コンフィグファイルは出力ファイルの配置を記述するために用いられます。 コンフィグファイルには２つの主項目があります。 それは目的となる構成のメモリ配置と、もう１つにメモリ領域へのセグメントの割り当てです。 追加として、いくつかの他の属性を指定できます。
Case is ignored for keywords, that is, section or attribute names, but it is not ignored for names and strings.
MEMORYセクションにてメモリ領域を定義します。
以下の例を見てください（これはC64の使用可能なのメモリ配置を定義しています）。
MEMORY {
RAM1: start = $0800, size = $9800;
ROM1: start = $A000, size = $2000;
RAM2: start = $C000, size = $1000;
ROM2: start = $E000, size = $2000;
}
見た通り, ２つのRAM領域と、２つのROM領域があります。 コロン前の名前は、アルファベットで始まる必要があり、残りはアルファベットもしくは数字である必要があります。 メモリ領域の名前は、セグメントを割り当てるときに使われます。 上述のように、これらの名前のために重要です。
上述の構文は、コンフィグファイルの全てのセクションで使われます。
名前(ROM1等)は識別子であり、セミコロンまでの残りのトークンはこの識別子の属性を指定します。
属性に値を割り当てるために等号を使用でき、属性を分けるためにカンマを使用でき、または両方を省略できます。
ただし、識別子の属性の終了を示すために、セミコロンは必ず使用しなければなりません。
上述のセクションは、以下のようにも記述できます。
# Start of memory section
MEMORY
{
RAM1:
start $0800
size $9800;
ROM1:
start $A000
size $2000;
RAM2:
start $C000
size $1000;
ROM2:
start $E000
size $2000;
}
もちろん、MEMORYセクションには、startやsizeだけでなく多くの属性があります。 startやsizeは強制的な属性であり、この意味は、各メモリ領域は必ずこれらの属性が定義されてなければなりません（リンカーがチェックします）。 他の属性については、次節以降にて記します。 気付いているかもしれないが、上の例にてコメントが使用されています。 コメントは、井桁（#）で始まり、それ以降は改行まで無視されます。
古いC64でプログラムを書き、それを実行したいと仮定します。
テスト目的では、RAM領域で実行される必要があります。
そのように、SEGMENTSセクションにて、メモリセクションをセグメントに割り当てます。
SEGMENTS {
CODE: load = RAM1, type = ro;
RODATA: load = RAM1, type = ro;
DATA: load = RAM1, type = rw;
BSS: load = RAM1, type = bss, define = yes;
}
ここでやっていることは、全てのセグメントがSEGMENTSセクションで
指定された順にRAM1のメモリ領域に入ることを、リンカに通知しています。
つまり、リンカーは最初にCODEセグメントを書き込み、
次にRODATAセグメント、DATAセグメントを書き込みます。
但し、BSSセグメントは書き込みません。
何故ならば、指定された各セグメントは、セグメント属性を指定することができます。
セグメント属性には、４つの属性があります。
ro 読み込みのみ（Raed Only）可能なことを意味します。
rw 読み書き（Raed Write）が可能なことを意味します。
bss 未初期化領域であることを意味します。
zp ゼロページ領域を意味します。
そのように、BBSと命名されたセグメントの属性をBBSに設定したため、
リンカーはこのセグメントが未初期化データであることを知り、ファイルへ出力しません。
以下は、重要なポイントです。
アセンブラでは、BSSセグメントは特別な意味を持ちません。
リンク時にBSS属性が指定されたとします。
このアプローチは、１つの固定されたBSSセグメントを持つよりはるかに柔軟性があり、
任意のセグメント数をサポートする設計上の判断の結果です。
もし、セグメントに「type = bss」が指定された場合、
リンカーはこのセグメントに未初期化（つまりゼロ）のデータしか含まれていないことを確認し、
そうでない場合、警告を発行します。
bss属性のセグメントが有用であるためには、
開発しているプログラムにおいて、なんとかしてその領域をクリアする必要があります
（これは、通常スタートアップコード内にて行われます。例えば、cc65用のスタートアップコードはその処理を行います）。
しかし、コードにおいてセグメントの開始位置、大きさは未知となります。
リンカーはそれら情報の取得を可能としますが、リンカーに要求する必要があります。BSS定義部で「define = yes」属性にて行っています。
__NAME_LOAD__ セグメントがロードされるアドレスが設定されます。
__NAME_RUN__ セグメントの実行アドレスが設定されます（実行アドレスは後項にて解説）。
__NAME_SIZE__ セグメントのサイズが設定されます。
セグメントの名前でNAMEを置き換え、上記の例ではBSSとなります。
これらのシンボルは、コードによってアクセスすることが可能です。
今、最初の３つのセグメントを書くため及び、最後のセグメント用のシンボルを生成するための設定を行い、１つの疑問がのこります。
それは、リンカーはどこにデータを出力するのか？
It would be very convenient to have the data in a file, wouldn't it?
