Atari Lynx specific information for cc65

Karri Kaksonen,
Ullrich von Bassewitz

2014-04-12
An overview over the Atari Lynx runtime system as it is implemented for the cc65 C compiler.

1. Overview

2. Building your first Hello World application

3. Binary format

4. Memory layout

5. Platform specific header files

6. Loadable drivers

7. Limitations

8. Cart access

9. License


1. Overview

This file contains an overview of the Atari Lynx runtime system as it comes with the cc65 C compiler. It describes the memory layout, Lynx specific header files, available drivers, and any pitfalls specific to that platform.

Please note that Lynx specific functions are just mentioned here, they are described in detail in the separate function reference. Even functions marked as "platform dependent" may be available on more than one platform. Please see the function reference for more information.

2. Building your first Hello World application

Here is a small traditional Hello World program for the Atari Lynx.

#include <lynx.h>
#include <tgi.h>
#include <6502.h> 

void main(void) {
  tgi_install(tgi_static_stddrv);
  tgi_init();
  CLI();
  while (tgi_busy())
    ;
  tgi_clear();
  tgi_setcolor(COLOR_GREEN);
  tgi_outtextxy(0, 0, "Hello World");
  tgi_updatedisplay();
  while (1)
    ;
}

The lynx.h contains all kind of system dependent things.

The tgi.h contains the graphics driver functions.

The 6502.h is needed for executing the CLI() command.

As the Atari Lynx does not have ASCII characters available you need to use the Tiny Graphics Interface library for producing letters on the screen.

The cc65 compiler suite has a graphics library called "Tiny Graphics Interface". This interface has some relocatable code. In order to use this in your own program you need to load it at run time.

Unfortunately the Lynx does not have a disk drive from where to load it. Therefore you must already load it at compile time. The easiest way is to automatically link it in statically from the Lynx C library.

cl65 -t lynx -o game.lnx main.c

This will create a bootable cart image called game.lnx

3. Binary format

The standard binary output format generated by the linker for the Lynx target is a cart image. By specifying the config file lynx-bll.cfg the linker will generate BLL download compatible binary files.

It is of course possible to change this behaviour by using a modified startup file and linker config.

The bootloader used in the cc65 lynx library uses a very minimal bootloader that does not check the cart or show a title screen.

The advantage of this bootloader is that it allows creation of cart images to many common formats.

Cart sizes

Block size Rom size Description
512 bytes  128k     Standard old games like Warbirds
1024 bytes 256k     Most common format for homebrew. Also newer games like Lemmings
2048 bytes 512k     Largest games like EOTB

4. Memory layout

cc65 generated programs with the default setup run with the I/O area and the kernal enabled, which gives a usable memory range of $200 - $C037.

Special locations:

  0000 - 00FF Zero page
  0100 - 01FF Machine stack

  A058 - C037 Collision buffer
  C038 - E017 Screen buffer 1
  E018 - FFF7 Screen buffer 0
  FFF8 - FFFF Hardware vectors

Text screen

No conio support is currently available for the Lynx.

Keyboard

The Lynx "flabode" keys, Opt 1, Pause and Opt 2 are implemented using the conio interface. The only characters the keyboard is able to produce are 'R' for Restart (Opt 1 + Pause), 'F' for flip (Opt 2 + Pause), 'P' for pause, '1' for Opt 1, '2' for Opt 2, '3' for Opt 1 + Opt 2 and '?' for all keys down at the same time.

Stack

The C runtime stack is located at $C037 (or $A057 if collision detection is enabled) and growing downwards.

Heap

The C heap is located at the end of the program and grows towards the C runtime stack.

Screen

The collision detection screen is at $A058 if it is enabled. The double buffered screens are at $C038 and $E018.

5. Platform specific header files

Programs containing Lynx specific code may use the lynx.h header file.

5.1 Lynx specific functions

5.2 Hardware access

The following pseudo variables declared in the lynx.h header file do allow access to hardware located in the address space. Some variables are structures, accessing the struct fields will access the chip registers.

MIKEY

The MIKEY structure allows access to MIKEY chip. See the _mikey.h header file located in the include directory for the declaration of the structure.

