1 <!doctype linuxdoc system>
5 <title>Atari specific information for cc65
7 <url url="mailto:shawnjefferson@24fightingchickens.com" name="Shawn Jefferson"> and<newline>
8 <url url="mailto:chris@groessler.org" name="Christian Groessler">
12 An overview over the Atari runtime system as it is implemented for the cc65 C
16 <!-- Table of contents -->
19 <!-- Begin the document -->
23 This file contains an overview of the Atari runtime system as it comes
24 with the cc65 C compiler. It describes the memory layout, Atari specific
25 header files, available drivers, and any pitfalls specific to that
28 The Atari runtime support comes in two flavors: <tt/atari/ and <tt/atarixl/.
29 The <tt/atari/ target supports all Atari 8-bit computers, the <tt/atarixl/ only
30 supports XL type or newer machines (excluding the 600XL).
32 The <tt/atarixl/ runtime makes the whole 64K of memory available, with the
33 exception of the I/O area at $D000 - $D7FF. Since the
34 <tt/atarixl/ runtime has some <ref name="limitations" id="limitations">, it is
35 recommended to use the <tt/atari/ target unless lack of memory dictates the
36 use of the <tt/atarixl/ target.
38 Please note that Atari specific functions are just mentioned here, they are
39 described in detail in the separate <url url="funcref.html" name="function
40 reference">. Even functions marked as "platform dependent" may be available on
41 more than one platform. Please see the function reference for more
45 <sect>Binary format<p>
47 The Atari DOS executable file format supports more than one load block (<it/chunk/).
49 The default binary output format generated by the linker for the
50 Atari target is a machine language program with a standard executable
51 header (FF FF <load chunk #1> ... <load chunk #n>).
52 A load chunk has the format [<2 byte start address> <2 bytes end address>
53 <chunk data>].
54 A run vector is added to the end of the
55 file ($02E0 $02E1 <run vector>) and is calculated using
56 the <tt/start/ label in crt0.s. (Technically the run vector is also a load chunk,
57 but is not regarded as such here.)
59 An <tt/atari/ program has two load chunks, an <tt/atarixl/ program has three load
60 chunks. The load chunks are defined in the linker configuration files. For more
61 detailed information about the load chunks see the chapter
62 <ref name="Technical details" id="techdetail">. For the discussion here it's
63 sufficient to know that the first load chunk(s) do preparation work and the
64 main part of the program is in the last load chunk.
66 The values determining the size of the main part of the program (the second load
67 chunk for <tt/atari/, the third load chunk for <tt/atarixl/) are calculated in
68 the crt0.s file from the __STARTUP_LOAD__ and __BSS_LOAD__ values.
69 Be aware of that if you create a custom linker config file and start moving segments around (see section
70 <ref name="Reserving a memory area inside the program" id="memhole">).
73 <sect>Memory layout<p>
75 <sect1><tt/atari/ target<p>
77 The default linker config file assumes that the BASIC ROM is disabled (or
78 the BASIC cartridge unplugged). This gives a usable memory range of
79 [$2000-$BC1F]. The library startup code examines the
80 current memory configuration, which depends on the size of the
81 installed memory and cartridges. It does so by using the value in
82 the MEMTOP ($2E5) variable as highest memory address the program
83 can use. The initial stack pointer, which is the upper bound of
84 memory used by the program, is set to this value, minus an optionally
85 defined __RESERVED_MEMORY__ value.
87 The default load address of $2000 can be changed by creating a custom
88 linker config file or by using the "--start-addr" cl65 command line
89 argument or the "--start-addr" or "-S" ld65 command line arguments.
91 Please note that the first load chunk (which checks the available memory)
92 will always be loaded at $2E00, regardless of the specified start
93 address. This address can only be changed by a custom linker config file.
99 The text screen depends on the installed memory size and cartridges
100 and can be obtained from the SAVMSC variable ($58).
103 The C runtime stack is located at MEMTOP and grows downwards,
104 regardless of how your linker config file is setup. This
105 accommodates the different memory configurations of the Atari
106 machines, as well as having a cartridge installed. You can override
107 this behaviour by writing your own crt0.s file and linking it to
108 your program (see also <ref name="Final note"
109 id="memhole_final_note">).
112 The C heap is located at the end of the program and grows towards the C
117 <sect1><tt/atarixl/ target<p>
119 The startup code rearranges the memory as follows:
122 <item>Sceen memory and display list are moved below the program start address.
123 <item>The ROM is disabled, making the memory in the areas [$C000-$CFFF]
124 and [$D800-$FFF9] available.
125 <item>Character generator data is copied from ROM to the CHARGEN location specified in the
126 linker config file. This is (in the default <tt/atarixl.cfg/ file) at the same address as
127 where it is in ROM ($E000, it can be changed, see <ref name="atarixl chargen location"
128 id="chargenloc">). With the character generator at $E000, there are two upper memory
129 areas available, [$D800-$DFFF] and [$E400-$FFF9].
132 With the default load address of $2400 this gives a usable memory range of
133 [$2400-$CFFF].
135 Please note that the first load chunk (which checks the system
136 compatibilty and available memory) will always be loaded at
137 $2E00, regardless of the specified start address. This address
138 can only be changed by a custom linker config file.
144 The text screen depends on the selected load address ($2400
145 by default), and resides directly before that address, rounded to the next
147 The screen memory's start address can be obtained from the SAVMSC variable
151 The C runtime stack is located at end of the MAIN memory area ($CFFF)
155 The C heap is located at the end of the program (end of BSS segment) and
156 grows towards the C runtime stack.
160 <sect>Linker configurations<p>
162 The ld65 linker comes with default config files for the Atari. There
163 are two targets for the Atari, <tt/atari/ and <tt/atarixl/.
164 The default config file for <tt/atari/ is selected with
165 <tt/-t atari/, and the default config file for <tt/atarixl/ is selected with
167 The Atari package comes with additional secondary linker config files which
168 can be used via <tt/-t atari -C <configfile>/ (for <tt/atari/ target) or
169 <tt/-t atarixl -C <configfile>/ (for <tt/atarixl/ target).
171 <sect1><tt/atari/ config files<p>
173 <sect2>default config file (<tt/atari.cfg/)<p>
175 The default configuration is tailored to C programs. It creates files
176 which have a default load address of $2000.
178 The files generated by this config file include the
179 <ref name="&dquot;system check&dquot;" id="syschk"> load chunk. It can
180 optionally be left out, see <ref name="Getting rid of the &dquot;system check&dquot; load chunk" id="nosyschk">.
182 <sect2><tt/atari-asm.cfg/<p>
184 This config file aims to give the assembler programmer maximum
185 flexibility. All program segments (<tt/CODE/, <tt/DATA/, etc.) are
188 By default it creates regular DOS executable files, which have a default
189 load address of $2E00. It's also possible to generate an image of
190 just the program data without EXE header, load address, or (auto-)start address.
191 To you so, you have to define the symbols <tt/__AUTOSTART__/ and <tt/__EXEHDR__/
192 when linking the program. Therefore, to generate a "plain" binary file, pass the
193 options "<tt/-D__AUTOSTART__=1 -D__EXEHDR__=1/" to the linker.
194 It's also possible to create a non auto-starting program file, by defining
195 only the <tt/__AUTOSTART__/ symbol. Such a program has to be run manually
196 after being loaded by DOS (for example by using the "M" option of DOS 2.5).
197 Defining only the <tt/__EXEHDR__/ symbol will create a (useless) file which
198 doesn't conform to the DOS executable file format (like a "plain" binary file)
199 but still has the "autostart" load chunk appended.
201 The sections of the file which the defines refer to (<tt/__AUTOSTART__/ for
202 the autostart trailer, <tt/__EXEHDR__/ for the EXE header and load address)
203 is <it/left out/, keep this in mind.
205 The values you assign to the two symbols <tt/__AUTOSTART__/ and <tt/__EXEHDR__/
208 <sect2><tt/atari-cart.cfg/<p>
210 This config file can be used to create 8K or 16K cartridges. It's suited both
211 for C and assembly language programs.
213 By default, an 8K cartridge is generated. To create a 16K cartridge, pass the
214 size of the cartridge to the linker, like "<tt/-D__CARTSIZE__=0x4000/".
215 The only valid values for <tt/__CARTSIZE__/ are 0x2000 and 0x4000.
217 The option byte of the cartridge can be set with the <tt/__CARTFLAGS__/
218 value, passed to the linker. The default value is $01, which means
219 that the cartridge doesn't prevent the booting of DOS.
221 The option byte will be located at address $BFFD. For more information
222 about its use, see e.g. "Mapping the Atari".
224 <sect2><tt/atari-cassette.cfg/<p>
226 This config file can be used to create cassette boot files. It's suited both
227 for C and assembly language programs.
229 The size of a cassette boot file is restricted to 32K. Larger programs
230 would need to be split in more parts and the parts to be loaded manually.
232 To write the generated file to a cassette, a utility to run
233 on an Atari is provided in the <tt/targetutil/ directory (<tt/w2cas.com/).
235 <sect1><tt/atarixl/ config files<p>
237 <sect2>default config file (<tt/atarixl.cfg/)<p>
239 The default configuration is tailored to C programs. It creates files
240 which have a default load address of $2400.
242 The files generated by this config file include the
243 <ref name="&dquot;system check&dquot;" id="syschkxl"> load chunk. It can
244 optionally be left out, see <ref name="Getting rid of the &dquot;system check&dquot; load chunk" id="nosyschk">.
246 <sect2><tt/atarixl-largehimem.cfg/<p>
248 This is the same as the default config file, but it rearranges the
249 high memory beneath the ROM into one large block. In order for this
250 config file to work, the runtime library has to be recompiled with a
251 special define. See the file <tt/libsrc/atari/Makefile.inc/ in the
254 The files generated by this config file include the
255 <ref name="&dquot;system check&dquot;" id="syschkxl"> load chunk. It can
256 optionally be left out, see <ref name="Getting rid of the &dquot;system check&dquot; load chunk" id="nosyschk">.
