virtual : Array [1..64000] of byte;


  unsigned char Virtual[64000];

would be a no-no, as you wouldn't have any space for your other variables.
What is the solution? I hear you enquiring minds cry. The answer : pointers!
Pointers to not use up the base 64k allocated to you by TP 6.0 [and C++], it
gets space from somewhere else in the base 640k memory of your computer. Here
is how you set them up :


  Type Virtual = Array [1..64000] of byte;  { The size of our Virtual Screen }
       VirtPtr = ^Virtual;                  { Pointer to the virtual screen  }

  VAR Virscr : VirtPtr;                     { Our first Virtual screen    

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