This file contains an overview of the Plus/4 runtime system as it comes with the cc65 C compiler. It describes the memory layout, Plus/4 specific header files, available drivers, and any pitfalls specific to that platform.
Please note that Plus/4 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.
Since the Plus/4 and the Commodore 16/116 are almost identical (the latter are missing the 6551 ACIA and do only have 16KB of memory), the C16 documentation is also worth a look. The difference between both cc65 targets is that the Plus/4 runtime uses banking to support full 64K RAM, while the C16 does not use banking and supports up to 32K RAM. Because banking is not needed, most C16 programs will be somewhat smaller than the same program compiled for the Plus/4. However, programs compiled for the C16 will always run on the Plus/4, while the reverse is not necessarily true.
The standard binary output format generated by the linker for the Plus/4 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 kernal and basic banked out. This gives a usable memory range of $1000 - $FD00. Having the kernal and basic ROMs banked out means, that no ROM entry points may be called directly from user code.
The text screen is located at $C00 (as in the standard setup).
The color RAM is located at $800 (standard location).
The C runtime stack is located at $FCFF and growing downwards.
The C heap is located at the end of the program and grows towards the C runtime stack.
Programs containing Plus/4 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
plus4.h and declares several functions
common to all CBM platforms.
Please note that most of the header file declarations from the
header file are shared between the C16 and Plus/4 configurations. For this
reason, most of it is located in a common header file named
There are currently no special Plus/4 functions.
Some functions are available for all (or at least most) of the Commodore machines. See the function reference for declaration and usage.
Some CPU related functions are available for some of the Commodore machines. See the function reference for declaration and usage.
The following pseudo variables declared in the
plus4.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.
TED structure allows access to the TED chip. See the
_ted.h header file located in the include directory for the
declaration of the structure.
A character array that mirrors the color RAM of the Plus/4 at $0800.
The names in the parentheses denote the symbols to be used for static linking of the drivers.
No graphics drivers are currently available for the Plus/4.
No extended memory drivers are currently available for the Plus/4.
Supports up to two joysticks connected to the standard joysticks port of the Plus/4.
No mouse drivers are currently available for the Plus/4.
Driver for the 6551 ACIA chip built into the Plus/4. Supports up to 19200 baud, hardware flow control (RTS/CTS) and interrupt driven receives. Note that because of the peculiarities of the 6551 chip transmits are not interrupt driven, and the transceiver blocks if the receiver asserts flow control because of a full buffer.
You need an adapter to use the builtin port, since the output levels available at the user port don't follow the RS232 standard.
Command line arguments can be passed to
main(). Since this is not
supported by BASIC, the following syntax was chosen:
RUN:REM ARG1 " ARG2 IS QUOTED" ARG3 "" ARG5
mainis the program name.
The program return code (low byte) is passed back to BASIC by use of the
The runtime for the Plus/4 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: