│         W E L C O M E         │
                   │  To the VGA Trainer Program   │ │
                   │              By               │ │
                   │      DENTHOR of ASPHYXIA      │ │ │
                   ╘═══════════════════════════════╛ │ │
                     ────────────────────────────────┘ │

                           --==[ PART 10 ]==--

■ Introduction

Wow! The trainer has finally reached part 10! This will also be the
first part introduced simultaneously to local BBS's and the INTERNET at
the same time! Yes folks, I put up a copy of previous tutorials onto
various ftp sites, and awaited the flames saying that the net.gurus
already knew this stuff, and why was I wasting disk space! The flames
did not appear (well, except for one), and I got some messages saying
keep it up, so from now on I will upload all future trainers to ftp
sites too (wasp.eng.ufl.edu , cs.uwp.edu etc.). I will also leave a
notice in the USENET groups comp.lang.pascal and comp.sys.ibm.pc.demos
when a new part is finished (Until enough people say stop ;-))

I can also be reached at my new E-Mail address,

Well, this tutorial is on Chain-4. When asked to do a trainer on
Chain-4, I felt that I would be walking on much travelled ground (I have
seen numerous trainers on the subject), but the people who asked me said
that they hadn't seen any, so could I do one anyway? Who am I to say no?

The sample program attached isn't that great, but I am sure that all you
people out there can immediately see the potential that Chain-4 holds.

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/Denthor/Asphyxia in private mail
                  on the ASPHYXIA BBS.
            2) Write to Denthor, EzE or Goth on Connectix.
            3) Write to :  Grant Smith
                           P.O.Box 270 Kloof
                           South Africa
            4) Call me (Grant Smith) at (031) 73 2129 (leave a message if you
                  call during varsity). Call +27-31-73-2129 if you call
                  from outside South Africa. (It's YOUR phone bill ;-))
            5) Write to smith9@batis.bis.und.ac.za in E-Mail.

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 Chain-4?

You people out there all have at least 256k vga cards. Most of you have
512k vga cards, and some have 1MB vga cards. But what you see on your
screen, as discussed in previous trainers, is 64k of data! What happened
to the other 192k??? Chain-4 is a method of using all 256k at one time.

The way this is done is simple. 1 screen = 64k. 64k * 4 = 256k.
Therefore, chain-4 allows you to write to four screens, while displaying
one of them. You can then move around these four screens to see the data
on them. Think of the Chain-4 screen as a big canvas. The viewport,
the bit you see out of, is a smaller rectangle which can be anywhere
over the bigger canvas.

     +----------------------------+ Chain-4 screen
     |          +--+              |
     |          |  | <- Viewport  |
     |          +--+              |
     |                            |

■ The size of the chain-4 screen

The Chain-4 screen, can be any size that adds up to 4 screens.

For example, it can be 4 screens across and one screen down, or one
screen across and 4 screens down, or two screens across and two screens
down, and any size in between.

In the sample program, the size is a constant. The size * 8 is how many
pixels across there are on the chain-4 screen, ie
   Size = 40   = 320 pixels across = 1 screen across, 4 screens down
   Size = 80   = 640 pixels across = 2 screens across, 2 screens down

We need to know the size of the screen for almost all dealings with the
Chain-4 screen, for obvious reasons.

■ Layout of the chain-4 screen, and accessing it

If you will remember all the way back to Part 1 of this series, I
explained that the memory layout of the MCGA screen is linear. Ie, the
top left hand pixel was pixel zero, the one to the right of it was
number one, the next one was number two etc. With Chain-4, things are
very different.

Chain-4 gets the 4 screens and chains them together (hence the name :)).
Each screen has a different plane value, and must be accessed
differently. The reason for this is that a segment of memory is only 64k
big, so that we could not fit the entire Chain-4 screen into one

All Chain-4 screens are accessed from $a000, just like in MCGA mode.
What we do is, before we write to the screen, find out what plane we are
writing to, set that plane, then plot the pixel. Here is how we find out
how far in to plot the pixel and what plane it is on :

 Instead of the linear model of MCGA mode, ie :
        │00│01│02│03│04│05│06│07│08│09│10│11│ ...

