This file contains an overview of the Atari 5200 runtime system as it comes with the cc65 C compiler. It describes the memory layout, Atari 5200 specific header files, available drivers, and any pitfalls specific to that platform.
Please note that Atari 5200 specific functions are just mentioned here, they are described in detail in the separate function reference. Even functions marked as "platform dependent" may be available on more than one platform. Please see the function reference for more information.
The binary output format generated by the linker for the Atari 5200 target is a cartridge image. It is of course possible to change this behaviour by using a modified startup file and linker config.
cc65 generated programs with the default setup use the RAM space from $021C to $3FFF. If you want to reserve memory for the display list and screen buffer you should define the __RESERVED_MEMORY__ linker variable. The number of bytes specified by __RESERVED_MEMORY__ are lowering the top of memory, therefore the available RAM memory for the program is $021C to $3FFF-__RESERVED_MEMORY__. The default linker config file sets __RESERVED_MEMORY__ to $1E0 to reserve space for an optional CONIO text screen.
Special locations:
The text screen is only enabled if any of the CONIO output functions is used in the program. Its size is 20x24 characters (Antic mode 6, BASIC mode 1) by default. The text screen is located at $3E00. The address of the screen memory is available at runtime in the variable SAVMSC ($001B).
If the program doesn't use any CONIO output functions it needs to setup its own display list.
The C runtime stack is located at $3FFF-__RESERVED_MEMORY__ and growing downwards.
The C heap is located at the end of the program and grows towards the C runtime stack.
Programs containing Atari 5200 specific code may use the atari5200.h header file.
This also includes access to operating system locations (e.g. hardware shadow registers) by a structure called
"OS".
The names are the usual ones you can find in system reference manuals. Example:
...
OS.sdmctl = 0x00; // screen off
OS.color4 = 14; // white frame
tics = OS.rtclok[1] // get ticks
...
The following pseudo variables declared in the atari5200.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.
GTIA_READ and GTIA_WRITEThe GTIA_READ structure allows read access to the GTIA. The
GTIA_WRITE structure allows write access to the GTIA.
See the _gtia.h header file located in the include directory
for the declaration of the structure.
POKEY_READ and POKEY_WRITEThe POKEY_READ structure allows read access to the POKEY. The
POKEY_WRITE structure allows write access to the POKEY.
See the _pokey.h header file located in the include directory
for the declaration of the structure.
ANTICThe ANTIC structure allows read access to the ANTIC.
See the _antic.h header file located in the include directory
for the declaration of the structure.
All drivers must be statically linked because no disk I/O is available. The names in the parentheses denote the symbols to be used for static linking of the drivers.
No graphics drivers are currently available for the Atari 5200.
No extended memory drivers are available for the Atari 5200.
atr5200std.joy (atr5200std_joy)A joystick driver for the standard Atari 5200 joystick is available. Depending on the version of the 5200 console, two or four joysticks can be attached.
No mouse drivers are available for the Atari 5200.
No serial drivers are available for the Atari 5200.
The atari5200 target uses Antic mode 6 (BASIC mode 1) for the console
screen by default. There are four colors available:
COLOR_WHITECOLOR_REDCOLOR_GREENCOLOR_BLACKNote that the COLOR_GREEN and COLOR_RED colors aren't
exactly the same colors as the ones with the same name on the
atari target.
They are the colors which are available as COLOR_LIGHTGREEN
and COLOR_LIGHTRED there.
One can set the color shadow registers directly with other colors.
Then the color defines from above will just become placeholders. In
this scenario it might be more convenient to use index values (0..3)
instead of the color defines. The index values specify which of the
system shadow color registers (COLOR0 .. COLOR3) to use.
The default console screen has a layout of 20x24 characters. An
alternative layout, 20x12, Antic mode 7, BASIC mode 2, is provided in
the file atari5200-conioscreen-20x12.o.
Using atari5200-conioscreen-20x12.o is as simple as placing it on
the linker command line like this:
cl65 -t atari5200 myprog.c atari5200-conioscreen-20x12.o
Disk I/O is not supported by the atari5200 target. This means that
you cannot use any of the following functions (and a few others):
AtariROMMaker (
https://www.wudsn.com/index.php/productions-atari800/tools/atarirommaker )
can be used to create a .CAR file from the binary ROM image cc65 generates.
This might be more convenient when working with emulators.
The 5200 ROM displays a splash screen at startup with the name of the game and the copyright year. The year information has a 'Year-2000' problem, the first two digits are fixed in the ROM and are always "19".
The runtime library provides a default game name which is "cc65
compiled". To change that, one has to link a file which puts data into
the "CARTNAME" segment.
For reference, here's the default version used by the cc65 library:
.export __CART_NAME__: absolute = 1
.macpack atari
.segment "CARTNAME"
scrcode " cc"
.byte '6' + 32, '5' + 32 ; use playfield 1
scrcode " compiled"
'__CART_NAME__' needs to be defined in order that the linker is
satisfied and doesn't try to include the version of the runtime library.
20 bytes are available in the CARTNAME segment (one line) for the
game/program name.
The century is hard-coded to 1900 by the ROM.
There are two digits which can be changed. For example "92" will give "1992" on the screen.
The default used by the runtime library is
.export __CART_YEAR__: absolute = 1
.segment "CARTYEAR"
.byte '9' + 32,'8' + 32 ; "98", using playfield 1
If the second byte of the year in the CARTYEAR segment is 255,
the cartridge is seen as a 'diagnostic' cartridge, and the splash
screen and most of the other startup initializations are bypassed.
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: