<title>Atari specific information for cc65
<author>Shawn Jefferson, <htmlurl url="mailto:shawnjefferson@24fightingchickens.com" name="shawnjefferson@24fightingchickens.com">
Christian Groessler, <htmlurl url="mailto:cpg@aladdin.de" name="cpg@aladdin.de">
-<date>16-Oct-2003
+<date>14-Sep-2004
<abstract>
An overview over the Atari runtime system as it is implemented for the cc65 C
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 <start address> <end address>). 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. 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 also.
+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<p>
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.
+ 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
These are defined to be Atari + number key.
+<sect1>Reserving a memory area inside the program<label id="memhole"><p>
+
+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.
+<p>
+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.
+<p>
+Each load chunk of the executable starts with a 4 byte header which
+defines its load address and size.
+<p>
+<sect2>Low code and high data example<p>
+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.
+<p>
+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).
+<p>
+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.
+<p>
+The user needs to create a customized linker config file which adds
+new memory areas and segments to hold the new EXE header and added
+load chunk header data. Also an assembly source file needs to be
+created which defines the contents of the new EXE header and the
+second load chunk header.
+<p>
+<p>
+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>
+<p>
+
+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.
+<p>
+The added NEXEHDR segment defines the correct EXE header. It puts only
+the CODE segment into load chunk #1 (RAMLO memory area).
+<p>
+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).
+<p>
+<p>
+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>
+<p>
+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<p>
+
+
+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"><p>
+
+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<p>
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">).
+name="uz@cc65.org"> or <htmlurl url="mailto:cpg@aladdin.de"
+name="cpg@aladdin.de">).
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