Commodore 128-specific information for cc65

Ullrich von Bassewitz,
Stefan A. Haubenthal


An overview over the C128 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 C128 runtime system as it comes with the cc65 C compiler. It describes the memory layout, C128-specific header files, available drivers, and any pitfalls specific to that platform.

Please note that C128-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 C128 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 the I/O area and the kernal ROM enabled. Note that this is a non standard memory layout, and that there is no "memory configuration index" for this layout. This means that special care has to be taken when changing the configuration, or calling any code that does this. The memory configuration register at $FF00 should be saved and restored instead of relying on the memory configuration index stored in the zero page.

The setup gives a usable memory range of $1C00 - $BFFF. Having just the kernal ROM mapped in means, that kernal entry points may be called directly, but using the BASIC ROM is not possible without additional code.

Special locations:

Text screen

The text screen is located at $400 (as in the standard setup).

Stack

The C runtime stack is located at $BFFF, and growing downwards.

Heap

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 C128-specific code may use the c128.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 c128.h and declares several functions common to all CBM platforms.

4.1 C128-specific functions

The functions listed below are special for the C128. See the function reference for declaration and usage.

4.2 C128-specific accelerator functions

The functions listed below are accelerator functions for the C128. See the function reference for declaration and usage.

4.3 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.

4.4 CBM specific CPU functions

Some CPU related functions are available for some of the Commodore machines. See the function reference for declaration and usage.

4.5 Hardware access

The following pseudo variables declared in the c128.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.

VIC

The VIC structure allows access to the VIC II (the graphics controller). See the _vic2.h header file located in the include directory for the declaration of the structure.

SID

The SID structure allows access to the SID (the sound interface device). See the _sid.h header file located in the include directory for the declaration of the structure.

VDC

The VDC structure allows access to the VDC (the video display controller). See the _vdc.h header file located in the include directory for the declaration of the structure.

CIA1, CIA2

Access to the two CIA (complex interface adapter) chips is available via the CIA1 and CIA2 variables. The structure behind these variables is explained in _6526.h.

COLOR_RAM

A character array that mirrors the color RAM of the C128 at $D800.

5. Loadable drivers

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 c128-vdc.tgi (c128_vdc_tgi).

Note: The graphics drivers for the VDC are incompatible with the extended memory drivers using the VDC memory!

c128-hi.tgi (c128_hi_tgi)

This driver features a resolution of 320×200 with two colors and an adjustable palette (that means that the two colors can be chosen out of a palette of the 16 VIC colors). Unlike BASIC 7.0, this driver puts its graphics data into the RAM behind the ROMs.

c128-vdc.tgi (c128_vdc_tgi)

This driver was written by Maciej Witkowiak. It uses the 80-column display, and features a resolution of 640×200 with two colors and an adjustable palette (that means that the two colors can be chosen out of the 16 VDC colors).

c128-vdc2.tgi (c128_vdc2_tgi)

This driver was written by Maciej Witkowiak. This driver uses the 80-column display, and features a resolution of 640×480 with two colors and an adjustable palette (that means that the two colors can be chosen out of the 16 VDC colors). The driver requires 64KB VDC RAM.

Note: The colors are translated from the definitions in the headers to correct VDC values; so, please use definitions or VIC color numbers only. Colors GRAY3 and BROWN are missing on the VDC; and, are translated to the two colors missing from the VIC palette.

5.2 Extended memory drivers

c128-efnram.emd (c128_efnram_emd)

Extended memory driver for the C128 External Function RAM. Written and contributed by Marco van den Heuvel.

c128-georam.emd (c128_georam_emd)

A driver for the GeoRam cartridge. The driver will always assume 2048 pages of 256 bytes each. There are no checks, so if your program knows better, just go ahead.

c128-ifnram.emd (c128_ifnram_emd)

Extended memory driver for the C128 Internal Function RAM. Written and contributed by Marco van den Heuvel.

c128-ram.emd (c128_ram_emd)

An extended memory driver for the RAM in page 1. The common memory area is excluded, so this driver supports 251 pages of 256 bytes each.

c128-ram2.emd (c128_ram2_emd)

An extended memory driver for the RAM in pages 1-3. The common memory area is excluded, so this driver supports up to 731 pages of 256 bytes each. The driver can be used as a full replacement for c128-ram.emd, because RAM in pages 2+3 is autodetected, but it's larger and there are not many machines with RAM in banks 2+3, so it has been made a separate driver. The additional code was contributed by Marco van den Heuvel.

c128-ramcart.emd (c128_ramcart_emd)

A driver for the RamCart 64/128 written and contributed by Maciej Witkowiak. Will test the hardware for the available RAM.

c128-reu.emd (c128_reu_emd)

A driver for the CBM REUs. The driver will test the connected REU to find out how much RAM is present.

c128-vdc.emd (c128_vdc_emd)

A driver for the VDC memory of the C128, written and contributed by Maciej Witkowiak. Autodetects the amount of memory available (16 or 64K), and offers 64 or 256 pages of 256 bytes each. Note: This driver is incompatible with any of the graphics drivers using the VDC!

5.3 Joystick drivers

The default drivers, joy_stddrv (joy_static_stddrv), point to c128-stdjoy.joy (c128_stdjoy_joy).

c128-ptvjoy.joy (c128_ptvjoy_joy)

Driver for the Protovision 4-player adapter originally written by Groepaz for the C64, and converted for the C128 by Uz. See Protovision shop for prices and building instructions. Up to four joysticks are supported.

c128-stdjoy.joy (c128_stdjoy_joy)

Supports up to two joysticks connected to the standard joysticks ports of the C128.

5.4 Mouse drivers

The default drivers, mouse_stddrv (mouse_static_stddrv), point to c128-1351.mou (c128_1351_mou).

c128-1351.mou (c128_1351_mou)

Supports a standard mouse connected to port #0 of the C128.

c128-inkwell.mou (c128_inkwell_mou)

Supports the Inkwell Systems lightpens, connected to port #0 of the C128. It can read both the one-button 170-C and the two-button 184-C pens. (It can read other lightpens and light-guns that send their button signal to the joystick left-button pin or the paddle Y [up/down] pin.) It works on only the 40-column screen.

c128-joy.mou (c128_joy_mou)

Supports a mouse emulated by a standard joystick, e.g. 1350 mouse, in port #1 of the C128.

c128-pot.mou (c128_pot_mou)

Supports a potentiometer device, e.g. Koala Pad, connected to port #1 of the C128.

5.5 RS232 device drivers

c128-swlink.ser (c128_swlink_ser)

Driver for the SwiftLink cartridge. Supports up to 38400 BPS, hardware flow control (RTS/CTS), and 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.

The driver uses the RS232 variables and buffers of the kernal (buffers at $C00 and $D00).

6. Limitations

7. Other hints

7.1 Passing arguments to the program

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

    RUN:REM ARG1 " ARG2 IS QUOTED" ARG3 "" ARG5

  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 Interrupts

The runtime for the C128 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.