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 __STARTUP_LOAD__ and __ZPSAVE_LOAD__ values, so keep
43 this in mind if you create a custom linker config file and start
44 moving segments around (see section
45 <ref name="Reserving a memory area inside the program" id="memhole">).
46 You can override this behaviour by creating your own crt0.s file and
47 linking it into your program. A run vector is added to the end of the
48 file ($02E0 <run vector>) and is calculated using
49 __STARTUP_LOAD__ in crt0.s.
52 <sect>Memory layout<p>
54 The default linker script assumes that the BASIC ROM is disabled (or
55 the BASIC cartridge unplugged). This gives a usable memory range from
56 $2E00 - $BC1F. The library startup code examines the
57 current memory configuration, which depends on the size of the
58 installed memory and cartridges present, by inspecting the value in
59 the MEMTOP ($2E5) variable. Then the initial stack pointer,
60 which indicates the upper bound of memory used, is adjusted. The
61 default load address of $2E00 was chosen to accommodate having
62 a DOS loaded and a driver that resides in low memory such as the 850
63 R: handler. You can override this behaviour by creating a custom
64 linker config file or by using the "--start-addr" cl65 command line
65 argument or the "--start-addr" or "-S" ld65 command line arguments.
71 The text screen depends on the installed memory size and cartridges
72 and can be obtained from the SAVMSC variable ($58).
75 The C runtime stack is located at MEMTOP and grows downwards,
76 regardless of how your linker config file is setup. This
77 accommodates the different memory configurations of the Atari
78 machines, as well as having a cartridge installed. You can override
79 this behaviour by writing your own crt0.s file and linking it to
80 your program (see also <ref name="Final note"
81 id="memhole_final_note">).
84 The C heap is located at the end of the program and grows towards the C
91 <sect>Platform specific header files<p>
93 Programs containing Atari specific code may use the <tt/atari.h/
97 <sect1>Atari specific functions<p>
99 The functions listed below are special for the Atari. See the <htmlurl
100 url="funcref.html" name="function reference"> for declaration and usage.
117 <sect1>Hardware access<p>
119 The following pseudo variables declared in the <tt/atari.h/ header
120 file do allow access to hardware located in the address space. Some
121 variables are structures, accessing the struct fields will access the
126 <tag><tt/GTIA_READ/ and <tt/GTIA_WRITE/</tag>
127 The <tt/GTIA_READ/ structure allows read access to the GTIA. The
128 <tt/GTIA_WRITE/ structure allows write access to the GTIA.
129 See the <tt/_gtia.h/ header file located in the include directory
130 for the declaration of the structure.
132 <tag><tt/POKEY_READ/ and <tt/POKEY_WRITE/</tag>
133 The <tt/POKEY_READ/ structure allows read access to the POKEY. The
134 <tt/POKEY_WRITE/ structure allows write access to the POKEY.
135 See the <tt/_pokey.h/ header file located in the include directory
136 for the declaration of the structure.
138 <tag><tt/ANTIC/</tag>
139 The <tt/ANTIC/ structure allows read access to the ANTIC.
140 See the <tt/_antic.h/ header file located in the include directory
141 for the declaration of the structure.
144 The <tt/PIA/ structure allows read access to the PIA 6520.
145 See the <tt/_pia.h/ header file located in the include directory
146 for the declaration of the structure.
152 <sect>Loadable drivers<p>
154 <sect1>Graphics drivers<p>
156 Currently there are no graphics drivers available for the Atari platform.
157 However, the runtime library provides a function named _graphics, with
158 a mode parameter just like the BASIC GRAPHICS command. This function will
159 switch to the requested graphics mode.
160 There are currently no functions available to access the graphics
161 memory. The access must be implemented manually.
163 Many graphics modes require more memory than the text screen which is
164 in effect when the program starts up. Therefore the programmer has to
165 tell the program beforehand the memory requirements of the graphics
166 modes the program intends to use.
167 This can be done by using the __RESERVED_MEMORY__ linker config
168 variable. The number specified there describes the number of bytes to
169 subtract from the top of available memory as seen from the runtime
170 library. This memory is then used by the screen buffer.
172 The numbers for the different graphics modes presented below should
173 only be seen as a rule of thumb. Since the screen buffer memory needs
174 to start at specific boundaries, the numbers depend on the current top
176 The following numbers were determined by a BASIC program.
180 graphics mode|reserved memory@<hline>
214 <caption>reserved memory required for different graphics modes
217 The values of "1" are needed because the graphics command crashes if
218 it doesn't have at least one byte available. This seems to be a bug of
221 <sect1>Extended memory drivers<p>
223 Currently there are no extended memory drivers available for the Atari
226 <sect1>Joystick drivers<p>
230 <tag><tt/ataristd.joy/</tag>
231 Supports up to four standard joysticks connected to the joystick ports of
234 <tag><tt/atarim8.joy/</tag>
235 Supports up to eight standard joysticks connected to a MultiJoy adapter.
241 <sect1>Mouse drivers<p>
243 Currently no drivers available (in fact, the API for loadable mouse drivers
244 does not exist). There is a static driver you can use.