259 <sect>Platform specific header files<p>
261 Programs containing Atari specific code may use the <tt/atari.h/
265 <sect1>Atari specific functions<p>
267 The functions and global variable listed below are special for the Atari.
268 See the <url url="funcref.html" name="function reference"> for declaration and usage.
278 <item>_is_cmdline_dos
287 <sect1>Hardware access<p>
289 The following pseudo variables declared in the <tt/atari.h/ header
290 file do allow access to hardware located in the address space. Some
291 variables are structures, accessing the struct fields will access the
296 <tag><tt/GTIA_READ/ and <tt/GTIA_WRITE/</tag>
297 The <tt/GTIA_READ/ structure allows read access to the GTIA. The
298 <tt/GTIA_WRITE/ structure allows write access to the GTIA.
299 See the <tt/_gtia.h/ header file located in the include directory
300 for the declaration of the structure.
302 <tag><tt/POKEY_READ/ and <tt/POKEY_WRITE/</tag>
303 The <tt/POKEY_READ/ structure allows read access to the POKEY. The
304 <tt/POKEY_WRITE/ structure allows write access to the POKEY.
305 See the <tt/_pokey.h/ header file located in the include directory
306 for the declaration of the structure.
308 <tag><tt/ANTIC/</tag>
309 The <tt/ANTIC/ structure allows read access to the ANTIC.
310 See the <tt/_antic.h/ header file located in the include directory
311 for the declaration of the structure.
314 The <tt/PIA/ structure allows read access to the PIA 6520.
315 See the <tt/_pia.h/ header file located in the include directory
316 for the declaration of the structure.
322 <sect>Loadable drivers<p>
324 The names in the parentheses denote the symbols to be used for static linking of the drivers.
327 <sect1>Graphics drivers<p>
329 <table><tabular ca="rrrr">
330 <tt/atari/|<tt/atarixl/|screen resolution|display pages@<hline>
331 <tt/atr3.tgi (atr3_tgi)/|<tt/atrx3.tgi (atrx3_tgi)/|40x24x4 (CIO mode 3, ANTIC mode 8)|1@
332 <tt/atr4.tgi (atr4_tgi)/|<tt/atrx4.tgi (atrx4_tgi)/|80x48x2 (CIO mode 4, ANTIC mode 9)|1@
333 <tt/atr5.tgi (atr5_tgi)/|<tt/atrx5.tgi (atrx5_tgi)/|80x48x4 (CIO mode 5, ANTIC mode A)|1@
334 <tt/atr6.tgi (atr6_tgi)/|<tt/atrx6.tgi (atrx6_tgi)/|160x96x2 (CIO mode 6, ANTIC mode B)|1@
335 <tt/atr7.tgi (atr7_tgi)/|<tt/atrx7.tgi (atrx7_tgi)/|160x96x4 (CIO mode 7, ANTIC mode D)|1@
336 <tt/atr8.tgi (atr8_tgi)/|<tt/atrx8.tgi (atrx8_tgi)/|320x192x2 (CIO mode 8, ANTIC mode F)|1@
337 <tt/atr8p2.tgi (atr8p2_tgi)/|<tt/atrx8p2.tgi (atrx8p2_tgi)/|320x192x2 (CIO mode 8, ANTIC mode F)|2@
338 <tt/atr9.tgi (atr9_tgi)/|<tt/atrx9.tgi (atrx9_tgi)/|80x192x16b (CIO mode 9, ANTIC mode F, GTIA mode $40)|1@
339 <tt/atr9p2.tgi (atr9p2_tgi)/|<tt/atrx9p2.tgi (atrx9p2_tgi)/|80x192x16b (CIO mode 9, ANTIC mode F, GTIA mode $40)|2@
340 <tt/atr10.tgi (atr10_tgi)/|<tt/atrx10.tgi (atrx10_tgi)/|80x192x9 (CIO mode 10, ANTIC mode F, GTIA mode $80)|1@
341 <tt/atr10p2.tgi (atr10p2_tgi)/|<tt/atrx10p2.tgi (atrx10p2_tgi)/|80x192x9 (CIO mode 10, ANTIC mode F, GTIA mode $80)|2@
342 <tt/atr11.tgi (atr11_tgi)/|<tt/atrx11.tgi (atrx11_tgi)/|80x192x16h (CIO mode 11, ANTIC mode F, GTIA mode $C0)|1@
343 <tt/atr14.tgi (atr14_tgi)/|<tt/atrx14.tgi (atrx14_tgi)/|160x192x2 (CIO mode 14, ANTIC mode C)|1@
344 <tt/atr15.tgi (atr15_tgi)/|<tt/atrx15.tgi (atrx15_tgi)/|160x192x4 (CIO mode 15, ANTIC mode E)|1@
345 <tt/atr15p2.tgi (atr15p2_tgi)/|<tt/atrx15p2.tgi (atrx15p2_tgi)/|160x192x4 (CIO mode 15, ANTIC mode E)|2
347 <!-- <caption>bla bla -->
351 Many graphics modes require more memory than the text screen which is
352 in effect when the program starts up. Therefore the programmer has to
353 tell the program beforehand the memory requirements of the graphics
354 modes the program intends to use.
