1 <!doctype linuxdoc system>
5 <title>Atari specific information for cc65
6 <author>Shawn Jefferson, <htmlurl
7 url="mailto:shawnjefferson@24fightingchickens.com"
8 name="shawnjefferson@24fightingchickens.com"> and
9 Christian Groessler, <htmlurl url="mailto:cpg@aladdin.de" name="cpg@aladdin.de">
13 An overview over the Atari runtime system as it is implemented for the cc65 C
17 <!-- Table of contents -->
20 <!-- Begin the document -->
24 This file contains an overview of the Atari runtime system as it comes
25 with the cc65 C compiler. It describes the memory layout, Atari specific
26 header files, available drivers, and any pitfalls specific to that
29 Please note that Atari specific functions are just mentioned here, they are
30 described in detail in the separate <htmlurl url="funcref.html" name="function
31 reference">. Even functions marked as "platform dependent" may be available on
32 more than one platform. Please see the function reference for more
36 <sect>Binary format<p>
38 The standard binary output format generated by the linker for the
39 Atari target is a machine language program with a standard executable
40 header (FF FF <2 byte start address> <2 bytes end address>
41 [program bytes]). These values are calculated in the crt0.s
42 file from the __CODE_LOAD__ and __BSS_LOAD__ values, so keep this in
43 mind if you create a custom linker config file and start moving
44 segments around (see section <ref name="Reserving a memory area inside the program" id="memhole">). You can
45 override this behaviour by creating your own crt0.s file and linking
46 it into your program. A run vector is added to the end of the file
47 ($02E0 <run vector>) and is calculated using
48 __CODE_LOAD__ in crt0.s.
51 <sect>Memory layout<p>
53 The default linker script assumes that the BASIC ROM is disabled (or
54 the BASIC cartridge unplugged). This gives a usable memory range from
55 $2E00 - $BC1F. The library startup code examines the
56 current memory configuration, which depends on the size of the
57 installed memory and cartridges present, by inspecting the value in
58 the MEMTOP ($2E5) variable. Then the initial stack pointer,
59 which indicates the upper bound of memory used, is adjusted. The load
60 address of $2E00 was chosen to accommodate having a DOS loaded
61 and a driver that resides in low memory such as the 850 R: handler.
62 You can override this behaviour by creating a custom linker config
69 The text screen depends on the installed memory size and cartridges
70 and can be obtained from the SAVMSC variable ($58).
73 The C runtime stack is located at MEMTOP and grows downwards,
74 regardless of how your linker config file is setup. This
75 accomodates the different memory configurations of the Atari
76 machines, as well as having a cartridge installed. You can override
77 this behaviour by writing your own crt0.s file and linking it to
78 your program (see also <ref name="Final note"
79 id="memhole_final_note">).
82 The C heap is located at the end of the program and grows towards the C
89 <sect>Platform specific header files<p>
91 Programs containing Atari specific code may use the <tt/atari.h/
95 <sect1>Atari specific functions<p>
97 The functions listed below are special for the Atari. See the <htmlurl
98 url="funcref.html" name="function reference"> for declaration and usage.
115 <sect1>Hardware access<p>
117 The following pseudo variables declared in the <tt/atari.h/ header
118 file do allow access to hardware located in the address space. Some
119 variables are structures, accessing the struct fields will access the
124 <tag><tt/GTIA_READ/ and <tt/GTIA_WRITE/</tag>
125 The <tt/GTIA_READ/ structure allows read access to the GTIA. The
126 <tt/GTIA_WRITE/ structure allows write access to the GTIA.
127 See the <tt/_gtia.h/ header file located in the include directory
128 for the declaration of the structure.
130 <tag><tt/POKEY_READ/ and <tt/POKEY_WRITE/</tag>
131 The <tt/POKEY_READ/ structure allows read access to the POKEY. The
132 <tt/POKEY_WRITE/ structure allows write access to the POKEY.
133 See the <tt/_pokey.h/ header file located in the include directory
134 for the declaration of the structure.
136 <tag><tt/ANTIC/</tag>
137 The <tt/ANTIC/ structure allows read access to the ANTIC.
138 See the <tt/_antic.h/ header file located in the include directory
139 for the declaration of the structure.
