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158 lines
5.2 KiB
Plaintext
158 lines
5.2 KiB
Plaintext
<!doctype linuxdoc system>
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<article>
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<title>Atari 7800 specific information for cc65
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<author>
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<url url="mailto:karri@sipo.fi" name="Karri Kaksonen"><newline>
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<abstract>
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An overview over the Atari 7800 runtime system as it is implemented
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for the cc65 C compiler.
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</abstract>
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<!-- Table of contents -->
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<toc>
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<!-- Begin the document -->
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<sect>Overview<p>
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This file contains an overview of the Atari 7800 runtime system as it
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comes with the cc65 C compiler. It describes the memory layout, Atari
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7800 specific header files and any pitfalls specific to that platform.
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<sect>Binary format<p>
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The default binary output format generated by the linker for the Atari
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7800 target is a 48K cartridge image.
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<sect>A78 header<p>
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There is lots of different cart hardware available for the atari7800.
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Some carts have ROM, RAM, sound hardware, non-volatile high score chips.
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In order to know what kind of hardware the cart build requires there is
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a header file of 128 bytes in front of the binary.
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The default build creates a cart file for a 48K rom cart without any
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extra features like the pokey audio chip or extra RAM.
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In order to make cc65 more user friendly the build will add the a78
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header automatically. This allows you to run the binary on emulators
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and flash carts on the real console.
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<sect>Encryption<p>
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In order to boot the game in a mode that supports atari7800 functions
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the cart must be encrypted after the linking phase.
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There is a program called sign7800 that can be used to sign the cart.
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The encryption is not required for running the cart on emulators.
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You can also run atari2600 games without encryption.
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<sect>Memory layout<p>
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cc65 generated programs with the default setup can use RAM from
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from $1800 to $203f.
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The 4k RAM is then mapped to zero page area.
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$2040 to $20ff is visible as zero page.
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After that we have a vero small RAM area that is unused.
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$2100 to $213f.
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Then we mirror a second block from the RAM to become the hardware stack.
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This would be from $2140 to $21ff.
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The C-stack starts at $2800 and it can grow down to $2200.
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size of the system stack can be customized by defining the
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__STACKSIZE__ linker variable.
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Special locations:
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<descrip>
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<tag/Stack/ The C runtime stack is located at $2800 -
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__STACKSIZE__ and growing downwards.
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<tag/Heap/ The C heap is located at $2200 and grows upwards.
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</descrip><p>
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<sect>Start-up condition<p>
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When powered-up, the Atari 7800 TIA registers contain random
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values. During the initialization phase, the start-up code needs to
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initialize the TIA registers to sound values (or else the console has
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an unpredictable behavior). In this implementation, zeros are written
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to all of TIA registers during the start-up phase.
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Note that RIOT registers (mostly timers) are left uninitialized, as
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they don't have any consequence on the console behavior.
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<sect>Platform specific header files<p>
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Programs containing Atari 7800 specific code may use the
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<tt/atari7800.h/ header file.
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The following pseudo variables declared in the <tt/atari7800.h/ header
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file allow access to the Atari 7800 TIA, MARIA & RIOT chips registers.
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<descrip>
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<tag><tt/TIA/</tag> The <tt/TIA/ structure allows read/write access
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to the Atari 7800 TIA chip registers. See the <tt/_tia.h/ header
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file located in the include directory for the declaration of the
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structure. Also refer to the Stella Programmer's Guide by Steve
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Wright for a detailed description of the chip and its registers.
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<tag><tt/RIOT/</tag> The <tt/RIOT/ structure allows read/write
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access to the Atari 7800 RIOT chip registers. See the
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<tt/_riot.h/ header file located in the include directory for the
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declaration of the structure. Also refer to the Stella Programmer's
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Guide by Steve Wright for a detailed description of the chip and its
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registers.
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<tag><tt/MARIA/</tag> The <tt/MARIA/ structure allows read/write
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access to the Atari 7800 MARIA chip registers. See the
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<tt/_maria.h/ header file located in the include directory for the
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declaration of the structure.
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</descrip><p>
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<sect>Loadable drivers<p>
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There are no drivers for the Atari 7800.
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<sect>Limitations<p>
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TBD
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<sect>Other hints<p>
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One may write a custom linker configuration file to tune the memory
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layout of a program. See the <tt/atari7800.cfg/ file in the cfg
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directory as a starting point.
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<sect>License<p>
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This software is provided 'as-is', without any expressed or implied
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warranty. In no event will the authors be held liable for any damages
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arising from the use of this software.
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Permission is granted to anyone to use this software for any purpose,
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including commercial applications, and to alter it and redistribute it
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freely, subject to the following restrictions:
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<enum>
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<item> The origin of this software must not be misrepresented; you must not
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claim that you wrote the original software. If you use this software
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in a product, an acknowledgment in the product documentation would be
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appreciated but is not required.
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<item> Altered source versions must be plainly marked as such, and must not
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be misrepresented as being the original software.
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<item> This notice may not be removed or altered from any source
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distribution.
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</enum>
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</article>
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