356 On the <tt/atari/ target his can be done by using the __RESERVED_MEMORY__
357 linker config variable. The number specified there describes the number
358 of bytes to subtract from the top of available memory as seen from the
359 runtime library. This memory is then used by the screen buffer.
361 On the <tt/atarixl/ target the screen memory resides below the program
362 load address. In order to reserve memory for a graphics mode, one
363 simply uses a higher program load address. There are restrictions on
364 selectable load addresses,
365 see <ref name="Selecting a good program load address" id="loadaddr">.
367 The numbers for the different graphics modes presented below should
368 only be seen as a rule of thumb. Since the screen buffer memory needs
369 to start at specific boundaries, the numbers depend on the current top
371 The following numbers were determined by a BASIC program.
375 graphics mode|reserved memory@<hline>
409 <caption>reserved memory required for different graphics modes
412 The values of "1" are needed because the graphics command crashes if
413 it doesn't have at least one byte available. This seems to be a bug of
416 Default drivers: <tt/atr8.tgi (atr8_tgi)/ and <tt/atrx8.tgi (atrx8_tgi)/.
418 <sect1>Extended memory drivers<p>
420 Currently there is only one extended memory driver. It manages the second 64K of a 130XE.
424 <tt/atari/|<tt/atarixl/@<hline>
425 <tt/atr130.emd (atr130_emd)/|<tt/atrx130.emd (atrx130_emd)/
429 <sect1>Joystick drivers<p>
431 Currently there are two joystick drivers available:
435 <tt/atari/|<tt/atarixl/|description@<hline>
436 <tt/atrstd.joy (atrstd_joy)/|<tt/atrxstd.joy (atrxstd_joy)/|Supports up to two/four standard joysticks connected to the joystick ports of the Atari. (Four on the pre-XL systems, two on XL or newer.)@
437 <tt/atrmj8.joy (atrmj8_joy)/|<tt/atrxmj8.joy (atrxmj8_joy)/|Supports up to eight standard joysticks connected to a MultiJoy adapter.
441 Default drivers: <tt/atrstd.joy (atrstd_joy)/ and <tt/atrxstd.joy (atrxstd_joy)/.
443 <sect1>Mouse drivers<p>
445 Currently there are five mouse drivers available:
449 <tt/atari/|<tt/atarixl/|description@<hline>
450 <tt/atrjoy.mou (atrjoy_mou)/|<tt/atrxjoy.mou (atrxjoy_mou)/|Supports a mouse emulated by a standard joystick.@
451 <tt/atrst.mou (atrst_mou)/|<tt/atrxst.mou (atrxst_mou)/|Supports an Atari ST mouse.@
452 <tt/atrami.mou (atrami_mou)/|<tt/atrxami.mou (atrxami_mou)/|Supports an Amiga mouse.@
453 <tt/atrtrk.mou (atrtrk_mou)/|<tt/atrxtrk.mou (atrxtrk_mou)/|Supports an Atari trakball.@
454 <tt/atrtt.mou (atrtt_mou)/|<tt/atrxtt.mou (atrxtt_mou)/|Supports an Atari touch tablet.
458 All mouse devices connect to joystick port #0.
460 Default drivers: <tt/atrst.mou (atrst_mou)/ and <tt/atrxst.mou (atrxst_mou)/.
462 <sect2>Mouse callbacks<p>
464 There are two mouse callbacks available.
466 The "text mode" callbacks (<tt/mouse_txt_callbacks/) display the mouse cursor as a "diamond" character
467 on the standard "GRAPHICS 0" text mode screen. The mouse cursor character can be changed by an
468 assembly file defining the character by exporting the zeropage symbol <tt/mouse_txt_char/.
469 The default file looks like this:
471 .export mouse_txt_char : zp = 96 ; 'diamond' screen code
474 The "P/M" callbacks (<tt/mouse_pm_callbacks/) use Player-Missile graphics for the mouse cursor.
475 The cursor shape can be changed, too, by an assembly file. Here's the default shape definition:
477 .export mouse_pm_bits
478 .export mouse_pm_height : zeropage
479 .export mouse_pm_hotspot_x : zeropage
480 .export mouse_pm_hotspot_y : zeropage
490 mouse_pm_height = * - mouse_pm_bits
491 ; hot spot is upper left corner
492 mouse_pm_hotspot_x = 0
493 mouse_pm_hotspot_y = 0
496 <tt/mouse_pm_bits/ defines the shape of the cursor, <tt/mouse_pm_height/ defines the number of
497 bytes in <tt/mouse_pm_bits/. <tt/mouse_pm_hotspot_x/ and <tt/mouse_pm_hotspot_y/ define the
498 position in the shape where "the mouse points to". When using this callback page #6 ($600
499 - $6FF) is used for the P/M graphics data and no P/M graphics can otherwise be used
500 by the program. The height of the shape (<tt/mouse_pm_height/)
501 must not exceed 32 lines since the callback routines cannot handle more than 32 lines.
