Mode 13h

                    嬪様様様様様様様様様様様様様様様
                             W E L C O M E         
                      To the VGA Trainer Program    
                                  By                
                          DENTHOR of ASPHYXIA        
                    塒様様様様様様様様様様様様様様様  
                      陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳 
                        陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳

                            --==[ PART 1 ]==--

  Introduction

 Hi there! This is Denthor of ASPHYXIA, AKA Grant Smith. This training
 program is aimed at all those budding young demo coders out there. I am
 assuming that the reader is fairly young, has a bit of basic Std. 6 math
 under his belt, has done a bit of programming before, probably in BASIC,
 and wants to learn how to write a demo all of his/her own.

 This I what I am going to do. I am going to describe how certain routines
 work, and even give you working source code on how you do it. The source
 code will assume that you have a VGA card that can handle the
 320x200x256 mode. I will also assume that you have Turbo Pascal 6.0 or
 above (this is because some of the code will be in Assembly language,
 and Turbo Pascal 6.0 makes this incredibly easy to use). By the end of
 the first "run" of sections, you will be able to code some cool demo
 stuff all by yourself. The info you need, I will provide to you, but it
 will be you who decides on the most spectacular way to use it.

 Why not download some of our demos and see what I'm trying to head you
 towards.

 I will be posting one part a week on the Mailbox BBS. I have the first
 "run" of sections worked out, but if you want me to also do sections on
 other areas of coding, leave a message to Grant Smith in private E-Mail,
 or start a conversation here in this conference. I will do a bit of
 moderating of a sort, and point out things that have been done wrong.

 In this, the first part, I will show you how you are supposed to set up
 your Pascal program, how to get into 320x200x256 graphics mode without a
 BGI file, and various methods of putpixels and a clearscreen utility.

 NOTE : I drop source code all through my explanations. You needn't try
        to grab all of it from all over the place, at the end of each part I
        add a little program that uses all the new routines that we have
        learned. If you do not fully understand a section, leave me
        private mail telling me what you don't understand or asking how I
        got something etc, and I will try to make myself clearer. One
        last thing : When you spot a mistake I have made in one of my
        parts, leave me mail and I will correct it post-haste.

 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
  Disclaimer

 Hi again, sorry that I have to add this, but here goes. All source code
 obtained from this series of instruction programs is used at your own
 risk. Denthor and the ASPHYXIA demo team hold no responsibility for any
 loss or damage suffered by anyone through the use of this code. Look
 guys, the code I'm going to give you has been used by us before in
 Demos, Applications etc, and we have never had any compliants of machine
 damage, but if something does go wrong with your computer, don't blame
 us. Sorry, but that's the way it is.

 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
  The MCGA mode and how you get into it in Pascal without a BGI

 Lets face it. BGI's are next to worthless for demo coding. It is
 difficult to find something that is slower then the BGI units for doing
 graphics. Another thing is, they wern't really meant for 256 color
 screens anyhow. You have to obtain a specific external 256VGA BGI to get
 into it in Pascal, and it just doesn't make the grade.

 So the question remains, how do we get into MCGA 320x200x256 mode in
 Pascal without a BGI? The answer is simple : Assembly language.
 Obviously assembly language has loads of functions to handle the VGA
 card, and this is just one of them. If you look in Norton Gides to
 Assembly Language, it says this ...

 ____________________________________________________________________
 INT 10h,  00h (0)        Set Video Mode

     Sets the video mode.

        On entry:      AH         00h
                       AL         Video mode

        Returns:       None

        Registers destroyed:      AX, SP, BP, SI, DI
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 This is all well and good, but what does it mean? It means that if you
 plug in the video mode into AL and call interrupt 10h, SHAZAM! you are
 in the mode of your choice. Now, the MCGA video mode is mode 13h, and
 here is how we do it in Pascal.

 Procedure SetMCGA;
 BEGIN
   asm
         mov     ax,0013h
         int     10h
   end;
 END;

 There you have it! One call to that procedure, and BANG you are in
 320x200x256 mode. We can't actually do anything in it yet, so to go back
 to text mode, you make the video mode equal to 03h, as seen below :

 Procedure SetText;
 BEGIN
   asm
         mov     ax,0003h
         int     10h
   end;
 END;

 BANG! We are back in text mode! Now, cry all your enquiring minds, what
 use is this? We can get into the mode, but how do we actually SHOW
 something on the screen? For that, you must move onto the next section
 ....

 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
   Clearing the screen to a specific color

 Now that we are in MCGA mode, how do we clear the screen. The answer is
 simple : you must just remember that the base adress of the screen is
 $a000. From $a000, the next 64000 bytes are what is actually displayed on
 the screen (Note : 320 * 200 = 64000). So to clear the screen, you just use
 the fillchar command (a basic Pascal command) like so :

       FillChar (Mem [$a000:0],64000,Col);

 What the mem command passes the Segment base and the Offset of a part of
 memory : in this case the screen base is the Segment, and we are starting
 at the top of the screen; Offset 0. The 64000 is the size of the screen
 (see above), and Col is a value between 0 and 255, which represents the
 color you want to clear the screen to.

 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
   Putting a pixel on the screen (two different methoods)

 If you look in Norton Guides about putting a pixel onto the screen, you
 will see this  :

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     Writes a pixel dot of a specified color at a specified screen
     coordinate.

     On entry:      AH         0Ch
                    AL         Pixel color
                    CX         Horizontal position of pixel
                    DX         Vertical position of pixel
                    BH         Display page number (graphics modes with more
                               than 1 page)

     Returns:       None

     Registers destroyed:      AX, SP, BP, SI, DI
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 As seen from our SetMCGA example, you would write this by doing the following:

 Procedure INTPutpixel (X,Y : Integer; Col : Byte);
 BEGIN
   asm
      mov        ah,0Ch
      mov        al,[col]
      mov        cx,[x]
      mov        dx,[y]
      mov        bx,[1]
      int        10h
   end;
 END;

 The X would be the X-Coordinate, the Y would be the Y-Coordinate, and the Col
 would be the color of the pixel to place. Note that MCGA has 256 colors,
 numbered 0 to 255. The startoff pallette is pretty grotty, and I will show
 you how to alter it in my next lesson, but for now you will have to hunt for
 colors that fit in for what you want to do. Luckily, a byte is 0 to 255, so
 that is what we pass to the col variable. Have a look at the following.

     CGA = 4 colours.
     4x4 = 16
     EGA = 16 colors.
     16x16 = 256
     VGA = 256 colors.
     Therefore an EGA is a CGA squared, and a VGA is an EGA squared ;-)

 Anyway, back to reality. Even though the abouve procedure is written in
 assembly language, it is slooow. Why? I hear your enquiring minds cry. The
 reason is simple : It uses interrupts (It calls INT 10h). Interrupts are
 sloooow ... which is okay for getting into MCGA mode, but not for trying
 to put down a pixel lickety-split. So, why not try the following ...

 Procedure MEMPutpixel (X,Y : Integer; Col : Byte);
 BEGIN
   Mem [VGA:X+(Y*320)]:=Col;
 END;

 The Mem command, as we have seen above, allows you to point at a certain
 point in memory ... the starting point is $a000, the base of the VGA's
 memory, and then we specify how far into this base memory we start.
 Think of the monitor this way. It starts in the top left hand corner at
 0. As you increase the number, you start to move across the screen to your
 right, until you reach 320. At 320, you have gone all the way across the
 screen and come back out the left side, one pixel down. This carries on
 until you reach 63999, at the bottom right hand side of the screen. This
 is how we get the equation X+(Y*320). For every increased Y, we must
 increment the number by 320. Once we are at the beginning of the Y line
 we want, we add our X by how far out we want to be. This gives us the
 exact point in memory that we want to be at, and then we set it equal to
 the pixel value we want.

 The MEM methood of putpixel is much faster, and it is shown in the sample
 program at the end of this lesson. The ASPHYXIA team uses neither putpixel;
 we use a DMA-Straight-To-Screen-Kill-Yer-Momma-With-An-Axe type putipixel
 which is FAST. We will give it out, but only to those of you who show us
 you are serious about coding. If you do do anything, upload it to me,
 I will be very interested to see it. Remember : If you do glean anything
 from these training sessions, give us a mention in your demos and UPLOAD
 YOUR DEMO TO US!

