This file contains an overview of the C64 runtime system as it comes with the cc65 C compiler. It describes the memory layout, C64-specific header files, available drivers, and any pitfalls specific to that platform.
Please note that C64-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.
The standard binary output format generated by the linker for the C64 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.
cc65 generated programs with the default setup run with the I/O area and the kernal ROM enabled (memory under the kernal may be used for graphics or as extended memory - see the sections about graphics and extended memory drivers). The BASIC ROM is disabled, which gives a usable memory range of $0800 - $CFFF. This means that kernal entry points may be called directly, but using the BASIC ROM is not possible without additional code.
The text screen is located at $400 (as in the standard setup).
The C runtime stack is located at $CFFF and growing downwards.
The C heap is located at the end of the program and grows towards the C runtime stack.
The ld65 linker comes with a default config file for the Commodore 64,
which is used via
-t c64. The
c64 package comes with additional secondary linker config files, which are
-t c64 -C <configfile>.
The default configuration is tailored to C programs. It supplies the load address and a small BASIC stub that starts the compiled program using a SYS command.
This configuration is made for assembler programmers who don't need a special
setup. The default start address is $801. It can be changed with the
linker command line option
--start-addr. All standard segments with the
zeropage are written to the output file and a two byte load
address is prepended.
To use this config file, assemble with
-t c64 and link with
c64-asm.cfg. The former will make sure that correct character translation is
in effect, while the latter supplies the actual config. When using
use both command line options.
Sample command line for
cl65 -o file.prg -t c64 -C c64-asm.cfg source.s
To generate code that loads to $C000:
cl65 -o file.prg --start-addr $C000 -t c64 -C c64-asm.cfg source.s
It is also possible to add a small BASIC header to the program, that uses SYS to jump to the program entry point (which is the start of the code segment). The advantage is that the program can be started using RUN.
To generate a program with a BASIC SYS header, use
cl65 -o file.prg -u __EXEHDR__ -t c64 -C c64-asm.cfg source.s
Please note that in this case a changed start address doesn't make sense, since the program must be loaded to the BASIC start address.
The C64 package comes with an alternative software driven 80 columns
c64-soft80.o which uses the memory under I/O between $D000
In memory constrained situations the memory from $400 to $7FF
can be made available to a program by calling
_heapadd ((void *) 0x0400, 0x0400);
at the beginning of
main(). Doing so is beneficial even if the program
doesn't use the the heap explicitly because loading a driver uses the heap implicitly.
c64-soft80.o is as simple as placing it on the linker command
line like this:
cl65 -t c64 myprog.c c64-soft80.o
Note that the soft80 conio driver is incompatible with the
c64-ram.emd (c64_ram_emd) extended memory driver and the
c64-hi.tgi (c64_hi_tgi) graphics driver.
In an (even more) memory constrained situation, a size optimized version of the software driven 80 columns module may be used, which only supports one common text color for the whole screen.
cl65 -t c64 myprog.c c64-soft80mono.o
Programs containing C64-specific code may use the
header files. Using the later may be an option when writing code for more than
one CBM platform, since it includes
c64.h and declares several functions
common to all CBM platforms.
The functions listed below are special for the C64. See the function reference for declaration and usage.
The functions listed below are accelerator functions for the C64. See the function reference for declaration and usage.
Some functions are available for all (or at least most) of the Commodore machines. See the function reference for declaration and usage.
The following pseudo variables declared in the
c64.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 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 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.
Access to the two CIA (complex interface adapter) chips is available via
CIA2 variables. The structure behind these variables
is explained in
A character array that mirrors the color RAM of the C64 at $D800.
The names in the parentheses denote the symbols to be used for static linking of the drivers.
Note: All available graphics drivers for the TGI interface will use the space below the I/O area and Kernal ROM; so, you can have hires graphics in the standard setup without any memory loss or need for a changed configuration.
You can use a mouse driver at the same time that you use a TGI driver. But, if
you want to see the default mouse pointer on the graphics screen, then you
explicitly must link a special object file into your program. It will put the
arrow into the "high RAM" area where the bitmaps are put. Its name is
cl65 -t c64 -o program-file main-code.c subroutines.s c64-tgimousedata.o
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 C64 colors).
Note that the graphics drivers are incompatible with the
c64-ram.emd (c64_ram_emd) extended memory driver and the
c64-soft80.o software 80-columns conio driver.
Extended memory driver for 65816 (eg SCPU) based extra RAM. Written and contributed by Marco van den Heuvel.
A driver for the C64 256K memory expansion. This driver offers 768 pages of 256 bytes each. Written and contributed by Marco van den Heuvel.
A driver for the Double Quick Brown Box cartridge. This driver offers 64 pages of 256 bytes each. Written and contributed by Marco van den Heuvel.
A driver for the Berkeley Softworks GeoRam cartridge. The driver will determine the available RAM from the connected cartridge. It supports 64KB up to 2048KB of RAM.
A driver for the ISEPIC cartridge. This driver offers just 8 pages of 256 bytes each. Written and contributed by Marco van den Heuvel.
A driver for the hidden RAM below the I/O area and kernal ROM. Supports 47 256 byte pages. Please note that this driver is incompatible with any of the graphics drivers, or the soft80 conio driver!
A driver for the RamCart 64/128 written and contributed by Maciej Witkowiak. Will test the hardware for the available RAM.
A driver for the CBM REUs. The driver will test the connected REU to find out how much RAM is present.
A driver for the VDC memory of the C128. Written and contributed by Maciej Witkowiak. Can be used if the program is running in C64 mode of the C128. Autodetects the amount of memory available (16 or 64K) and offers 64 or 256 pages of 256 bytes each.
A driver for the C64 D2TV (the second or PAL version). This driver offers indeed 7680 pages of 256 bytes each.
The default drivers,
joy_stddrv (joy_static_stddrv), point to
Driver for the Digital Excess & Hitmen adapter contributed by Groepaz. See http://www.digitalexcess.de/downloads/productions.php on instructions how to build one. Up to four joysticks are supported.
Driver for the Protovision 4-player adapter contributed by Groepaz. See Protovision shop for prices and building instructions. Up to four joysticks are supported.
Supports up to two standard joysticks connected to the joysticks port of the C64.
Supports one joystick emulated by the numberpad of the C128 in C64 mode, the firebutton is labeled "5" and ENTER.
You can use these drivers in text-mode or graphics-mode (TGI) programs. See the description of the graphics drivers.
The default drivers,
mouse_stddrv (mouse_static_stddrv), point to
Supports a standard mouse connected to port #0 of the C64.
Supports the Inkwell Systems lightpens, connected to port #0 of the C64. 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.)
Supports a mouse emulated by a standard joystick, e.g. 1350 mouse, in port #1 of the C64.
Supports a potentiometer device, e.g. Koala Pad, connected to port #1 of the C64.
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 realtime clock functions use the CIA1 TOD clock. As that clock only stores
the time but not the date, the date set by
clock_settime() is simply stored
inside the C library for retrieval in the same program via
For an Esc, press CTRL and the
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
main()is the program name.
The program return code (low byte) is passed back to BASIC by use of the
The runtime for the C64 uses routines marked as
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
feature in the
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: