Oric Atmos-specific information for cc65

Ullrich von Bassewitz,
Stefan A. Haubenthal,
Greg King

An overview over the Atmos 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 Atmos runtime system as it comes with the cc65 C compiler. It describes the memory layout, Atmos-specific header files, available drivers, and any pitfalls specific to that platform.

Please note that Atmos-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 standard binary output format generated by the linker for the Atmos target is a machine language program with a one-line BASIC stub that jumps to the machine-language part through CALL. It has one sacrificial byte attached to the end (a bug in the Oric ROM means that BASIC can put a variable on top of the last byte that was loaded). It has a 24-byte tape header. A file can be CLOADed as a BASIC program, and started by typing RUN. The standard load address is $501.

3. Memory layout

In the standard setup, cc65-generated programs use the memory from $0501 to $9800; so, nearly 37K of memory (including the stack) is available. ROM calls are possible without further precautions.

If your program needs more memory, and it won't use TGI graphics, then you can use the ld65 command-line option, -D __GRAB__=1, when building the program, to include the graphics screen RAM. Then, nearly 44K of memory ($0501 to $B400) is available.

Special locations:


The C runtime stack is located at $97FF (or $B3FF), and grows 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 Atmos-specific code may use the atmos.h header file.

4.1 Atmos-specific functions

The functions listed below are special for the Atmos. See the function reference for declaration and usage.

4.2 Hardware access

The following pseudo variables declared in the atmos.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.


Access to the VIA (Versatile Interface Adapter) chip is available via the VIA variable. The structure behind this variable is explained in _6522.h.

5. Loadable drivers

Note: Since the Atmos doesn't have working disk I/O (see section "Limitations"), the available drivers cannot be loaded at runtime (so the term "loadable drivers" is somewhat misleading). Instead, the drivers have to be statically linked. While this may seem overhead, it has two advantages:

  1. The interface is identical to the one used for other platforms and to the one for the Atmos once it has disk I/O.
  2. Once disk I/O is available, existing code can be changed to load drivers at runtime with almost no effort.

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

5.1 Graphics drivers

The default drivers, tgi_stddrv (tgi_static_stddrv), point to atmos-240-200-2.tgi (atmos_240_200_2_tgi).

atmos-228-200-3.tgi (atmos_228_200_3_tgi)

This driver was written by Greg King and Stefan Haubenthal. It features a resolution of 228×200 with a palette of two colors that can be chosen from the Atmos's eight colors. The driver supports a third palette-"color" that actually "flips" the pixel (it becomes the other color) that is on the screen under the graphics cursor.

atmos-240-200-2.tgi (atmos_240_200_2_tgi)

This driver was written by Stefan Haubenthal and Greg King. It features a resolution of 240×200 with black and white colors. It is the default graphics driver for the Atmos.

5.2 Extended memory drivers

No extended memory drivers are currently available for the Atmos.

5.3 Joystick drivers

atmos-pase.joy (atmos_pase_joy)

Supports two standard joysticks connected to a P.A.S.E. / Altai interface of the Atmos.

atmos-ijk.joy (atmos_ijk_joy)

Supports two standard joysticks connected to an IJK interface of the Atmos.

5.4 Mouse drivers

No mouse drivers are currently available for the Atmos.

5.5 RS232 device drivers

atmos-acia.ser (atmos_acia_ser)

Driver for the Telestrat integrated serial controller and the Atmos with a serial add-on. Supports up to 19200 baud, requires hardware flow control (RTS/CTS) and does interrupt driven receives. Note that, because of the peculiarities of the 6551 chip, together with the use of the NMI, transmits are not interrupt driven; and, the transceiver blocks if the receiver asserts flow control because of a full buffer.

6. Limitations

6.1 Disk I/O

The existing library for the Atmos doesn't implement C file I/O. There are hacks for the read() and write() routines in place, which will make functions work that read from stdin and write to stdout and stderr (such as printf()). However, those functions have some shortcomings which won't be fixed, because they're going to be replaced anyway.

To be more concrete, that limitation means that you cannot use any of the following functions (and a few others):

7. Other hints

7.1 Function keys

They are defined to be FUNCT + a number key.

7.2 Capitals lock

The keyboard's "CAPS Lock" mode is turned off while the program is running. The previous mode (usually, CAPS Lock turned on [because Oric BASIC keywords must be UPPER-case]) is restored when the program stops.

7.3 Passing arguments to the program

Command-line arguments can be passed to main(). Since that is not supported directly by BASIC, the following syntax was chosen:

    RUN:REM arg1 " ARG2 IS QUOTED" ARG3 "" ARG5

  1. You must turn CAPS lock off (tap CTRL-T) when you want to type lower-case arguments (but, RUN and REM must be UPPER-case).
  2. Arguments are separated by spaces.
  3. Arguments may be quoted.
  4. Leading and trailing spaces around an argument are ignored. Spaces within a quoted argument are allowed.
  5. The first argument passed to main() is the program name.
  6. A maximum number of 10 arguments (including the program name) are supported.

7.4 Automatic starting

Usually, a cc65-built program just will sit quietly in memory, after it is CLOADed. It waits for you to start it (by typing BASIC's RUN command). But, if you want to create a program that will start running immediately after it is loaded, then you can use the linker command-line option -D __AUTORUN__=$C7.

7.5 Interrupts

The runtime for the Atmos uses routines marked as .INTERRUPTOR for interrupt handlers. Such routines must be written as simple machine language subroutines and will be called automatically by the interrupt handler code when they are linked into a program. See the discussion of the .CONDES feature in the assembler manual.

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.