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How to port Atari800 to new platforms
=====================================
This is a guide for people who intend to create a new version of Atari800
for their computer or operating system (called "target system" throughout
this document).
Prerequisites
-------------
The target system must have a raw computing power at least comparable
to a Pentium 100, otherwise the emulator will run slower than the original
8-bit Atari. Other hardware requirements are usually easier to fulfill:
a few megs RAM, a few megs disk space, a display, some keys and an audio
output.
You should have a working compiler of the C programming language
for the target system. The compiler, however, can run on another system
(such as PC), which is called cross-compiling and is common when developing
for small devices.
You should understand at least the basics of programming in C.
The more you know, the better.
Getting started
---------------
Try compiling an existing version of Atari800 for a system different
from your target system, just to learn how this works.
See the INSTALL file for instructions.
Try compiling something for your target system: a "Hello world" program
or (preferrably) a game. In case of problems, ask people who develop
for your target system. If you are not an expert of programming
the target system, get in touch with more experienced developers,
as you may need their advice sooner or later.
Compile-time configuration
--------------------------
Before you start feeding your C compiler with *.c files,
you need a file named "config.h". This file specifies the system
to compile for, the C library functions that are available and features
to include. There are two methods of creating this file: manually
and using the configure script.
Using the configure script is recommended, because it makes your port
easier to maintain for people who have never seen your target system.
Yet if you find it too hard to use the configure script, go on with
editing config.h by hand and consider using the configure script later.
The configure script is works well on Unix and GNU/Linux systems,
but can be awkward to use on Windows, especially if you use
an Integrated Development Environment and not command-line tools.
The configure script is the file named "configure" that should be run
by a Unix shell program such as "bash". The file is hardly human-readable.
It is generated from the source file "configure.ac" by a tool called
"autoconf". Another tool "autoheader" turns "configure.ac" into
"config.h.in", which is a template for "config.h" used by "configure".
"autogen.sh" is a shortcut that simply runs "autoheader" and "autoconf".
If you opt to edit "config.h" by hand, make a copy of "config.h.in"
under the name "config.h" and edit it changing some #undef's into #define's.
Consult the comments and other config.h files created this way, in "dc",
"win32/msc" and "wince/port" directories.
Compiling for the first time
----------------------------
Try starting with something simple. Disable sound emulation,
either by passing "--without-sound" to the configure script or by #undef'ing
sound items in "config.h".
The easiest way to start is to compile a "simple" version of the emulator,
that does not use any external libraries/interfaces for emulation of video,
audio and input, and uses only standard C library functions. With the
configure script you specify "--without-video --without-sound". If editing
"config.h", "#define BASIC 1" and figure out which files to compile.
The "simple" version does not support graphics or joysticks and has a very
limited keyboard input. Most likely it will appear as a blank screen when
run. That's fine. Edit the runtime configuration file providing paths to
Atari ROM images and change "DISABLE_BASIC" to "0". If you're lucky, you'll
see the "READY" prompt of Atari BASIC.
Implementing platform-specific part
-----------------------------------
Create a new source file with your platform-specific code.
You have several examples to copy-and-paste from.
The entry point to the program normally looks like this:
int main(int argc, char **argv)
{
/* initialise Atari800 core */
if (!Atari800_Initialise(&argc, argv))
return 3;
/* main loop */
for (;;) {
key_code = Atari_Keyboard();
Atari800_Frame();
if (display_screen)
Atari_DisplayScreen();
}
}
You need to implement functions specified in "platform.h".
These functions should display graphics on screen and read keyboard
and joysticks/mouse in a platform-specific way.
Graphics
--------
Atari_Initialise() is the function that does platform-specific initialization.
You may need to switch the graphics mode in this function.
Atari_DisplayScreen() should update the screen with the contents
of the atari_screen array. atari_screen is declared in screen.h as (ULONG *),
but it is really a pointer to a rectangular array of bytes. There are
240 (ATARI_HEIGHT) rows, 384 (ATARI_WIDTH) bytes each. While there are exactly
240 rows to display, only 336 middle columns of the 384 are meaningful.
Do never display more than 336 middle columns! Although the remaining columns
may appear black when you start the emulator, there's no such guarantee
when the emulator is running.
It is fine to display only 320 middle columns if that's easier for you than 336.
You can also crop the Atari graphics from the top and the bottom, because most
programs use just the 192 middle lines. If your display has considerably lower
resolution, you need to provide your own bitmap rescaling.
Bytes in atari_screen[] are Atari color codes (0-15 = shades of gray,
16-31 = shades of brown/yellow, etc). 8-bit Ataris have a fixed palette
of 256 colors. You can get 8-bit RGB components of Atari colors
with the macros Palette_GetR(code), Palette_GetG(code) and Palette_GetB(code),
defined in "colours.h".
Keyboard input
--------------
There are three variables that are read by the emulation core and should be
set by your platform-specific code:
int key_code;
int key_shift;
int key_consol;
key_code is the Atari scancode of the currently pressed keystroke, or one
of the emulator-defined special values, such as AKEY_NONE, AKEY_COLDSTART,
AKEY_BREAK, etc. Atari scancodes are 8-bit values with 6 low bits containing
the code of a regular key and two high bits reflecting the status of Shift
and Control modifiers. See input.h or some Atari documentation for details.
Codes that are special to the emulator are AKEY_NONE (which means no key
pressed, or just the Shift or the Control) and AKEY_* values defined
in "atari.h".
key_shift should be TRUE when any Shift key is pressed, FALSE otherwise.
In 8-bit Atari world the term "console keys" refers to the three function keys
(Start, Select and Option) commonly used in games. The keys are independent
of each other and of all other keys. The state of each console key should
be reflected by one bit in key_consol (cleared if the key is pressed).
See "input.h".
Joystick input
--------------
Four Atari joysticks are emulated, but few Atari games support more than two.
Each joystick has one fire button. Your Atari_TRIG() is passed the joystick
number (0-3) and should return 0 if the corresponding fire button is pressed
and 1 if it's not.
The joysticks are "digital", that is, they can be moved in one of 8 directions
from the center position. Joystick positions are passed in a bit obscure way.
Atari_PORT(0) should return the position of joysticks 0 and 1, Atari_PORT(1)
- the position of joysticks 2 and 3. The returned values are 8-bit,
four bits per joystick. The acceptable values of the four bits are defined
as STICK_* constants in "input.h".
Mouse input
-----------
Nearly all Atari programs are controlled by keyboard or joysticks.
Other controllers were much less popular. However, it is easy to support
them in your version of Atari800.
Paddles, touch tablet, light pen and other devices can be emulated
by a pointing device such as mouse. In the main emulation loop your code
should update the variables mouse_delta_x, mouse_delta_y and mouse_butons.
mouse_delta_x and mouse_delta_y are *relative* positions, that is,
how far the mouse was moved since last time. mouse_buttons is composed
of 3 bits that represent left, right and middle mouse button (1 = pressed).
To test mouse input, select "Light pen" as the emulated device
and press right mouse button to enable the mouse pointer maintained
by the emulator.
Final words
-----------
That's the end of this guide. I hope it's useful for developers new
to Atari800. Suggestions about what to add or improve are welcome.
If you are serious about porting Atari800, please join our mailing list.
See the documentation or the web page for details.
Happy hacking!
Piotr Fusik
November 5, 2006
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