142 The <tt/PIA/ structure allows read access to the PIA 6520.
143 See the <tt/_pia.h/ header file located in the include directory
144 for the declaration of the structure.
150 <sect>Loadable drivers<p>
152 <sect1>Graphics drivers<p>
154 Currently there are no graphics drivers available for the Atari platform.
156 <sect1>Extended memory drivers<p>
158 Currently there are no extended memory drivers available for the Atari
161 <sect1>Joystick drivers<p>
165 <tag><tt/atari-stdjoy.joy/</tag>
166 Supports up to four standard joysticks connected to the joystick ports of
173 <sect1>Mouse drivers<p>
175 Currently no drivers available (in fact, the API for loadable mouse drivers
176 does not exist). There is a static driver you can use.
179 <sect1>RS232 device drivers<p>
181 Currently there are no RS232 loadable drivers available for the Atari
182 platform. There is a static driver you can use.
191 <sect1>Function keys<p>
193 These are defined to be Atari + number key.
195 <sect1>Reserving a memory area inside the program<label id="memhole"><p>
197 The Atari 130XE maps its additional memory into CPU memory in 16K
198 chunks at address $4000 to $7FFF. One might want to
199 prevent this memory area from being used by cc65. Other reasons to
200 prevent the use of some memory area could be the buffers for display
201 lists and screen memory.
203 The Atari executable format allows holes inside a program, e.g. one
204 part loads into $2E00 to $3FFF, going below the reserved
205 memory area (assuming a reserved area from $4000 to
206 $7FFF), and another part loads into $8000 to
209 Each load chunk of the executable starts with a 4 byte header which
210 defines its load address and size.
212 <sect2>Low code and high data example<p>
213 Goal: Create an executable with 2 load chunks which doesn't use the
214 memory area from $4000 to $7FFF. The CODE segment of
215 the program should go below $4000 and the DATA and RODATA
216 segments should go above $7FFF.
218 The main problem is that the EXE header generated by the cc65 runtine
219 lib is wrong. It defines a single load chunk with the sizes/addresses
220 of the CODE, RODATA, and DATA segments (the whole user program).
222 The contents of the EXE header come from the EXEHDR segment, which is
223 defined in crt0.s. This cannot be changed w/o modifiying and
224 recompiling the cc65 atari runtime lib. Therefore the original EXE
225 header must be discarded. It will be replaced by a user created
228 The user needs to create a customized linker config file which adds
229 new memory areas and segments to hold the new EXE header and added
230 load chunk header data. Also an assembly source file needs to be
231 created which defines the contents of the new EXE header and the
232 second load chunk header.
235 This is a modified cc65 Atari linker configuration file (split.cfg):
238 ZP: start = $82, size = $7E, type = rw, define = yes;
240 HEADER: start = $0000, size = $6, file = %O; # first load chunk
241 RAMLO: start = $2E00, size = $1200, file = %O;
243 BANK: start = $4000, size = $4000, file = "";
245 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
246 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
249 EXEHDR: load = BANK, type = wprot;
251 NEXEHDR: load = HEADER, type = wprot; # first load chunk
252 CODE: load = RAMLO, type = wprot, define = yes;
254 CHKHDR: load = SECHDR, type = wprot; # second load chunk
255 RODATA: load = RAM, type = wprot, define = yes;
256 DATA: load = RAM, type = rw, define = yes;
257 BSS: load = RAM, type = bss, define = yes;
259 ZEROPAGE: load = ZP, type = zp;
260 AUTOSTRT: load = RAM, type = wprot; # defines program entry point
263 CONDES: segment = RODATA,
265 label = __CONSTRUCTOR_TABLE__,
266 count = __CONSTRUCTOR_COUNT__;
267 CONDES: segment = RODATA,
269 label = __DESTRUCTOR_TABLE__,
270 count = __DESTRUCTOR_COUNT__;
273 __STACKSIZE__ = $800; # 2K stack
278 A new memory area BANK was added which describes the reserved area.
279 It gets loaded with the contents of the old EXEHDR segment. But the
280 memory area isn't written to the output file. This way the contents of
281 the EXEHDR segment get discarded.
283 The added NEXEHDR segment defines the correct EXE header. It puts only
284 the CODE segment into load chunk #1 (RAMLO memory area).
