Atari 5200 specific information for cc65

Christian Groessler

An overview over the Atari 5200 runtime system as it is implemented for the cc65 C compiler.

1. Overview

2. Binary format

3. Memory layout

4. Platform specific header files

5. Loadable drivers

6. Limitations

7. Other hints

8. License

1. Overview

This file contains an overview of the Atari 5200 runtime system as it comes with the cc65 C compiler. It describes the memory layout, Atari 5200 specific header files, available drivers, and any pitfalls specific to that platform.

Please note that Atari 5200 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. Binary format

The binary output format generated by the linker for the Atari 5200 target is a cartridge image. It is of course possible to change this behaviour by using a modified startup file and linker config.

3. Memory layout

cc65 generated programs with the default setup use the RAM space from $021C to $3FFF. If you want to reserve memory for the display list and screen buffer you should define the __RESERVED_MEMORY__ linker variable. The number of bytes specified by __RESERVED_MEMORY__ are lowering the top of memory, therefore the available RAM memory for the program is $021C to $3FFF-__RESERVED_MEMORY__. The default linker config file sets __RESERVED_MEMORY__ to $1E0 to reserve space for an optional CONIO text screen.

Special locations:

Text screen

The text screen is only enabled if any of the CONIO output functions is used in the program. Its size is 20x24 characters (Antic mode 6, BASIC mode 1) by default. The text screen is located at $3E00. The address of the screen memory is available at runtime in the variable SAVMSC ($001B).

If the program doesn't use any CONIO output functions it needs to setup its own display list.


The C runtime stack is located at $3FFF-__RESERVED_MEMORY__ and growing downwards.


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

4. Platform specific header files

Programs containing Atari 5200 specific code may use the atari5200.h header file.

This also includes access to operating system locations (e.g. hardware shadow registers) by a structure called "OS". The names are the usual ones you can find in system reference manuals. Example:

    OS.sdmctl = 0x00;                       // screen off
    OS.color4 = 14;                         // white frame
    tics = OS.rtclok[1]                     // get ticks

4.1 Atari 5200 specific functions

4.2 Hardware access

The following pseudo variables declared in the atari5200.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.


The GTIA_READ structure allows read access to the GTIA. The GTIA_WRITE structure allows write access to the GTIA. See the _gtia.h header file located in the include directory for the declaration of the structure.


The POKEY_READ structure allows read access to the POKEY. The POKEY_WRITE structure allows write access to the POKEY. See the _pokey.h header file located in the include directory for the declaration of the structure.


The ANTIC structure allows read access to the ANTIC. See the _antic.h header file located in the include directory for the declaration of the structure.

5. Loadable drivers

All drivers must be statically linked because no disk I/O is available. The names in the parentheses denote the symbols to be used for static linking of the drivers.

5.1 Graphics drivers

No graphics drivers are currently available for the Atari 5200.

5.2 Extended memory drivers

No extended memory drivers are available for the Atari 5200.

5.3 Joystick drivers (atr5200std_joy)

A joystick driver for the standard Atari 5200 joystick is available. Depending on the version of the 5200 console, two or four joysticks can be attached.

5.4 Mouse drivers

No mouse drivers are available for the Atari 5200.

5.5 RS232 device drivers

No serial drivers are available for the Atari 5200.

6. Limitations

6.1 Direct console I/O

The atari5200 target uses Antic mode 6 (BASIC mode 1) for the console screen by default. There are four colors available:

Note that the COLOR_GREEN and COLOR_RED colors aren't exactly the same colors as the ones with the same name on the atari target. They are the colors which are available as COLOR_LIGHTGREEN and COLOR_LIGHTRED there.

One can set the color shadow registers directly with other colors. Then the color defines from above will just become placeholders. In this scenario it might be more convenient to use index values (0..3) instead of the color defines. The index values specify which of the system shadow color registers (COLOR0 .. COLOR3) to use.

The default console screen has a layout of 20x24 characters. An alternative layout, 20x12, Antic mode 7, BASIC mode 2, is provided in the file atari5200-conioscreen-20x12.o.

Using atari5200-conioscreen-20x12.o is as simple as placing it on the linker command line like this:

cl65 -t atari5200 myprog.c atari5200-conioscreen-20x12.o

6.2 Disk I/O

Disk I/O is not supported by the atari5200 target. This means that you cannot use any of the following functions (and a few others):

7. Other hints

7.1 CAR format

AtariROMMaker ( ) can be used to create a .CAR file from the binary ROM image cc65 generates. This might be more convenient when working with emulators.

7.2 Changing the splash screen

The 5200 ROM displays a splash screen at startup with the name of the game and the copyright year. The year information has a 'Year-2000' problem, the first two digits are fixed in the ROM and are always "19".

Changing the game name

The runtime library provides a default game name which is "cc65 compiled". To change that, one has to link a file which puts data into the "CARTNAME" segment.

For reference, here's the default version used by the cc65 library:

.export         __CART_NAME__: absolute = 1
.macpack        atari
.segment        "CARTNAME"
                scrcode "   cc"
                .byte   '6' + 32, '5' + 32      ; use playfield 1
                scrcode " compiled"

'__CART_NAME__' needs to be defined in order that the linker is satisfied and doesn't try to include the version of the runtime library.

20 bytes are available in the CARTNAME segment (one line) for the game/program name.

Changing the copyright year / changing the cartridge type

The century is hard-coded to 1900 by the ROM.

There are two digits which can be changed. For example "92" will give "1992" on the screen.

The default used by the runtime library is

.export         __CART_YEAR__: absolute = 1
.segment        "CARTYEAR"
                .byte   '9' + 32,'8' + 32       ; "98", using playfield 1

If the second byte of the year in the CARTYEAR segment is 255, the cartridge is seen as a 'diagnostic' cartridge, and the splash screen and most of the other startup initializations are bypassed.

8. 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.