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<title>Commodore Plus/4 specific information for cc65
<author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
<date>2003-12-14

<abstract>
An overview over the Plus/4 runtime system as it is implemented for the cc65 C
compiler.
</abstract>

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<sect>Overview<p>

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 <htmlurl url="funcref.html" name="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 <htmlurl
url="c16.html" name="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.


<sect>Binary format<p>

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. 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.


<sect>Memory layout<p>

cc65 generated programs with the default setup run with the kernal and basic
banked out. This gives a usable memory range of &dollar;1000 - &dollar;FD00.
Having the kernal and basic ROMs banked out means, that no ROM entry points
may be called directly from user code.

Special locations:

<descrip>
  <tag/Text screen/
  The text screen is located at &dollar;C00 (as in the standard setup).

  <tag/Color RAM/
  The color RAM is located at &dollar;800 (standard location).

  <tag/Stack/
  The C runtime stack is located at &dollar;FCFF and growing downwards.

  <tag/Heap/
  The C heap is located at the end of the program and grows towards the C
  runtime stack.

</descrip><p>



<sect>Platform specific header files<p>

Programs containing Plus/4 specific code may use the <tt/plus4.h/ or <tt/cbm.h/
header files. Using the later may be an option when writing code for more than
one CBM platform, since it includes <tt/plus4.h/ and declares several functions
common to all CBM platforms.

Please note that most of the header file declarations from the <tt/plus4.h/
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 <tt/cbm264.h/.



<sect1>Plus/4 specific functions<p>

There are currently no special Plus/4 functions.


<sect1>CBM specific functions<p>

Some functions are available for all (or at least most) of the Commodore
machines. See the <htmlurl url="funcref.html" name="function reference"> for
declaration and usage.

<itemize>
<item>cbm_close
<item>cbm_closedir
<item>cbm_k_setlfs
<item>cbm_k_setnam
<item>cbm_k_load
<item>cbm_k_save
<item>cbm_k_open
<item>cbm_k_close
<item>cbm_k_readst
<item>cbm_k_chkin
<item>cbm_k_ckout
<item>cbm_k_basin
<item>cbm_k_bsout
<item>cbm_k_clrch
<item>cbm_load
<item>cbm_open
<item>cbm_opendir
<item>cbm_read
<item>cbm_readdir
<item>cbm_save
<item>cbm_write
<item>get_tv
</itemize>


<sect1>Hardware access<p>

The following pseudo variables declared in the <tt/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.

<descrip>

  <tag><tt/TED/</tag>
  The <tt/TED/ structure allows access to the TED chip. See the
  <tt/_ted.h/ header file located in the include directory for the
  declaration of the structure.

  <tag><tt/COLOR_RAM/</tag>
  A character array that mirrors the color RAM of the Plus/4 at &dollar;0800.

</descrip><p>



<sect>Loadable drivers<p>

<sect1>Graphics drivers<p>

No graphics drivers are currently available for the Plus/4.


<sect1>Extended memory drivers<p>

No extended memory drivers are currently available for the Plus/4.



<sect1>Joystick drivers<p>

<descrip>

  <tag><tt/plus4-stdjoy.joy/</tag>
  Supports up to two joysticks connected to the standard joysticks port of
  the Plus/4.

</descrip><p>



<sect1>Mouse drivers<p>

Currently no drivers available (in fact, the API for loadable mouse drivers
does not exist).


<sect1>RS232 device drivers<p>

<descrip>

  <tag><tt/plus4-stdser.ser/</tag>
  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.

</descrip><p>



<sect>Limitations<p>



<sect>Other hints<p>

<sect1>Passing arguments to the program<p>

Command line argument passing is currently not supported for the Plus/4.


<sect1>Program return code<p>

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


<sect1>Interrupts<p>

The runtime for the Plus/4 uses routines marked as <tt/.CONDES/ type 2 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 <tt/.CONDES/
feature in the <htmlurl url="ca65.html" name="assembler manual">.



<sect>Bugs/Feedback<p>

If you have problems using the library, if you find any bugs, or if you're
doing something interesting with it, I would be glad to hear from you. Feel
free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
name="uz@cc65.org">).



<sect>License<p>

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:

<enum>
<item>	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.
<item>	Altered source versions must be plainly marked as such, and must not
	be misrepresented as being the original software.
<item>	This notice may not be removed or altered from any source
	distribution.
</enum>

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