 Well, after this is the sample program; have fun with it, UNDERSTAND it,
 and next week I will start on fun with the pallette.

 See you all later,
     - Denthor

 {$X+}   (* This is a handy little trick to know. If you put this at the top
            of your program, you do not have to set a variable when calling
            a function, i.e. you may just say 'READKEY' instead of
            'CH:=READKEY'                                                *)

 USES Crt;           (* This has a few nice functions in it, such as the
                        READKEY command.                                 *)

 CONST VGA = $a000;  (* This sets the constant VGA to the segment of the
                        VGA screen.                                      *)

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 Procedure SetMCGA;  { This procedure gets you into 320x200x256 mode. }
 BEGIN
   asm
      mov        ax,0013h
      int        10h
   end;
 END;

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 Procedure SetText;  { This procedure returns you to text mode.  }
 BEGIN
   asm
      mov        ax,0003h
      int        10h
   end;
 END;

 {陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳}
 Procedure Cls (Col : Byte);
    { This clears the screen to the specified color }
 BEGIN
   Fillchar (Mem [$a000:0],64000,col);
 END;

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 Procedure INTPutpixel (X,Y : Integer; Col : Byte);
    { This puts a pixel on the screen using interrupts. }
 BEGIN
   asm
      mov        ah,0Ch
      mov        al,[col]
      mov        cx,[x]
      mov        dx,[y]
      mov        bx,[1]
      int        10h
   end;
 END;

 {陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳}
 Procedure TestINTPutpixel;
    { This tests out the speed of the INTPutpixel procedure. }
 VAR loop1,loop2 : Integer;
 BEGIN
   For loop1:=0 to 319 do
     For loop2:=0 to 199 do
       INTPutpixel (loop1,loop2,Random (256));
   Readkey;
   Cls (0);
 END;

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 Procedure MEMPutpixel (X,Y : Integer; Col : Byte);
   { This puts a pixel on the screen by writing directly to memory. }
 BEGIN
   Mem [VGA:X+(Y*320)]:=Col;
 END;

 {陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳}
 Procedure TestMEMPutpixel;
    { This tests out the speed of the MEMPutpixel procedure. }
 VAR loop1,loop2 : Integer;
 BEGIN
   For loop1:=0 to 319 do
     For loop2:=0 to 199 do
       MEMPutpixel (loop1,loop2,Random (256));
   Readkey;
   Cls (0);
 END;

 {陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳}
 BEGIN    (* Of the main program *)
   ClrScr;               { This clears the text Screen (CRT unit) }
   Writeln ('What will happen is that I will clear the screen twice. After');
   Writeln ('each clear screen you will have to hit a key. I will then fill');
   Writeln ('the screen twice with randomlly colored pixels using two different');
   Writeln ('methoods, after each of which you will have to hit a key. I will');
   Writeln ('then return you to text mode.');
   Writeln; Writeln;
   Write ('Hit any kay to continue ...');
   Readkey;

   SetMCGA;
   CLS (32);
   Readkey;
   CLS (90);
   Readkey;
   TestINTPutpixel;
   TestMEMPutpixel;
   SetText;

   Writeln ('All done. This concludes the first sample program in the ASPHYXIA');
   Writeln ('Training series. You may reach DENTHOR under the name of GRANT');
   Writeln ('SMITH on the MailBox BBS, or leave a message to ASPHYXIA on the');
   Writeln ('ASPHYXIA BBS. Get the numbers from Roblist, or write to :');
   Writeln ('             Grant Smith');
   Writeln ('             P.O. Box 270');
   Writeln ('             Kloof');
   Writeln ('             3640');
   Writeln ('I hope to hear from you soon!');
   Writeln; Writeln;
   Write   ('Hit any key to exit ...');
   Readkey;
 END.     (* Of the main program *)

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Date this article was posted to GameDev.net: 7/16/1999
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Denthor's Asphyxia Tutorials

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