286 The header for the second load chunk comes from the new CHKHDR
287 segment. It puts the RODATA and DATA segments into load chunk #2 (RAM
291 The contents of the new NEXEHDR and CHKHDR segments come from this
294 .import __CODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__
295 .import __DATA_LOAD__, __RODATA_LOAD__
298 .word $FFFF ; EXE file magic number
301 .word __CODE_LOAD__ + __CODE_SIZE__ - 1
304 ; 2nd load chunk (contains with AUTOSTRT in fact a 3rd load chunk)
305 .word __RODATA_LOAD__
306 .word __BSS_LOAD__ - 1
311 cl65 -t atari -C split.cfg -o prog.com prog.c split.s
314 <sect2>Low data and high code example<p>
317 Goal: Put RODATA and DATA into low memory and CODE with BSS into high
322 ZP: start = $82, size = $7E, type = rw, define = yes;
324 HEADER: start = $0000, size = $6, file = %O; # first load chunk
325 RAMLO: start = $2E00, size = $1200, file = %O;
327 BANK: start = $4000, size = $4000, file = "";
329 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
330 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
333 EXEHDR: load = BANK, type = wprot; # discarded old EXE header
335 NEXEHDR: load = HEADER, type = wprot; # first load chunk
336 RODATA: load = RAMLO, type = wprot, define = yes;
337 DATA: load = RAMLO, type = rw, define = yes;
339 CHKHDR: load = SECHDR, type = wprot; # second load chunk
340 CODE: load = RAM, type = wprot, define = yes;
341 BSS: load = RAM, type = bss, define = yes;
343 ZEROPAGE: load = ZP, type = zp;
344 AUTOSTRT: load = RAM, type = wprot; # defines program entry point
347 CONDES: segment = RODATA,
349 label = __CONSTRUCTOR_TABLE__,
350 count = __CONSTRUCTOR_COUNT__;
351 CONDES: segment = RODATA,
353 label = __DESTRUCTOR_TABLE__,
354 count = __DESTRUCTOR_COUNT__;
357 __STACKSIZE__ = $800; # 2K stack
361 New contents for NEXEHDR and CHKHDR are needed (split2.s):
363 .import __CODE_LOAD__, __BSS_LOAD__, __DATA_SIZE__
364 .import __DATA_LOAD__, __RODATA_LOAD__
368 .word __RODATA_LOAD__
369 .word __DATA_LOAD__ + __DATA_SIZE__ - 1
373 .word __BSS_LOAD__ - 1
378 cl65 -t atari -C split2.cfg -o prog.com prog.c split2.s
381 <sect2>Final note<label id="memhole_final_note"><p>
383 There are two other memory areas which don't appear directly in the
384 linker script. They are the stack and the heap.
386 The cc65 runtime lib places the stack location at the end of available
387 memory. This is dynamically set from the MEMTOP system variable at
388 startup. The heap is located in the area between the end of the BSS
389 segment and the top of the stack as defined by __STACKSIZE__.
391 If BSS and/or the stack shouldn't stay at the end of the program,
392 some parts of the cc65 runtime lib need to be replaced/modified.
394 common/_heap.s defines the location of the heap and atari/crt0.s
395 defines the location of the stack by initializing sp.
398 <sect>Bugs/Feedback<p>
400 If you have problems using the library, if you find any bugs, or if you're
401 doing something interesting with it, I would be glad to hear from you. Feel
402 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
403 name="uz@cc65.org"> or <htmlurl url="mailto:cpg@aladdin.de"
404 name="cpg@aladdin.de">).
410 This software is provided 'as-is', without any expressed or implied
411 warranty. In no event will the authors be held liable for any damages
412 arising from the use of this software.
414 Permission is granted to anyone to use this software for any purpose,
415 including commercial applications, and to alter it and redistribute it
416 freely, subject to the following restrictions:
419 <item> The origin of this software must not be misrepresented; you must not
420 claim that you wrote the original software. If you use this software
421 in a product, an acknowledgment in the product documentation would be
422 appreciated but is not required.
423 <item> Altered source versions must be plainly marked as such, and must not
424 be misrepresented as being the original software.
425 <item> This notice may not be removed or altered from any source