247 <sect1>RS232 device drivers<p>
249 Currently there are no RS232 loadable drivers available for the Atari
250 platform. There is a static driver you can use.
256 <sect>DIO implementation<label id="dio"><p>
258 The Atari supports disk drives with either 128 or 256 byte sectors.
259 The first three sectors of any disk are always 128 bytes long though. This is
260 because the system can only boot from 128 bytes sectors.
262 Therefore the DIO read and write functions transfer only 128 bytes
263 for sectors 1 to 3, regardless of the type of diskette.
266 <sect>CONIO implementation<label id="conio"><p>
268 The console I/O is speed optimized therefore support for XEP80 hardware
269 or f80.com software is missing. Of course you may use stdio.h functions.
274 <sect1>Function keys<p>
276 Function keys are mapped to Atari + number key.
278 <sect1>Reserving a memory area inside a program<label id="memhole"><p>
280 The Atari 130XE maps its additional memory into CPU memory in 16K
281 chunks at address $4000 to $7FFF. One might want to
282 prevent this memory area from being used by cc65. Other reasons to
283 prevent the use of some memory area could be to reserve space for the
284 buffers for display lists and screen memory.
286 The Atari executable format allows holes inside a program, e.g. one
287 part loads into $2E00 to $3FFF, going below the reserved
288 memory area (assuming a reserved area from $4000 to
289 $7FFF), and another part loads into $8000 to
292 Each load chunk of the executable starts with a 4 byte header which
293 defines its load address and size. In the following linker scripts
294 these headers are named HEADER and SECHDR (for the MEMORY layout), and
295 accordingly NEXEHDR and CHKHDR (for the SEGMENTS layout).
297 <sect2>Low code and high data example<p>
298 Goal: Create an executable with 2 load chunks which doesn't use the
299 memory area from $4000 to $7FFF. The CODE segment of
300 the program should go below $4000 and the DATA and RODATA
301 segments should go above $7FFF.
303 The main problem is that the EXE header generated by the cc65 runtime
304 lib is wrong. It defines a single load chunk with the sizes/addresses
305 of the STARTUP, LOWCODE, INIT, CODE, RODATA, and DATA segments (the whole user
308 The contents of the EXE header come from the EXEHDR segment, which is
309 defined in crt0.s. This cannot be changed without modifying and
310 recompiling the cc65 atari runtime lib. Therefore the original EXE
311 header must be discarded. It will be replaced by a user created
312 one. The discarding is done by assigning the EXEHDR segment to the
313 BANK memory area. The BANK memory area is discarded in the new linker
314 script (written to file "").
316 The user needs to create a customized linker config file which adds
317 new memory areas and segments to hold the new EXE header and the
318 header data for the second load chunk. Also an assembly source file
319 needs to be created which defines the contents of the new EXE header
320 and the second load chunk header.
323 This is an example of a modified cc65 Atari linker configuration file
327 __STACKSIZE__ = $800; # 2K stack
328 __RESERVED_MEMORY__: value = $0000, weak = yes;
331 STARTADDRESS: default = $2E00;
334 ZP: start = $82, size = $7E, type = rw, define = yes;
336 HEADER: start = $0000, size = $6, file = %O; # first load chunk
337 RAMLO: start = %S, size = $4000 - %S, file = %O;
339 BANK: start = $4000, size = $4000, file = "";
341 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
342 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
345 EXEHDR: load = BANK, type = ro;
347 NEXEHDR: load = HEADER, type = ro; # first load chunk
348 STARTUP: load = RAMLO, type = ro, define = yes;
349 LOWCODE: load = RAMLO, type = ro, define = yes, optional = yes;
350 INIT: load = RAMLO, type = ro, optional = yes;
351 CODE: load = RAMLO, type = ro, define = yes;
353 CHKHDR: load = SECHDR, type = ro; # second load chunk
354 RODATA: load = RAM, type = ro, define = yes;
355 DATA: load = RAM, type = rw, define = yes;
356 BSS: load = RAM, type = bss, define = yes;
357 ZPSAVE: load = RAM, type = bss, define = yes;
359 ZEROPAGE: load = ZP, type = zp;
360 AUTOSTRT: load = RAM, type = ro; # defines program entry point
363 CONDES: segment = RODATA,
365 label = __CONSTRUCTOR_TABLE__,
366 count = __CONSTRUCTOR_COUNT__;
367 CONDES: segment = RODATA,
369 label = __DESTRUCTOR_TABLE__,
370 count = __DESTRUCTOR_COUNT__;
375 A new memory area BANK was added which describes the reserved area.
376 It gets loaded with the contents of the old EXEHDR segment. But the
377 memory area isn't written to the output file. This way the contents of
378 the EXEHDR segment get discarded.