503 The default callbacks definition (<tt/mouse_def_callbacks/) is an alias for the "P/M" callbacks.
505 <sect1>RS232 device drivers<p>
507 Currently there is one RS232 driver. It uses the R: device (therefore
508 an R: driver needs to be installed) and was tested with the 850
513 <tt/atari/|<tt/atarixl/@<hline>
514 <tt/atrrdev.ser (atrrdev_ser)/|<tt/atrxrdev.ser (atrxrdev_ser)/
521 <sect1><tt/atarixl/<#if output="info|latex2e"> limitations</#if><label id="limitations"<p>
524 <item>The display is cleared at program start and at program termination. This is a side
525 effect of relocating the display memory below the program start address.
526 <item>Not all possible CIO and SIO functions are handled by the runtime stub code which banks
527 the ROM in and out. All functions used by the runtime library are handled, though.
528 <item>The <tt/_sys()/ function is not supported.
529 <item>It is not compatible with DOSes or other programs using the memory below the ROM.
532 <sect>DIO implementation<label id="dio"><p>
534 The Atari supports disk drives with either 128 or 256 byte sectors.
535 The first three sectors of any disk are always 128 bytes long though. This is
536 because the system can only boot from 128 bytes sectors.
538 Therefore the DIO read and write functions transfer only 128 bytes
539 for sectors 1 to 3, regardless of the type of diskette.
542 <sect>CONIO implementation<label id="conio"><p>
544 The console I/O is speed optimized therefore support for XEP80 hardware
545 or f80.com software is missing. Of course you may use stdio.h functions.
548 <sect>Technical details<label id="techdetail"><p>
550 <sect1><tt/atari/<#if output="info|latex2e"> details</#if><p>
552 <sect2><#if output="info|latex2e"><tt/atari/ </#if>Load chunks<p>
554 An <tt/atari/ program contains two load chunks.
557 <item>"system check"<label id="syschk">&nl;
558 This load chunk is always loaded at address $2E00, and checks if the system has
559 enough memory to run the program. It also checks if the program start address is not
560 below MEMLO. If any of the checks return false, the loading of the program is aborted.&nl;
561 The contents of this chunk come from the SYSCHKCHNK memory area of the linker config file.
562 <item>main program&nl;
563 This load chunk is loaded at the selected program start address (default $2000) and
564 contains all of the code and data of the program.&nl;
565 The contents of this chunk come from the MAIN memory area of the linker config file.
569 <sect1><tt/atarixl/<#if output="info|latex2e"> details</#if><p>
571 <sect2>General operation<p>
573 The <tt/atarixl/ target banks out the ROM while the program is running in
574 order to make more memory available to the program.
576 The screen memory is by default located at the top of available memory,
577 $BFFF if BASIC is not enabled, $9FFF if BASIC is enabled.
578 Therefore, in order to create a largest possible continuous memory area,
579 the screen memory is moved below the program load address. This gives
580 a memory area from <program load addr> to $CFFF.
582 The startup code installs wrappers for interrupt handlers and ROM routines.
583 When an interrupt or call to a ROM routine happens, the wrappers enable the
584 ROM, call the handler or routine, and disable the ROM again.
586 The "wrapping" of the ROM routines is done by changing the ROM entry
587 point symbols in <tt/atari.inc/ to point to the wrapper functions.
589 For ROM functions which require input or output buffers, the wrappers
590 copy the data as required to buffers in low memory.
592 <sect2><#if output="info|latex2e"><tt/atarixl/ </#if>Load chunks<label id="xlchunks"><p>
594 An <tt/atarixl/ program contains three load chunks.
597 <item>"system check"<label id="syschkxl">&nl;
598 This load chunk is always loaded at address $2E00, and checks if the system is
599 suitable for running the program. It also checks if there is enough room between MEMLO
600 and the program start address to move the text mode screen buffer there. If any of the
601 checks return false, the loading of the program is aborted.&nl;
602 The contents of this chunk come from the SYSCHKCHNK memory area of the linker config file.
603 <item>"shadow RAM prepare"&nl;
604 The second load chunk gets loaded to the selected program load address (default $2400).
605 It moves the screen memory below the program load address, copies the character generator
606 from ROM to its new place in RAM, and copies the parts of the program which reside in
607 high memory below the ROM to their place. The high memory parts are included in this load chunk.&nl;
608 At the beginning of this load chunk there is a .bss area, which is not part of the
609 EXE file. Therefore the on-disk start address of this load chunk will be higher than the
610 selected start address. This .bss area (segment LOWBSS) contains the buffers for the
611 double buffering of ROM input and output data. If you add contents to this segment be aware
612 that the contents won't be zero initialized by the startup code.&nl;
613 The contents of this chunk come from the SRPREPCHNK memory area of the linker config file.
614 <item>main program&nl;
615 This load chunk is loaded just above the LOWBSS segment, replacing the code of
616 the previous load chunk. It contains all remaining code and data sections of the program,
617 including the startup code.&nl;
618 The contents of this chunk come from the RAM memory area of the linker config file.
621 <sect2>Moving screen memory below the program start address<p>
623 When setting a graphics mode, the ROM looks at the RAMTOP location. RAMTOP
624 describes the amount of installed memory in pages (RAMTOP is only one byte).
