Document the symbols section and the new -D and --define options

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<title>Commodore VIC20 (aka VC20) specific information for cc65
<author>Ullrich von Bassewitz, <htmlurl url="mailto:uz@cc65.org" name="uz@cc65.org">
Stefan A. Haubenthal, <htmlurl url="mailto:polluks@sdf.lonestar.org" name="polluks@sdf.lonestar.org">
<date>2004-09-13

<abstract>
An overview over the VIC20 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 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 <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.


<sect>Binary format<p>

The standard binary output format generated by the linker for the VIC20 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 I/O area and the
kernal and BASIC ROM enabled (RAM at &dollar;A000 - &dollar;BFFF may be used
for the heap), which gives a usable memory range of &dollar;1000 - &dollar;1DFF.
All ROM entry points may be called directly without additional code.

Special locations:

<descrip>
  <tag/Text screen/
  The text screen is located at &dollar;1E00 (as in the standard setup).

  <tag/Stack/
  The C runtime stack is located at &dollar;1DFF 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 VIC20 specific code may use the <tt/vic20.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/vic20.h/ and declares several functions
common to all CBM platforms.


<sect1>VIC20 specific functions<p>

There are currently no special VIC20 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/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.

<descrip>

  <tag><tt/VIC/</tag>
  The <tt/VIC/ structure allows access to the VIC (the graphics
  controller). See the <tt/_vic.h/ header file located in the include
  directory for the declaration of the structure.

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

  <tag><tt/COLOR_RAM/</tag>
  A character array that mirrors the color RAM of the VIC20 at &dollar;9600.

</descrip><p>



<sect>Loadable drivers<p>

<sect1>Graphics drivers<p>

No graphics drivers are currently available for the VIC20.


<sect1>Extended memory drivers<p>

No extended memory drivers are currently available for the VIC20.


<sect1>Joystick drivers<p>

<descrip>

  <tag><tt/vic20-stdjoy.joy/</tag>
  Supports one standard joystick connected to the joysticks port of the VIC20.

</descrip><p>



<sect1>Mouse drivers<p>

No mouse drivers are currently available for the VIC20.


<sect1>RS232 device drivers<p>

No VIC1011 drivers are currently available for the VIC20.



<sect>Limitations<p>



<sect>Other hints<p>

<sect1>Passing arguments to the program<p>

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

<tscreen><verb>
    RUN:REM ARG1 " ARG2 IS QUOTED" ARG3 "" ARG5
</verb></tscreen>

<enum>
<item>Arguments are separated by spaces.
<item>Arguments may be quoted.
<item>Leading and trailing spaces around an argument are ignored. Spaces within
      a quoted argument are allowed.
<item>The first argument passed to <tt/main/ is the program name.
<item>A maximum number of 10 arguments (including the program name) are
      supported.
</enum>


<sect1>Program return code<p>

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


<sect1>Using extended memory<p>

The extended memory at $A000 may be added to the heap by using the following
code:

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


<sect1>Interrupts<p>

The runtime for the VIC20 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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