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<title>Atari specific information for cc65
<author>Shawn Jefferson, <htmlurl url="mailto:shawnjefferson@24fightingchickens.com" name="shawnjefferson@24fightingchickens.com">
Christian Groessler, <htmlurl url="mailto:cpg@aladdin.de" name="cpg@aladdin.de">
<date>16-Oct-2003

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

Please note that Atari 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
Atari target is a machine language program with a standard executable
header (FF FF &lt;start address&gt; &lt;end address&gt;). These values are
calculated in the crt0.s file from the __CODE_LOAD__ and __BSS_LOAD__
values, so keep this in mind if you create a custom linker config file
and start moving segments around.  You can override this behaviour by
creating your own crt0.s file and linking it into your program.  A run
vector is added to the end of the file (&dollar;02E0 &lt;run vector&gt;)
and is calculated using __CODE_LOAD__ in crt0.s also.


<sect>Memory layout<p>

The default linker script assumes that the BASIC ROM is disabled (or
the BASIC cartridge unplugged). This gives a usable memory range from
&dollar;2E00 - &dollar;BC1F. The library startup code examines the
current memory configuration, which depends on the size of the
installed memory and cartridges present, by inspecting the value in
the MEMTOP (&dollar;2E5) variable. Then the initial stack pointer,
which indicates the upper bound of memory used, is adjusted. The load
address of &dollar;2E00 was chosen to accommodate having a DOS loaded
and a driver that resides in low memory such as the 850 R: handler.
You can override this behaviour by creating a custom linker config
file.

Special locations:

<descrip>
  <tag/Text screen/
  The text screen depends on the installed memory size and cartridges
  and can be obtained from the SAVMSC variable (&dollar;58).

  <tag/Stack/
  The C runtime stack is located at MEMTOP and grows downwards,
  regardless of how your linker config file is setup.  This
  accomodates the different memory configurations of the Atari
  machines, as well as having a cartridge installed.  You can override
  this behaviour by writing your own crt0.s file and linking it to
  your program.

  <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 Atari specific code may use the <tt/atari.h/
header file.


<sect1>Atari specific functions<p>

The functions listed below are special for the Atari. See the <htmlurl
url="funcref.html" name="function reference"> for declaration and usage.

<itemize>
<item>_gtia_mkcolor
<item>get_ostype
<item>get_tv
<item>getcolor
<item>getdefdev
<item>graphics
<item>rest_vecs
<item>save_vecs
<item>scroll
<item>setcolor
<item>setcolor_low
</itemize>


<sect1>Hardware access<p>

The following pseudo variables declared in the <tt/atari.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/GTIA_READ/ and <tt/GTIA_WRITE/</tag>
  The <tt/GTIA_READ/ structure allows read access to the GTIA. The
  <tt/GTIA_WRITE/ structure allows write access to the GTIA.
  See the <tt/_gtia.h/ header file located in the include directory
  for the declaration of the structure.

  <tag><tt/POKEY_READ/ and <tt/POKEY_WRITE/</tag>
  The <tt/POKEY_READ/ structure allows read access to the POKEY. The
  <tt/POKEY_WRITE/ structure allows write access to the POKEY.
  See the <tt/_pokey.h/ header file located in the include directory
  for the declaration of the structure.

  <tag><tt/ANTIC/</tag>
  The <tt/ANTIC/ structure allows read access to the ANTIC.
  See the <tt/_antic.h/ header file located in the include directory
  for the declaration of the structure.

  <tag><tt/PIA/</tag>
  The <tt/PIA/ structure allows read access to the PIA 6520.
  See the <tt/_pia.h/ header file located in the include directory
  for the declaration of the structure.

</descrip><p>



<sect>Loadable drivers<p>

<sect1>Graphics drivers<p>

Currently there are no graphics drivers available for the Atari platform.

<sect1>Extended memory drivers<p>

Currently there are no extended memory drivers available for the Atari
platform.

<sect1>Joystick drivers<p>

<descrip>

  <tag><tt/atari-stdjoy.joy/</tag>
  Supports up to four standard joysticks connected to the joystick ports of
  the Atari.

</descrip><p>



<sect1>Mouse drivers<p>

Currently no drivers available (in fact, the API for loadable mouse drivers
does not exist). There is a static driver you can use.


<sect1>RS232 device drivers<p>

Currently there are no RS232 loadable drivers available for the Atari
platform. There is a static driver you can use.


<sect>Limitations<p>



<sect>Other hints<p>



<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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