625 The screen memory and display list are placed immediately below RAMTOP.
627 Now in order to relocate the screen memory to lower memory, the startup code
628 puts a value into RAMTOP which causes the ROM routines to allocate the display
629 memory below the program start address and then it issues a ROM call to setup
630 the regular text mode.
632 <sect2>Selecting a good program load address<label id="loadaddr"><p>
634 Due to the movement of the screen memory below the program start, there are some
635 load addresses which are sub-optimal because they waste memory or prevent a
636 higher resolution graphics mode from being enabled.
638 There are restrictions at which addresses screen memory (display buffer and display
639 list) can be placed. The display buffer cannot cross a 4K boundary and a display
640 list cannot cross a 1K boundary.
642 The startup code takes this into account when moving the screen memory down.
643 If the program start address (aligned to the next lower page boundary) minus
644 the screen buffer size would result in a screen buffer which spans a 4K
645 boundary, the startup code lowers RAMTOP to this 4K boundary.&nl;
646 The size of the screen buffer in text mode is 960 ($3C0) bytes. So, for
647 example, a selected start address of $2300 would span the 4K boundary
648 at $2000. The startup code would adjust the RAMTOP value in such way that
649 the screen memory would be located just below this boundary (at $1C40).
650 This results in the area [$2000-$22FF] being wasted.
651 Additionally, the program might fail to load since the lowest address used
652 by the screen memory could be below MEMLO. (The lowest address used in this
653 example would be at $1C20, where the display list would allocated.)
655 These calculations are performed by the startup code (in the first two
656 load chunks), but the startup code only takes the default 40x24 text mode
657 into account. If the program later wants to load TGI drivers which set
658 a more memory consuming graphics mode, the user has to pick a higher
660 Using higher resolution modes there is a restriction in the ROM that it
661 doesn't expect RAMTOP to be at arbitrary values. The Atari memory modules
662 came only in 8K or 16K sizes, so the ROM expects RAMTOP to only have
663 values in 8K steps. Therefore, when using the highest resolution modes
664 the program start address must be at an 8K boundary.
667 <sect2>Character generator location<label id="chargenloc"><p>
669 The default <tt/atarixl/ linker config file (<tt/atarixl.cfg/) leaves the
670 character generator location at the same address where it is in ROM
671 ($E000). This has the disadvatage to split the upper memory into
672 two parts ([$D800-$DFFF] and
673 [$E400-$FFF9]). For applications which
674 require a large continuous upper memory area, an alternative linker
675 config file (<tt/atarixl-largehimem.cfg/) is provided. It relocates the
676 character generator to $D800, providing a single big upper
677 memory area at [$DC00-$FFF9].
679 With the character generator at a different address than in ROM, the routines
680 which enable and disable the ROM also have to update the chargen pointer.
681 This code is not enabled by default. In order to enable it,
682 uncomment the line which sets CHARGEN_RELOC in <tt/libsrc/atari/Makefile.inc/
683 and recompile the <tt/atarixl/ runtime library.
688 <sect1>Function keys<p>
690 Function keys are mapped to Atari + number key.
693 <sect1>Passing arguments to the program<p>
695 Command line arguments can be passed to <tt/main()/ when the used DOS supports it.
698 <item>Arguments are separated by spaces.
699 <item>Leading and trailing spaces around an argument are ignored.
700 <item>The first argument passed to <tt/main/ is the program name.
701 <item>A maximum number of 16 arguments (including the program name) are
708 The runtime for the Atari uses routines marked as <tt/.INTERRUPTOR/ for
709 interrupt handlers. Such routines must be written as simple machine language
710 subroutines and will be called automatically by the VBI handler code
711 when they are linked into a program. See the discussion of the <tt/.CONDES/
712 feature in the <url url="ca65.html" name="assembler manual">.
714 Please note that on the Atari targets the <tt/.INTERRUPTOR/s are being
715 run in NMI context. The other targets run them in IRQ context.
717 <sect1>Reserving a memory area inside a program<label id="memhole"><p>
719 (This section is primarily applicable to the <tt/atari/ target, but the
720 principles apply to <tt/atatixl/ as well.)
722 The Atari 130XE maps its additional memory into CPU memory in 16K
723 chunks at address $4000 to $7FFF. One might want to
724 prevent this memory area from being used by cc65. Other reasons to
725 prevent the use of some memory area could be to reserve space for the
726 buffers for display lists and screen memory.