380 The newly added NEXEHDR segment defines the correct EXE header. It
381 puts the STARTUP, LOWCODE, INIT, and CODE segments, which are the
382 segments containing only code, into load chunk #1 (RAMLO memory area).
384 The header for the second load chunk comes from the new CHKHDR
385 segment. It puts the RODATA, DATA, BSS, and ZPSAVE segments into load
386 chunk #2 (RAM memory area).
389 The contents of the new NEXEHDR and CHKHDR segments come from this
392 .import __CODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__
393 .import __DATA_LOAD__, __RODATA_LOAD__, __STARTUP_LOAD__
397 .word __STARTUP_LOAD__
398 .word __CODE_LOAD__ + __CODE_SIZE__ - 1
401 .word __RODATA_LOAD__
402 .word __BSS_LOAD__ - 1
407 cl65 -t atari -C split.cfg -o prog.com prog.c split.s
410 <sect2>Low data and high code example<p>
413 Goal: Put RODATA and DATA into low memory and STARTUP, LOWCODE, INIT,
414 CODE, BSS, ZPSAVE into high memory (split2.cfg):
418 __STACKSIZE__ = $800; # 2K stack
419 __RESERVED_MEMORY__: value = $0000, weak = yes;
422 STARTADDRESS: default = $2E00;
425 ZP: start = $82, size = $7E, type = rw, define = yes;
427 HEADER: start = $0000, size = $6, file = %O; # first load chunk
428 RAMLO: start = %S, size = $4000 - %S, file = %O;
430 BANK: start = $4000, size = $4000, file = "";
432 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
433 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
436 EXEHDR: load = BANK, type = ro; # discarded old EXE header
438 NEXEHDR: load = HEADER, type = ro; # first load chunk
439 RODATA: load = RAMLO, type = ro, define = yes;
440 DATA: load = RAMLO, type = rw, define = yes;
442 CHKHDR: load = SECHDR, type = ro; # second load chunk
443 STARTUP: load = RAM, type = ro, define = yes;
444 INIT: load = RAM, type = ro, optional = yes;
445 CODE: load = RAM, type = ro, define = yes;
446 ZPSAVE: load = RAM, type = bss, define = yes;
447 BSS: load = RAM, type = bss, define = yes;
449 ZEROPAGE: load = ZP, type = zp;
450 AUTOSTRT: load = RAM, type = ro; # defines program entry point
453 CONDES: segment = RODATA,
455 label = __CONSTRUCTOR_TABLE__,
456 count = __CONSTRUCTOR_COUNT__;
457 CONDES: segment = RODATA,
459 label = __DESTRUCTOR_TABLE__,
460 count = __DESTRUCTOR_COUNT__;
464 New contents for NEXEHDR and CHKHDR are needed (split2.s):
466 .import __STARTUP_LOAD__, __ZPSAVE_LOAD__, __DATA_SIZE__
467 .import __DATA_LOAD__, __RODATA_LOAD__
471 .word __RODATA_LOAD__
472 .word __DATA_LOAD__ + __DATA_SIZE__ - 1
475 .word __STARTUP_LOAD__
476 .word __ZPSAVE_LOAD__ - 1
481 cl65 -t atari -C split2.cfg -o prog.com prog.c split2.s
484 <sect2>Final note<label id="memhole_final_note"><p>
486 There are two other memory areas which don't appear directly in the
487 linker script. They are the stack and the heap.
489 The cc65 runtime lib places the stack location at the end of available
490 memory. This is dynamically set from the MEMTOP system variable at
491 startup. The heap is located in the area between the end of the BSS
492 segment and the top of the stack as defined by __STACKSIZE__.
494 If BSS and/or the stack shouldn't stay at the end of the program,
495 some parts of the cc65 runtime lib need to be replaced/modified.
497 common/_heap.s defines the location of the heap and atari/crt0.s
498 defines the location of the stack by initializing sp.
501 <sect>Bugs/Feedback<p>
503 If you have problems using the library, if you find any bugs, or if you're
504 doing something interesting with it, I would be glad to hear from you. Feel
505 free to contact me by email (<htmlurl url="mailto:uz@cc65.org"
506 name="uz@cc65.org"> or <htmlurl url="mailto:chris@groessler.org"
507 name="chris@groessler.org"> ).
513 This software is provided 'as-is', without any expressed or implied
514 warranty. In no event will the authors be held liable for any damages
515 arising from the use of this software.
517 Permission is granted to anyone to use this software for any purpose,
518 including commercial applications, and to alter it and redistribute it
519 freely, subject to the following restrictions:
522 <item> The origin of this software must not be misrepresented; you must not
523 claim that you wrote the original software. If you use this software
524 in a product, an acknowledgment in the product documentation would be
525 appreciated but is not required.
526 <item> Altered source versions must be plainly marked as such, and must not
527 be misrepresented as being the original software.
528 <item> This notice may not be removed or altered from any source