<!doctype linuxdoc system>

<article>

<title>Atari specific information for cc65
<author>Shawn Jefferson, <htmlurl
url="mailto:shawnjefferson@24fightingchickens.com"
name="shawnjefferson@24fightingchickens.com"> and 
Christian Groessler, <htmlurl url="mailto:cpg@aladdin.de" name="cpg@aladdin.de">
<date>04-Sep-2005

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

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<toc>

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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;2 byte start address&gt; &lt;2 bytes end address&gt;
&lsqb;program bytes&rsqb;). 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 (see section <ref name="Reserving a memory area inside the program" id="memhole">).  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.


<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 (see also <ref name="Final note"
  id="memhole_final_note">).

  <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>get_ostype
<item>get_tv
<item>_gtia_mkcolor
<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.
However, the runtime library provides a function named _graphics, with
a mode parameter just like the BASIC GRAPHICS command. This function will
switch to the requested graphics mode.
There are currently no functions available to access the graphics
memory. The access must be implemented manually.

Many graphics modes require more memory than the text screen which is
in effect when the program starts up. Therefore the programmer has to
tell the program beforehand the memory requirements of the graphics
modes the program intends to use. 
This can be done by using the __RESERVED_MEMORY__ linker config
variable. The number specified there describes the number of bytes to
subtract from the top of available memory as seen from the runtime
library. This memory is then used by the screen buffer.

The numbers for the different graphics modes presented below should
only be seen as a rule of thumb. Since the screen buffer memory needs
to start at specific boundaries, the numbers depend on the current top
of available memory.
The following numbers were determined by a BASIC program.

<table>
<tabular ca="rr">
graphics mode|reserved memory@<hline>
0|1@
1|1@
2|1@
3|1@
4|1@
5|182@
6|1182@
7|3198@
8|7120@
9|7146@
10|7146@
11|7146@
12|162@
13|1@
14|3278@
15|7120@
16|1@
17|1@
18|1@
19|1@
20|1@
21|184@
22|1192@
23|3208@
24|7146@
25|7146@
26|7146@
27|7146@
28|160@
29|1@
30|3304@
31|7146
</tabular>
<caption>reserved memory required for different graphics modes
</table>

The values of "1" are needed because the graphics command crashes if
it doesn't have at least one byte available. This seems to be a bug of
the Atari ROM code.

<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>DIO implementation<label id="dio"><p>

The Atari supports disk drives with either 128 or 256 byte sectors.
The first three sectors of any disk are always 128 bytes long though. This is
because the system can only boot from 128 bytes sectors.

Therefore the DIO read and write functions transfer only 128 bytes
for sectors 1 to 3, regardless of the type of diskette.


<sect>Other hints<p>

<sect1>Function keys<p>

These are defined to be Atari + number key.

<sect1>Reserving a memory area inside a program<label id="memhole"><p>

The Atari 130XE maps its additional memory into CPU memory in 16K
chunks at address &dollar;4000 to &dollar;7FFF. One might want to
prevent this memory area from being used by cc65. Other reasons to
prevent the use of some memory area could be the buffers for display
lists and screen memory.
<p>
The Atari executable format allows holes inside a program, e.g. one
part loads into &dollar;2E00 to &dollar;3FFF, going below the reserved
memory area (assuming a reserved area from &dollar;4000 to
&dollar;7FFF), and another part loads into &dollar;8000 to
&dollar;BC1F.
<p>
Each load chunk of the executable starts with a 4 byte header which
defines its load address and size.
<p>
<sect2>Low code and high data example<p>
Goal: Create an executable with 2 load chunks which doesn't use the
memory area from &dollar;4000 to &dollar;7FFF. The CODE segment of
the program should go below &dollar;4000 and the DATA and RODATA
segments should go above &dollar;7FFF.
<p>
The main problem is that the EXE header generated by the cc65 runtine
lib is wrong. It defines a single load chunk with the sizes/addresses
of the LOWCODE, INIT, CODE, RODATA, and DATA segments (the whole user
program).
<p>
The contents of the EXE header come from the EXEHDR segment, which is
defined in crt0.s. This cannot be changed w/o modifiying and
recompiling the cc65 atari runtime lib. Therefore the original EXE
header must be discarded. It will be replaced by a user created
one.
<p>
The user needs to create a customized linker config file which adds
new memory areas and segments to hold the new EXE header and the
header data for the second load chunk. Also an assembly source file
needs to be created which defines the contents of the new EXE header
and the second load chunk header.
<p>
<p>
This is a modified cc65 Atari linker configuration file (split.cfg):
<tscreen><verb>
MEMORY {
    ZP: start = $82, size = $7E, type = rw, define = yes;

    HEADER: start = $0000, size = $6, file = %O;        # first load chunk
    RAMLO: start = $2E00, size = $1200, file = %O;

    BANK: start = $4000, size = $4000, file = "";

    SECHDR: start = $0000, size = $4, file = %O;        # second load chunk
    RAM: start = $8000, size = $3C20, file = %O;        # $3C20: matches upper bound $BC1F
    TRAILER: start = $0000, size = $0006, file = %O;
}
SEGMENTS {
    EXEHDR: load = BANK, type = ro;

    NEXEHDR: load = HEADER, type = ro;                  # first load chunk
    LOWCODE: load = RAMLO, type = ro, define = yes, optional = yes;
    INIT: load = RAMLO, type = ro, optional = yes;
    CODE: load = RAMLO, type = ro, define = yes;

    CHKHDR: load = SECHDR, type = ro;                   # second load chunk
    RODATA: load = RAM, type = ro, define = yes;
    DATA: load = RAM, type = rw, define = yes;
    BSS: load = RAM, type = bss, define = yes;

