cc65/doc/atari7800.sgml

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<article>
<title>Atari 7800 specific information for cc65
<author>
<url url="mailto:karri@sipo.fi" name="Karri Kaksonen"><newline>

<abstract>
An overview over the Atari 7800 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 7800 runtime system as it
comes with the cc65 C compiler. It describes the memory layout, Atari
7800 specific header files and any pitfalls specific to that platform.

<sect>Binary format<p>

The default binary output format generated by the linker for the Atari
7800 target is a 48K cartridge image.

<sect>A78 header<p>

There is lots of different cart hardware available for the atari7800.
Some carts have ROM, RAM, sound hardware, non-volatile high score chips.
In order to know what kind of hardware the cart build requires there is
a header file of 128 bytes in front of the binary.

The default build creates a cart file for a 48K rom cart without any
extra features like the pokey audio chip or extra RAM.

In order to make cc65 more user friendly the build will add the a78
header automatically. This allows you to run the binary on emulators
and flash carts on the real console.

<sect>Encryption<p>

In order to boot the game in a mode that supports atari7800 functions
the cart must be encrypted after the linking phase.
There is a program called sign7800 that can be used to sign the cart.
The encryption is not required for running the cart on emulators.
You can also run atari2600 games without encryption.

<sect>Memory layout<p>

cc65 generated programs with the default setup can use RAM from
from &dollar;1800 to &dollar;203f.
The 4k RAM is then mapped to zero page area.
&dollar;2040 to &dollar;20ff is visible as zero page.
After that we have a vero small RAM area that is unused.
&dollar;2100 to &dollar;213f.
Then we mirror a second block from the RAM to become the hardware stack.
This would be from &dollar;2140 to &dollar;21ff.

The C-stack starts at &dollar;2800 and it can grow down to &dollar;2200.

size of the system stack can be customized by defining the
__STACKSIZE__ linker variable.

Special locations:

<descrip>
  <tag/Stack/ The C runtime stack is located at &dollar;2800 -
    __STACKSIZE__ and growing downwards.

  <tag/Heap/ The C heap is located at &dollar;2200 and grows upwards.

</descrip><p>

<sect>Start-up condition<p>

When powered-up, the Atari 7800 TIA registers contain random
values. During the initialization phase, the start-up code needs to
initialize the TIA registers to sound values (or else the console has
an unpredictable behavior). In this implementation, zeros are written
to all of TIA registers during the start-up phase.

Note that RIOT registers (mostly timers) are left uninitialized, as
they don't have any consequence on the console behavior.

<sect>Platform specific header files<p>

Programs containing Atari 7800 specific code may use the
<tt/atari7800.h/ header file.

The following pseudo variables declared in the <tt/atari7800.h/ header
file allow access to the Atari 7800 TIA, MARIA & RIOT chips registers.

<descrip>

  <tag><tt/TIA/</tag> The <tt/TIA/ structure allows read/write access
  to the Atari 7800 TIA chip registers. See the <tt/_tia.h/ header
  file located in the include directory for the declaration of the
  structure. Also refer to the Stella Programmer's Guide by Steve
  Wright for a detailed description of the chip and its registers.

  <tag><tt/RIOT/</tag> The <tt/RIOT/ structure allows read/write
  access to the Atari 7800 RIOT chip registers. See the
  <tt/_riot.h/ header file located in the include directory for the
  declaration of the structure. Also refer to the Stella Programmer's
  Guide by Steve Wright for a detailed description of the chip and its
  registers.

  <tag><tt/MARIA/</tag> The <tt/MARIA/ structure allows read/write
  access to the Atari 7800 MARIA chip registers. See the
  <tt/_maria.h/ header file located in the include directory for the
  declaration of the structure.

</descrip><p>


<sect>Loadable drivers<p>

There are no drivers for the Atari 7800.


<sect>Limitations<p>

TBD


<sect>Other hints<p>

One may write a custom linker configuration file to tune the memory
layout of a program. See the <tt/atari7800.cfg/ file in the cfg
directory as a starting point.


<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>