SUZY

The SUZY structure allows access to SUZY chip. See the _suzy.h header file located in the include directory for the declaration of the structure.

6. Loadable drivers

The names in the parentheses denote the symbols to be used for static linking of the drivers.

6.1 Graphics drivers

lynx-160-102-16.tgi (lynx_160_102_16_tgi)

A TGI driver for the standard graphics mode (160×102 in 16 colors).

The TGI driver is implemented as an interrupt driven dual buffering device. To use it as a single-buffer device set draw page and view page to the same value 0 or 1;

The TGI driver has a few Lynx-specific extensions.

Calling tgi_sprite(spr) or tgi_ioctl(0, spr) will display a standard Lynx sprite on screen.

Calling tgi_flip() or tgi_ioctl(1, 0) will do a flip screen.

Calling tgi_setbgcolor(bgcolor) or tgi_ioctl(2, bgindex) will set the text background color to the index defined by bgindex. If bgindex is 0 then the background color is transparent.

To set the framerate of the display hardware call tgi_setframerate(rate) or tgi_ioctl(3, rate). The supported framerates are 50, 60 and 75 frames per second. Actually there is no real reason to use anything else than 75 frames per second.

To check if the drawing engine is busy with the previous swap you can call tgi_busy or tgi_ioctl(4, 0). It returns 0 if idle and 1 if busy

To update displays you can call tgi_updatedisplay() or tgi_ioctl(4, 1) it will wait for the next VBL interrupt and set the draw buffer to the view buffer. The draw buffer is also changed to (drawbuffer xor 1).

You can also enable or disable collision detection by a call to tgi_setcollisiondetection(active) or tgi_ioctl(5, active). The collision result is located before the sprite structure by default in this driver.

In order to reserve memory for the collision detection buffer you need to specify lynx-coll.cfg as the configuration file to the linker.

6.2 Extended memory drivers

No extended memory drivers are currently available for the Lynx.

6.3 Joystick drivers

lynx-stdjoy.joy (lynx_stdjoy_joy)

A joystick driver for the standard buttons.

6.4 Mouse drivers

No mouse drivers are currently available for the Lynx.

6.5 RS232 device drivers

lynx-comlynx.ser (lynx_comlynx_ser)

A serial driver for the ComLynx port.

The ComLynx port has Tx and Rx wired together. Every byte is sent to all connected Lynxes. Only one Lynx can send at a time. There is no protocol created for communication. You are on your own.

If the Lynx returns framing error then it is likely that another Lynx is sending data at the same time.

The Lynx can also send a break and receive a break. The Lynx break is recognized if the bit is down for 24 bit cycles or more.

To send a break you just set the break bit. The length of the break depends on how long this bit is down.

The driver supports the baudrates:

The parity bit supports MARK and SPACE. It also supports EVEN and ODD parity but the parity bit is included in the calculation. Most of us don't want it this way. But there is nothing we can do about it.

The Lynx hardware will always check parity on incoming traffic. Currently the driver cannot receive data from standard PC's due to this parity bug. For working with Lynx to Lynx communication use EVEN parity.

To send data to standard PC's use MARK or SPACE as parity setting.

There is always only one stop bit. And the data length is always 8 bits.

We have no handshaking available. Even software handshake is impossible as ComLynx has only one wire for the data.

Both transmit and receive are interrupt driven.

7. Limitations

8. Cart access

At this point in time there is no support for the cart filesystem yet. I have a lynx-cart-demo example project that uses an interrupt driven display, has support for the cart filesystem and an abcmusic sound module.

At some point in time we may find a way to rewrite these to fit the way the cc65 drivers require. But for the time being you can create less portable applications using these Lynx specific modules in lynx-cart-demo.

9. License

This software is provided 'as-is', without any expressed or implied warranty. In no event will the authors be held liable for any damages arising from the use of this software.

Permission is granted to anyone to use this software for any purpose, including commercial applications, and to alter it and redistribute it freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not claim that you wrote the original software. If you use this software in a product, an acknowledgment in the product documentation would be appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.