728 The Atari executable format allows holes inside a program, e.g. one
729 part loads into $2E00 to $3FFF, going below the reserved
730 memory area (assuming a reserved area from $4000 to
731 $7FFF), and another part loads into $8000 to
734 Each load chunk of the executable starts with a 4 byte header which
735 defines its load address and size. In the following linker config files
736 these headers are named HEADER and SECHDR (for the MEMORY layout), and
737 accordingly NEXEHDR and CHKHDR (for the SEGMENTS layout).
739 <sect2>Low code and high data example<p>
740 Goal: Create an executable with 2 load chunks which doesn't use the
741 memory area from $4000 to $7FFF. The CODE segment of
742 the program should go below $4000 and the DATA and RODATA
743 segments should go above $7FFF.
745 The main problem is that the EXE header generated by the cc65 runtime
746 lib is wrong. It defines a single load chunk with the sizes/addresses
747 of the STARTUP, LOWCODE, ONCE, CODE, RODATA, and DATA segments, in
748 fact, the whole user program (we're disregarding the "system check"
751 The contents of the EXE header come from the EXEHDR and MAINHDR segments.
752 The EXEHDR segment just contains the $FFFF value which is required
753 to be the first bytes of the EXE file.&nl;
754 The MAINHDR are defined in in crt0.s. This cannot be changed without
755 modifying and recompiling the cc65 atari runtime library. Therefore
756 the original contents of this segment must be discarded and be
757 replaced by a user created one. This discarding is done by assigning the
758 MAINHDR segment to the (new introduced) DISCARD memory area. The DISCARD memory area is
759 thrown away in the new linker config file (written to file "").
760 We add a new FSTHDR segment for the chunk header of the first chunk.
762 The user needs to create a customized linker config file which adds
763 new memory areas and segments to hold the new header data for the first load
764 chunk and the header data for the second load chunk. Also an assembly source file
765 needs to be created which defines the contents of the new header data
766 for the two load chunks.
769 This is an example of a modified cc65 Atari linker configuration file
773 __STACKSIZE__: value = $800 type = weak; # 2K stack
774 __RESERVED_MEMORY__: value = $0000, type = weak;
777 STARTADDRESS: default = $2E00;
780 ZP: start = $82, size = $7E, type = rw, define = yes;
782 HEADER: start = $0000, size = $2, file = %O; # first load chunk
784 FSTHDR: start = $0000, size = $4, file = %O; # second load chunk
785 RAMLO: start = %S, size = $4000 - %S, file = %O;
787 DISCARD: start = $4000, size = $4000, file = "";
789 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
790 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
793 EXEHDR: load = HEADER, type = ro;
795 MAINHDR: load = DISCARD, type = ro;
797 NEXEHDR: load = FSTHDR, type = ro; # first load chunk
798 STARTUP: load = RAMLO, type = ro, define = yes;
799 LOWCODE: load = RAMLO, type = ro, define = yes, optional = yes;
800 ONCE: load = RAMLO, type = ro, optional = yes;
801 CODE: load = RAMLO, type = ro, define = yes;
803 CHKHDR: load = SECHDR, type = ro; # second load chunk
804 RODATA: load = RAM, type = ro, define = yes;
805 DATA: load = RAM, type = rw, define = yes;
806 BSS: load = RAM, type = bss, define = yes;
808 ZEROPAGE: load = ZP, type = zp;
809 AUTOSTRT: load = RAM, type = ro; # defines program entry point
812 CONDES: segment = ONCE,
814 label = __CONSTRUCTOR_TABLE__,
815 count = __CONSTRUCTOR_COUNT__;
816 CONDES: segment = RODATA,
818 label = __DESTRUCTOR_TABLE__,
819 count = __DESTRUCTOR_COUNT__;
824 A new memory area DISCARD was added.
825 It gets loaded with the contents of the (now unused) MAINHDR segment. But the
826 memory area isn't written to the output file. This way the contents of
827 the MAINHDR segment get discarded.
829 The newly added NEXEHDR segment defines the correct chunk header for the
830 first intended load chunk. It
831 puts the STARTUP, LOWCODE, ONCE, and CODE segments, which are the
832 segments containing only code, into load chunk #1 (RAMLO memory area).
834 The header for the second load chunk comes from the new CHKHDR
835 segment. It puts the RODATA, DATA, BSS, and ZPSAVE segments into load
836 chunk #2 (RAM memory area).
