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>20-Feb-2005 <abstract> An overview over the Atari runtime system as it is implemented for the cc65 C compiler. </abstract>  <toc>  <sect>Overview 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 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 <2 byte start address> <2 bytes end address> [program bytes]). 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 ($02E0 <run vector>) and is calculated using __CODE_LOAD__ in crt0.s. <sect>Memory layout The default linker script assumes that the BASIC ROM is disabled (or the BASIC cartridge unplugged). This gives a usable memory range from $2E00 - $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 ($2E5) variable. Then the initial stack pointer, which indicates the upper bound of memory used, is adjusted. The load address of $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 ($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> <sect>Platform specific header files Programs containing Atari specific code may use the <tt/atari.h/ header file. <sect1>Atari specific functions 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 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> <sect>Loadable drivers <sect1>Graphics drivers Currently there are no graphics drivers available for the Atari platform. <sect1>Extended memory drivers Currently there are no extended memory drivers available for the Atari platform. <sect1>Joystick drivers <descrip> <tag><tt/atari-stdjoy.joy/</tag> Supports up to four standard joysticks connected to the joystick ports of the Atari. </descrip> <sect1>Mouse drivers 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 Currently there are no RS232 loadable drivers available for the Atari platform. There is a static driver you can use. <sect>Limitations <sect>DIO implementation<label id="dio"> 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 <sect1>Function keys These are defined to be Atari + number key. <sect1>Reserving a memory area inside a program<label id="memhole"> The Atari 130XE maps its additional memory into CPU memory in 16K chunks at address $4000 to $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. The Atari executable format allows holes inside a program, e.g. one part loads into $2E00 to $3FFF, going below the reserved memory area (assuming a reserved area from $4000 to $7FFF), and another part loads into $8000 to $BC1F. Each load chunk of the executable starts with a 4 byte header which defines its load address and size. <sect2>Low code and high data example Goal: Create an executable with 2 load chunks which doesn't use the memory area from $4000 to $7FFF. The CODE segment of the program should go below $4000 and the DATA and RODATA segments should go above $7FFF. 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 CODE, RODATA, and DATA segments (the whole user program). 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. 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. 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 } SEGMENTS { EXEHDR: load = BANK, type = wprot; NEXEHDR: load = HEADER, type = wprot; # first load chunk CODE: load = RAMLO, type = wprot, define = yes; CHKHDR: load = SECHDR, type = wprot; # second load chunk RODATA: load = RAM, type = wprot, define = yes; DATA: load = RAM, type = rw, define = yes; BSS: load = RAM, type = bss, define = yes; ZEROPAGE: load = ZP, type = zp; AUTOSTRT: load = RAM, type = wprot; # 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 } </verb></tscreen> 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. The added NEXEHDR segment defines the correct EXE header. It puts only the CODE segment into load chunk #1 (RAMLO memory area). 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). The contents of the new NEXEHDR and CHKHDR segments come from this file (split.s): <tscreen><verb> .import __CODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__ .import __DATA_LOAD__, __RODATA_LOAD__ .segment "NEXEHDR" .word $FFFF ; EXE file magic number ; 1st load chunk .word __CODE_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> 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 Goal: Put RODATA and DATA into low memory and CODE with 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 } SEGMENTS { EXEHDR: load = BANK, type = wprot; # discarded old EXE header NEXEHDR: load = HEADER, type = wprot; # first load chunk RODATA: load = RAMLO, type = wprot, define = yes; DATA: load = RAMLO, type = rw, define = yes; CHKHDR: load = SECHDR, type = wprot; # second load chunk CODE: load = RAM, type = wprot, define = yes; BSS: load = RAM, type = bss, define = yes; ZEROPAGE: load = ZP, type = zp; AUTOSTRT: load = RAM, type = wprot; # 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 } </verb></tscreen> New contents for NEXEHDR and CHKHDR are needed (split2.s): <tscreen><verb> .import __CODE_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 __CODE_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"> 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 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 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>