223cc6685e
git-svn-id: svn://svn.compuextreme.de/Viitor/V963/Viitor_cc65@5933 504e572c-2e33-0410-9681-be2bf7408885
250 lines
8.4 KiB
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250 lines
8.4 KiB
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<TITLE>Atari specific information for cc65: Other hints</TITLE>
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<A HREF="atari.html#toc8">Contents</A>
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<H2><A NAME="s8">8.</A> <A HREF="atari.html#toc8">Other hints</A></H2>
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<H2><A NAME="ss8.1">8.1</A> <A HREF="atari.html#toc8.1">Function keys</A>
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</H2>
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<P>These are defined to be Atari + number key.</P>
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<H2><A NAME="memhole"></A> <A NAME="ss8.2">8.2</A> <A HREF="atari.html#toc8.2">Reserving a memory area inside a program</A>
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</H2>
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<P>The Atari 130XE maps its additional memory into CPU memory in 16K
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chunks at address $4000 to $7FFF. One might want to
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prevent this memory area from being used by cc65. Other reasons to
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prevent the use of some memory area could be the buffers for display
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lists and screen memory.</P>
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<P>The Atari executable format allows holes inside a program, e.g. one
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part loads into $2E00 to $3FFF, going below the reserved
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memory area (assuming a reserved area from $4000 to
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$7FFF), and another part loads into $8000 to
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$BC1F.</P>
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<P>Each load chunk of the executable starts with a 4 byte header which
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defines its load address and size.</P>
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<H3>Low code and high data example</H3>
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<P>Goal: Create an executable with 2 load chunks which doesn't use the
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memory area from $4000 to $7FFF. The CODE segment of
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the program should go below $4000 and the DATA and RODATA
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segments should go above $7FFF.</P>
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<P>The main problem is that the EXE header generated by the cc65 runtine
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lib is wrong. It defines a single load chunk with the sizes/addresses
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of the LOWCODE, INIT, CODE, RODATA, and DATA segments (the whole user
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program).</P>
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<P>The contents of the EXE header come from the EXEHDR segment, which is
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defined in crt0.s. This cannot be changed w/o modifiying and
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recompiling the cc65 atari runtime lib. Therefore the original EXE
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header must be discarded. It will be replaced by a user created
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one.</P>
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<P>The user needs to create a customized linker config file which adds
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new memory areas and segments to hold the new EXE header and the
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header data for the second load chunk. Also an assembly source file
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needs to be created which defines the contents of the new EXE header
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and the second load chunk header.</P>
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<P>This is a modified cc65 Atari linker configuration file (split.cfg):
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<BLOCKQUOTE><CODE>
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<PRE>
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MEMORY {
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ZP: start = $82, size = $7E, type = rw, define = yes;
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HEADER: start = $0000, size = $6, file = %O; # first load chunk
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RAMLO: start = $2E00, size = $1200, file = %O;
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BANK: start = $4000, size = $4000, file = "";
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SECHDR: start = $0000, size = $4, file = %O; # second load chunk
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RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
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TRAILER: start = $0000, size = $0006, file = %O;
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}
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SEGMENTS {
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EXEHDR: load = BANK, type = ro;
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NEXEHDR: load = HEADER, type = ro; # first load chunk
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LOWCODE: load = RAMLO, type = ro, define = yes, optional = yes;
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INIT: load = RAMLO, type = ro, optional = yes;
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CODE: load = RAMLO, type = ro, define = yes;
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CHKHDR: load = SECHDR, type = ro; # second load chunk
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RODATA: load = RAM, type = ro, define = yes;
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DATA: load = RAM, type = rw, define = yes;
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BSS: load = RAM, type = bss, define = yes;
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ZEROPAGE: load = ZP, type = zp;
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AUTOSTRT: load = TRAILER, type = ro; # defines program entry point
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}
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FEATURES {
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CONDES: segment = RODATA,
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type = constructor,
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label = __CONSTRUCTOR_TABLE__,
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count = __CONSTRUCTOR_COUNT__;
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CONDES: segment = RODATA,
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type = destructor,
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label = __DESTRUCTOR_TABLE__,
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count = __DESTRUCTOR_COUNT__;
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}
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SYMBOLS {
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__STACKSIZE__ = $800; # 2K stack
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__RESERVED_MEMORY__: value = $0, weak = yes;
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}
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>A new memory area BANK was added which describes the reserved area.
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It gets loaded with the contents of the old EXEHDR segment. But the
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memory area isn't written to the output file. This way the contents of
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the EXEHDR segment get discarded.</P>
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<P>The added NEXEHDR segment defines the correct EXE header. It puts only
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the CODE segment into load chunk #1 (RAMLO memory area).</P>
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<P>The header for the second load chunk comes from the new CHKHDR
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segment. It puts the RODATA and DATA segments into load chunk #2 (RAM
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memory area).</P>
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<P>The contents of the new NEXEHDR and CHKHDR segments come from this
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file (split.s):
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<BLOCKQUOTE><CODE>
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<PRE>
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.import __LOWCODE_LOAD__, __BSS_LOAD__, __CODE_SIZE__
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.import __CODE_LOAD__, __DATA_LOAD__, __RODATA_LOAD__
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.segment "NEXEHDR"
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.word $FFFF ; EXE file magic number
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; 1st load chunk
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.word __LOWCODE_LOAD__
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.word __CODE_LOAD__ + __CODE_SIZE__ - 1
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.segment "CHKHDR"
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; 2nd load chunk (contains with AUTOSTRT in fact a 3rd load chunk)
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.word __RODATA_LOAD__
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.word __BSS_LOAD__ - 1
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Compile with
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<BLOCKQUOTE><CODE>
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<PRE>
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cl65 -t atari -C split.cfg -o prog.com prog.c split.s
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H3>Low data and high code example</H3>
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<P>Goal: Put RODATA and DATA into low memory and LOWCODE, INIT, CODE, BSS
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into high memory (split2.cfg):</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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MEMORY {
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ZP: start = $82, size = $7E, type = rw, define = yes;
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HEADER: start = $0000, size = $6, file = %O; # first load chunk
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RAMLO: start = $2E00, size = $1200, file = %O;
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BANK: start = $4000, size = $4000, file = "";
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SECHDR: start = $0000, size = $4, file = %O; # second load chunk
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RAM: start = $8000, size = $3C20, file = %O; # $3C20: matches upper bound $BC1F
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TRAILER: start = $0000, size = $0006, file = %O;
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}
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SEGMENTS {
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EXEHDR: load = BANK, type = ro; # discarded old EXE header
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NEXEHDR: load = HEADER, type = ro; # first load chunk
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RODATA: load = RAMLO, type = ro, define = yes;
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DATA: load = RAMLO, type = rw, define = yes;
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CHKHDR: load = SECHDR, type = ro; # second load chunk
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LOWCODE: load = RAM, type = ro, define = yes, optional = yes;
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INIT: load = RAM, type = ro, optional = yes;
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CODE: load = RAM, type = ro, define = yes;
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BSS: load = RAM, type = bss, define = yes;
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ZEROPAGE: load = ZP, type = zp;
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AUTOSTRT: load = TRAILER, type = ro; # defines program entry point
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}
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FEATURES {
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CONDES: segment = RODATA,
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type = constructor,
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label = __CONSTRUCTOR_TABLE__,
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count = __CONSTRUCTOR_COUNT__;
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CONDES: segment = RODATA,
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type = destructor,
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label = __DESTRUCTOR_TABLE__,
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count = __DESTRUCTOR_COUNT__;
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}
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SYMBOLS {
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__STACKSIZE__ = $800; # 2K stack
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__RESERVED_MEMORY__: value = $0, weak = yes;
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}
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>New contents for NEXEHDR and CHKHDR are needed (split2.s):
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<BLOCKQUOTE><CODE>
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<PRE>
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.import __LOWCODE_LOAD__, __BSS_LOAD__, __DATA_SIZE__
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.import __DATA_LOAD__, __RODATA_LOAD__
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.segment "NEXEHDR"
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.word $FFFF
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.word __RODATA_LOAD__
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.word __DATA_LOAD__ + __DATA_SIZE__ - 1
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.segment "CHKHDR"
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.word __LOWCODE_LOAD__
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.word __BSS_LOAD__ - 1
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Compile with
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<BLOCKQUOTE><CODE>
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<PRE>
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cl65 -t atari -C split2.cfg -o prog.com prog.c split2.s
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H3><A NAME="memhole_final_note"></A> Final note</H3>
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<P>There are two other memory areas which don't appear directly in the
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linker script. They are the stack and the heap.</P>
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<P>The cc65 runtime lib places the stack location at the end of available
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memory. This is dynamically set from the MEMTOP system variable at
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startup. The heap is located in the area between the end of the BSS
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segment and the top of the stack as defined by __STACKSIZE__.</P>
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<P>If BSS and/or the stack shouldn't stay at the end of the program,
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some parts of the cc65 runtime lib need to be replaced/modified.</P>
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<P>common/_heap.s defines the location of the heap and atari/crt0.s
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defines the location of the stack by initializing sp.</P>
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<HR>
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<A HREF="atari-9.html">Next</A>
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<A HREF="atari-7.html">Previous</A>
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<A HREF="atari.html#toc8">Contents</A>
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