    ZEROPAGE: load = ZP, type = zp;
    AUTOSTRT: load = TRAILER, type = ro;                # defines program entry point
}
FEATURES {
    CONDES: segment = RODATA,
            type = constructor,
            label = __CONSTRUCTOR_TABLE__,
            count = __CONSTRUCTOR_COUNT__;
    CONDES: segment = RODATA,
            type = destructor,
            label = __DESTRUCTOR_TABLE__,
            count = __DESTRUCTOR_COUNT__;
}
SYMBOLS {
    __STACKSIZE__ = $800;       # 2K stack
    __RESERVED_MEMORY__: value = $0, weak = yes;
}
</verb></tscreen>
<p>

A new memory area BANK was added which describes the reserved area.
It gets loaded with the contents of the old EXEHDR segment. But the
memory area isn't written to the output file. This way the contents of
the EXEHDR segment get discarded.
<p>
The added NEXEHDR segment defines the correct EXE header. It puts only
the CODE segment into load chunk #1 (RAMLO memory area).
<p>
The header for the second load chunk comes from the new CHKHDR
segment. It puts the RODATA and DATA segments into load chunk #2 (RAM
memory area).
<p>
<p>
The contents of the new NEXEHDR and CHKHDR segments come from this
file (split.s):
<tscreen><verb>
        .import __LOWCODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__
        .import __CODE_LOAD__, __DATA_LOAD__, __RODATA_LOAD__

        .segment "NEXEHDR"
        .word    $FFFF          ; EXE file magic number
        ; 1st load chunk
        .word    __LOWCODE_LOAD__
        .word    __CODE_LOAD__ + __CODE_SIZE__ - 1

        .segment "CHKHDR"
        ; 2nd load chunk (contains with AUTOSTRT in fact a 3rd load chunk)
        .word    __RODATA_LOAD__
        .word    __BSS_LOAD__ - 1
</verb></tscreen>
<p>
Compile with
<tscreen><verb>
cl65 -t atari -C split.cfg -o prog.com prog.c split.s
</verb></tscreen>

<sect2>Low data and high code example<p>


Goal: Put RODATA and DATA into low memory and LOWCODE, INIT, CODE, BSS
into high memory (split2.cfg):

<tscreen><verb>
MEMORY {
    ZP: start = $82, size = $7E, type = rw, define = yes;

    HEADER: start = $0000, size = $6, file = %O;        # first load chunk
    RAMLO: start = $2E00, size = $1200, file = %O;

    BANK: start = $4000, size = $4000, file = "";

    SECHDR: start = $0000, size = $4, file = %O;        # second load chunk
    RAM: start = $8000, size = $3C20, file = %O;        # $3C20: matches upper bound $BC1F
    TRAILER: start = $0000, size = $0006, file = %O;
}
SEGMENTS {
    EXEHDR: load = BANK, type = ro;                     # discarded old EXE header

    NEXEHDR: load = HEADER, type = ro;                  # first load chunk
    RODATA: load = RAMLO, type = ro, define = yes;
    DATA: load = RAMLO, type = rw, define = yes;

    CHKHDR: load = SECHDR, type = ro;                   # second load chunk
    LOWCODE: load = RAM, type = ro, define = yes, optional = yes;
    INIT: load = RAM, type = ro, optional = yes;
    CODE: load = RAM, type = ro, define = yes;
    BSS: load = RAM, type = bss, define = yes;

    ZEROPAGE: load = ZP, type = zp;
    AUTOSTRT: load = TRAILER, type = ro;                # defines program entry point
}
FEATURES {
    CONDES: segment = RODATA,
            type = constructor,
            label = __CONSTRUCTOR_TABLE__,
            count = __CONSTRUCTOR_COUNT__;
    CONDES: segment = RODATA,
            type = destructor,
            label = __DESTRUCTOR_TABLE__,
            count = __DESTRUCTOR_COUNT__;
}
SYMBOLS {
    __STACKSIZE__ = $800;       # 2K stack
    __RESERVED_MEMORY__: value = $0, weak = yes;
}
</verb></tscreen>

New contents for NEXEHDR and CHKHDR are needed (split2.s):
<tscreen><verb>
        .import __LOWCODE_LOAD__, __BSS_LOAD__, __DATA_SIZE__
        .import __DATA_LOAD__, __RODATA_LOAD__

        .segment "NEXEHDR"
        .word    $FFFF
        .word    __RODATA_LOAD__
        .word    __DATA_LOAD__ + __DATA_SIZE__ - 1

        .segment "CHKHDR"
        .word    __LOWCODE_LOAD__
        .word    __BSS_LOAD__ - 1
</verb></tscreen>

Compile with
<tscreen><verb>
cl65 -t atari -C split2.cfg -o prog.com prog.c split2.s
</verb></tscreen>

<sect2>Final note<label id="memhole_final_note"><p>

There are two other memory areas which don't appear directly in the
linker script. They are the stack and the heap.

The cc65 runtime lib places the stack location at the end of available
memory. This is dynamically set from the MEMTOP system variable at
startup. The heap is located in the area between the end of the BSS
segment and the top of the stack as defined by __STACKSIZE__.

If BSS and/or the stack shouldn't stay at the end of the program,
some parts of the cc65 runtime lib need to be replaced/modified.

common/_heap.s defines the location of the heap and atari/crt0.s
defines the location of the stack by initializing sp.


<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"> or <htmlurl url="mailto:cpg@aladdin.de"
name="cpg@aladdin.de">).



<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>

</article>
