Commodore VIC20 (aka VC20 aka VIC1001) specific information for cc65

Ullrich von Bassewitz,
Stefan A. Haubenthal

An overview over the VIC20 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 VIC20 runtime system as it comes with the cc65 C compiler. It describes the memory layout, VIC20-specific header files, available drivers, and any pitfalls specific to that platform.

Please note that VIC20-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 VIC20 target is a machine language program with a one line BASIC stub, which calls the machine language part via SYS. This means that a program can be loaded as BASIC program and started with RUN. 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 run with unexpanded memory (RAM at $A000 - $BFFF may be used for the heap), which gives a usable memory range of $1000 - $1DFF. All ROM entry points may be called directly without additional code.

Special locations:

Text screen

The text screen is located at $1E00 (as in the standard setup).


The C runtime stack is located at $1DFF 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 VIC20-specific code may use the vic20.h or cbm.h header files. Using the later may be an option when writing code for more than one CBM platform, since it includes vic20.h and declares several functions common to all CBM platforms.

4.1 VIC20-specific functions

There are currently no special VIC20 functions.

4.2 CBM-specific functions

Some functions are available for all (or at least most) of the Commodore machines. See the function reference for declaration and usage.

4.3 Hardware access

The following pseudo variables declared in the vic20.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 VIC structure allows access to the VIC (the graphics controller). See the _vic.h header file located in the include directory for the declaration of the structure.


Access to the two VIA (versatile interface adapter) chips is available via the VIA1 and VIA2 variables. The structure behind these variables is explained in _6522.h.


A character array that mirrors the color RAM of the VIC20 at $9600.

5. Loadable drivers

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

5.1 Graphics drivers

vic20-hi.tgi (vic20_hi_tgi)

This driver features a resolution of 160×192 with two colors. The background can be chosen from a sixteen-color palette. The foreground can be chosen from an eight-color palette.

The driver will use memory from addresses $1000 to $1FFF as a graphics buffer. Therefore, the VIC-20 must have, at least, 8K of expansion RAM.

Programs that use this driver must be linked by the vic20-tgi.cfg configuration file. It will link a special header into the program. That header will do the housekeeping that's needed by TGI.

An example command line:

  cl65 -D DYN_DRV=0 -t vic20 -C vic20-tgi.cfg samples/mandelbrot.c

When the program starts, it will move itself up in RAM, to make room for the buffer. When the program finishes, it will reset the BASIC interpreter. That means that graphics pictures won't be preserved between the executions of programs. Also, the graphics buffer shares RAM with the text screen. If a picture must be saved, then a program must put it somewhere else (such as a disk file) before returning to the text mode.

5.2 Extended memory drivers

vic20-rama.emd (vic20_rama_emd)

A driver for any RAM at $A000-$BFFF. Supports 32 256 byte pages. Written and contributed by Marco van den Heuvel.

vic20-georam.emd (vic20_georam_emd)

A driver for the Berkeley Softworks GeoRam cartridge connected by means of the MasC=erade c64 cartridge adapter. The driver will determine the available RAM from the connected cartridge. It supports 64KB up to 2048KB of RAM.

5.3 Joystick drivers

The default drivers, joy_stddrv (joy_static_stddrv), point to (vic20_stdjoy_joy). (vic20_stdjoy_joy)

Supports one standard joystick connected to the joysticks port of the VIC20. (vic20_ptvjoy_joy)

Driver for the Protovision 4-player adapter contributed by Groepaz. See the Protovision shop for prices and building instructions. Up to three joysticks are supported.

5.4 Mouse drivers

No mouse drivers are currently available for the VIC20.

5.5 RS232 device drivers

No VIC1011 drivers are currently available for the VIC20.

6. Limitations

6.1 Escape code

The CTRL key cannot be used to type most control characters, entering an Esc is not possible.

7. Other hints

7.1 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:


  1. Arguments are separated by spaces.
  2. Arguments may be quoted.
  3. Leading and trailing spaces around an argument are ignored. Spaces within a quoted argument are allowed.
  4. The first argument passed to main() is the program name.
  5. A maximum number of 10 arguments (including the program name) are supported.

7.2 Program return code

The program return code (low byte) is passed back to BASIC by use of the ST variable.

7.3 Using extended memory

BLK5 memory may be added to the heap by using the following code:

    /* Check for the existence of RAM */
    if (PEEK(0xA000) == POKE(0xA000, PEEK(0xA000)+1)) {
        /* Add it to the heap */
        _heapadd ((void *) 0xA000, 0x2000);

7.4 Interrupts

The runtime for the VIC20 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.