WHITE LIGHTNING
by Oasis Software


INTRODUCTION
by Mike Butler

White Lightning is a high level graphics development system for the
Spectrum 48k. It is aimed primarily at the user who has commercial games
writing in mind and has the patience to learn a sizeable new language. It
is not a games designer and stunning results probably won't be produced
overnight, but it does have the power and flexibility to produce software
of a commercial standard (with a little perseverance!). [...]

Assembly language has three advantages over high level languages: speed,
flexibility and compactness. During the running of an arcade game, the
processor spends most of its time manipulating screen data, and if the
appropriate commands are implemented in the language, the execution
"overhead" is very small. Add to this the fact that considerable time has
been spent on the routines themselves to optimise execution speed, and we
feel most machine code programmers would be hard pressed to better White
Lightning for speed. As far as flexibility is concerned, White Lightning
has almost 300 commands as well as access to BASIC and machine language if
required. A lot of the tricky routines like rotations and enlargements are
already implemented for you. As far as compactness goes, Forth itself
produces very compact code, but there is, of course, the overhead of the
language itself. Assembly language has four major drawbacks. Firstly,
you've got to learn it. Having mastered machine code, program development
is very slow compared with a typical high level language; there is no
"crash protection" whatsoever; and to produce effective results you need
a fairly intimate knowledge of the machine you're working with.

BASIC has several points in its favour; these are: excellent crash
protection, extremely readable source code and a relatively short learning
curve. These features make BASIC a very good introduction to programming
for the hobbyist, but for the serious games writer the language is
insufficient in terms of both its speed and flexibility.

Because White Lightning is Forth based it has virtually the speed of
machine code, no knowledge of the machine is required, the source code is
relatively readable, and it is fairly well protected from crashing.

[...]


SPRITE DEVELOPMENT

Included with White Lightning is a sprite generator. This comes complete
with a pre-defined character set which, when suitably combined, makes up
to 167 full characters. The pre-defined characters cover Asteroids,
Pac-Man, Scramble, Defender, Space Invaders, City Bomber, Lunar Lander,
Frogger, Centipede, Donkey Kong and many more. You can use them as they
are, customise them, or design up to 255 of your own sprites. The
development software allows you to reflect, spin or invert. When you have
finished work, or between sessions, the whole lot can be simply saved to
tape.


IDEAL

The main part of the package is the White Lightning language itself. The
language can be thought of as being divided into two parts: firstly, there
is a super-fast integer Forth, which conforms to a standard Fig-Forth; but
secondly, and of most importance to games designers, there is the IDEAL
sub-language. IDEAL stands for "Interrupt Driven Extendible Animation
Language". IDEAL has a dictionary of over 100 words, which can be freely
mixed with Forth, or, as we shall see, can be accessed from BASIC.


Interrupt Driven

Forth/IDEAL words can be executed under interrupt; this means that
programs can be run in foreground and background at the same time.
Suppose, for instance, the program you are writing involves a scrolling
backdrop, which has been defined in a sprite 6 screens wide. A program can
be run in background to handle the scrolling backdrop, and a separate
program written in foreground to control all of the characters which move
within the backdrop. This will free the user from complex timing
calculations to get a smooth scroll and is one of the most powerful
features of the entire package. Background words can be executed up to 50
times a second.


Extendible

Forth is extendible and was chosen as the most suitable host language for
IDEAL because of this extremely useful feature. New words can be defined
in terms of any of the Forth/IDEAL words, or your own previously defined
words. This means that you can create diagonal scrolls, for instance, by
combining individual scrolls.


Animation Language

Very careful planning went into the designing of the IDEAL animation
language, to make it as portable as possible between micros. If you've
written a very successful program for one micro, you can move the same
code across without too much difficulty. The only major changes necessary
are changes to accommodate the different screen formats; ie. the number of
columns and rows. You don't need to worry about the complexities of
various memory mappings; IDEAL does that for you. Some implementations will
not allow attributes to be moved separately from pixel data, so to be
certain of higher portability, move attributes and data together.


ACCESS FROM BASIC

If you are not familiar with Forth and want to get reasonable software
quickly, you can access the IDEAL language from BASIC. Programs will no
longer be portable and you won't get quite the same speed and polish, but
perfectly good programs can be, and have been, written this way. More
memory will be used for BASIC source, so bear all this in mind before
deciding to put off learning Forth! Most of the useful BASIC commands that
handle sound and graphics, such as DRAW, CIRCLE, BEEP and so on, have been
implemented in Spectra Forth. Whenever possible, call them from Forth and
not BASIC. The interpretation and initial floating point manipulation of
BASIC commands are avoided, and commands will therefore execute more
rapidly from Forth.


OPERATING INSTRUCTIONS

1. Rewind the tape marked "White Lightning". Disconnect Interface 1
   if fitted.
2. Load using LOAD "" then stop the tape. White Lightning will auto-run.
3. Once loaded, you will get the prompt "LOAD SPRITES Y/N".
4. If it's the first time you've used the package, type Y to load the
   demonstration sprites, which will follow directly after White Lightning
   on the same tape. There is a section of data before the sprites which
   doesn't load, but don't worry about this; this is information used by
   the sprite development package. Once loaded, the "LOAD SOURCE Y/N"
   prompt will appear. Press N to enter White Lightning or, if you wish to
   load some source code, place the appropriate cassette in the recorder,
   press PLAY and then type Y.
5. To RUN the demonstrations, just LOAD using LOAD "" and the program will
   auto-run.
6. To RUN the Sprite Development Software just type LOAD "" and again the
   program will auto-run.


[...]


-- end of introduction