839 The contents of the new NEXEHDR and CHKHDR segments come from this
842 .import __CODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__
843 .import __DATA_LOAD__, __RODATA_LOAD__, __STARTUP_LOAD__
846 .word __STARTUP_LOAD__
847 .word __CODE_LOAD__ + __CODE_SIZE__ - 1
850 .word __RODATA_LOAD__
851 .word __BSS_LOAD__ - 1
856 cl65 -t atari -C split.cfg -o prog.com prog.c split.s
859 <sect2>Low data and high code example<p>
862 Goal: Put RODATA and DATA into low memory and STARTUP, LOWCODE, ONCE,
863 CODE, BSS, ZPSAVE into high memory (split2.cfg):
867 __STACKSIZE__: value = $800 type = weak; # 2K stack
868 __RESERVED_MEMORY__: value = $0000, type = weak;
871 STARTADDRESS: default = $2E00;
874 ZP: start = $82, size = $7E, type = rw, define = yes;
876 HEADER: start = $0000, size = $2, file = %O; # first load chunk
878 FSTHDR: start = $0000, size = $4, file = %O; # second load chunk
879 RAMLO: start = %S, size = $4000 - %S, file = %O;
881 DISCARD: start = $4000, size = $4000, file = "";
883 SECHDR: start = $0000, size = $4, file = %O; # second load chunk
884 RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
887 EXEHDR: load = HEADER, type = ro; # discarded old EXE header
889 MAINHDR: load = DISCARD, type = ro;
891 NEXEHDR: load = FSTHDR, type = ro; # first load chunk
892 RODATA: load = RAMLO, type = ro, define = yes;
893 DATA: load = RAMLO, type = rw, define = yes;
895 CHKHDR: load = SECHDR, type = ro; # second load chunk
896 STARTUP: load = RAM, type = ro, define = yes;
897 ONCE: load = RAM, type = ro, optional = yes;
898 CODE: load = RAM, type = ro, define = yes;
899 BSS: load = RAM, type = bss, define = yes;
901 ZEROPAGE: load = ZP, type = zp;
902 AUTOSTRT: load = RAM, type = ro; # defines program entry point
905 CONDES: segment = ONCE,
907 label = __CONSTRUCTOR_TABLE__,
908 count = __CONSTRUCTOR_COUNT__;
909 CONDES: segment = RODATA,
911 label = __DESTRUCTOR_TABLE__,
912 count = __DESTRUCTOR_COUNT__;
916 New contents for NEXEHDR and CHKHDR are needed (split2.s):
918 .import __STARTUP_LOAD__, __BSS_LOAD__, __DATA_SIZE__
919 .import __DATA_LOAD__, __RODATA_LOAD__
922 .word __RODATA_LOAD__
923 .word __DATA_LOAD__ + __DATA_SIZE__ - 1
926 .word __STARTUP_LOAD__
927 .word __BSS_LOAD__ - 1
932 cl65 -t atari -C split2.cfg -o prog.com prog.c split2.s
935 <sect2>Final note<label id="memhole_final_note"><p>
937 There are two other memory areas which don't appear directly in the
938 linker config file. They are the stack and the heap.
940 The cc65 runtime lib places the stack location at the end of available
941 memory. This is dynamically set from the MEMTOP system variable at
942 startup. The heap is located in the area between the end of the BSS
943 segment and the top of the stack as defined by __STACKSIZE__.
945 If BSS and/or the stack shouldn't stay at the end of the program,
946 some parts of the cc65 runtime lib need to be replaced/modified.
948 common/_heap.s defines the location of the heap and atari/crt0.s
949 defines the location of the stack by initializing sp.
952 <sect1>Upgrading from an older cc65 version<p>
954 If you are using a customized linker config file you might get some errors
955 regarding the MAINHDR segment. Like this:
958 ld65: Error: Missing memory area assignment for segment `MAINHDR'
961 The old "HEADER" memory description contained six bytes: $FFFF
962 and the first and last memory addess of the program. For the "system
963 check" load chunk this had to be split into two memory assigments. The
964 "HEADER" now only contains the $FFFF. The main program's first
965 and last memory address were moved to a new segment, called "MAINHDR",
966 which in the new linker config file goes into its own memory area (also
967 called "MAINHDR").&nl;&nl;
968 A simple way to adapt your old linker config file is to add the
969 following line to the "SEGMENTS" section:
972 MAINHDR: load = HEADER, type = ro;
977 <sect1>Getting rid of the "system check" load chunk<label id="nosyschk"><p>
979 If, for some reason, you don't want to include the "system check" load
980 chunk, you can do so by defining the symbol <tt/__SYSTEM_CHECK__/ when linking the
981 program. The "system check" chunk doesn't include vital parts of the
982 program. So if you don't want the system checks, it is save to leave them out.
983 This is probably mostly interesting for debugging.
985 When using cl65, you can leave it out with this command line:
988 cl65 -Wl -D__SYSTEM_CHECK__=1 <arguments>
991 The value you assign to <tt/__SYSTEM_CHECK_/ doesn't matter. If the
992 <tt/__SYSTEM_CHECK__/ symbol is defined, the load chunk won't be included.
997 This software is provided 'as-is', without any expressed or implied
998 warranty. In no event will the authors be held liable for any damages
999 arising from the use of this software.
1001 Permission is granted to anyone to use this software for any purpose,
1002 including commercial applications, and to alter it and redistribute it
1003 freely, subject to the following restrictions:
1006 <item> The origin of this software must not be misrepresented; you must not
1007 claim that you wrote the original software. If you use this software
1008 in a product, an acknowledgment in the product documentation would be
1009 appreciated but is not required.
1010 <item> Altered source versions must be plainly marked as such, and must not
1011 be misrepresented as being the original software.
1012 <item> This notice may not be removed or altered from any source