 Each plane of the Chain-4 screen accesses the memory in this way :

       Plane 0 :
        │00│  │  │  │01│  │  │  │02│  │  │  │ ...

       Plane 1 :
        │  │00│  │  │  │01│  │  │  │02│  │  │ ...

       Plane 2 :
        │  │  │00│  │  │  │01│  │  │  │02│  │ ...

       Plane 3 :
        │  │  │  │00│  │  │  │01│  │  │  │02│ ...

In this way, by choosing the right plane to write to, we can access all
of the 256k of memory available to us. The plane that we write to can
easily be found by the simple calculation of  x mod 4, and the x
coordinate is also found by  x div 4. We work out our y by multiplying
it by the size of our chain-4 screen.

NOTE : It is possible to write to all four planes at once by setting the
       correct port values.

■ Uses of Chain-4

The uses of Chain-4 are many. One could write data to one screen, then
flip to it (the move_to command is almost instantaneous). This means
that 64k of memory does not need to be set aside for a virtual screen,
you are using the vga cards memory instead!

Scrolling is much easier to code for in Chain-4 mode.

It is possible to "tweak" the mode into other resolutions. In our demo,
our vectors were in 320x240 mode, and our dot vectors were in 320x400

The main disadvantage of chain-4 as I see it is the plane swapping,
which can be slow. With a bit of clever coding however, these can be
kept down to a minimum.

■ The sample programs

The first sample program is GFX.PAS. This is a until in which I have
placed most of our routines from previous tuts. All the procedures and
variables you can see under the INTERFACE section can be used in any
program with GFX in the USES clause. In other words, I could do this :


  cls (vaddr,0);

This program would compile perfectly. What I suggest you do is this :
Rename the file to a name that suites you (eg your group name), change
the first line of the unit to that name, then add all useful procedures
etc. to the unit. Make it grow :-).

The second file is the sample program (note the USES GFX,crt; up near
the top!). The program is easy to understand and is documented. The bit
that I want to draw your attention to is the constant, BIT. Because I
am distributing this file to many places in text form, not binary form,
I could not just add a .CEL file with the program. So what I did was
write some text in one color then saved it as a .CEL . I then wrote a
ten line program that did the following : Moving from left to right, it
counted how many pixels were of color zero, then saved the byte value to
an array. When it came across color one, is counted for how long that
went on then saved the byte value and saved it to an array and so on.
When it was finished, I converted the array into a text file in the
CONST format. Not too cunning, but I thought I had better explain it ;-)

■ In closing

There are other documents and sample programs available on Chain-4 and
it's like : Try XLIB for one...

Finally! Some BBS's have joined my BBS list! (Okay, only two new ones,
but it's a start ;-)) All you international BBS's! If you will regularly
download the tuts from an FTP site, give me your names!

I own a car. The car's name is Bob. A few days ago, Bob was in an
accident, and now has major damage to his front. Knowing insurance, I
probably won't get much, probably nothing (the other guy wasn't insured,
and I am only 18 :( ). I will probably have to find work in order to pay
for my repairs. The point to this meandering is this : I am upset, so if
you think you are getting a quote, you can just forget it.

Oh, well. Life goes on!

See you next time,
  - Denthor

These fine BBS's carry the ASPHYXIA DEMO TRAINER SERIES : (alphabetical)

║BBS Name                  ║Telephone No.   ║Open ║Msg║File║Past║
║ASPHYXIA BBS #1           ║(031) 765-5312  ║ALL  ║ * ║ *  ║ *  ║
║ASPHYXIA BBS #2           ║(031) 765-6293  ║ALL  ║ * ║ *  ║ *  ║
║Connectix BBS             ║(031) 266-9992  ║ALL  ║   ║ *  ║ *  ║
║POP!                      ║(012) 661-1257  ║ALL  ║   ║ *  ║ *  ║
║Pure Surf BBS             ║(031) 561-5943  ║A/H  ║   ║ *  ║ *  ║

For international users : If you live outside the Republic of South
Africa, do the following : Dial +27, dont dial the first 0, but dial
the rest of the number. Eg, for the ASPHYXIA BBS : +27-31-765-5312

Open = Open at all times or only A/H
Msg  = Available in message base
File = Available in file base
Past = Previous Parts available