上述のように指定されたファイルが無いと、実際には、上述のような簡易の設定では必要とされません。
領域データを書き込むためのファイル名を与えるために、メモリ領域に指定可能な追加属性"file"があります。
もしファイル名が与えられていない場合、リンカーはデフォルトのファイル名を割り当てます。
これは"a.out"か、コマンドラインに-oオプションで与えられた名前です。
デフォルトの動作では目的のために問題ないので、上記の例では属性を使用しませんでした。
今度はそれを見てみましょう。
"file"属性（キーワードは、"FILE"と記述する事も可能です）は、データが書かれるファイルを二重引用符(")で囲まれた文字列で指定します。
同じファイル名を複数回指定することができ、その場合、そのファイル名を持つすべてのメモリ領域のデータは、MEMORYセクションで定義された順に書き込まれます。
上述のように、MEMORYセクションで同じファイル名を使用してみましょう。
MEMORY {
RAM1: start = $0800, size = $9800, file = %O;
ROM1: start = $A000, size = $2000, file = "rom1.bin";
RAM2: start = $C000, size = $1000, file = %O;
ROM2: start = $E000, size = $2000, file = "rom2.bin";
}
ここで使用される％Oは、明示的にデフォルトの動作を指定する方法です。
%Oはデフォルトの出力名を含む文字列（引用符を含む）、
つまり"a.out"または、コマンドラインで -oオプションで指定した名前に置き換えられます。
%O is replaced by a string (including the quotes) that contains the default output name,
that is, "a.out" or the name specified with the -o option on the command line.
Into this file, the
linker will first write any segments that go into RAM1, and will append
then the segments for RAM2, because the memory areas are given in this
order. So, for the RAM areas, nothing has really changed.
We've not used the ROM areas, but we will do that below, so we give the file
names here. Segments that go into ROM1 will be written to a file named
"rom1.bin", and segments that go into ROM2 will be written to a file
named "rom2.bin". The name given on the command line is ignored in both cases.
Assigning an empty file name for a memory area will discard the data written to it. This is useful, if the a memory area has segments assigned that are empty (for example because they are of type bss). In that case, the linker will create an empty output file. This may be suppressed by assigning an empty file name to that memory area.
Let us look now at a more complex example. Say, you've successfully tested your new "Super Operating System" (SOS for short) for the C64, and you will now go and replace the ROMs by your own code. When doing that, you face a new problem: If the code runs in RAM, we need not to care about read/write data. But now, if the code is in ROM, we must care about it. Remember the default segments (you may of course specify your own):
CODE read only code
RODATA read only data
DATA read/write data
BSS uninitialized data, read/write
Since BSS is not initialized, we must not care about it now, but what
about DATA? DATA contains initialized data, that is, data that was
explicitly assigned a value. And your program will rely on these values on
startup. Since there's no other way to remember the contents of the data
segment, than storing it into one of the ROMs, we have to put it there. But
unfortunately, ROM is not writable, so we have to copy it into RAM before
running the actual code.
The linker cannot help you copying the data from ROM into RAM (this must be done by the startup code of your program), but it has some features that will help you in this process.
First, you may not only specify a "load" attribute for a segment, but
also a "run" attribute. The "load" attribute is mandatory, and, if
you don't specify a "run" attribute, the linker assumes that load area
and run area are the same. We will use this feature for our data area:
SEGMENTS {
CODE: load = ROM1, type = ro;
RODATA: load = ROM2, type = ro;
DATA: load = ROM2, run = RAM2, type = rw, define = yes;
BSS: load = RAM2, type = bss, define = yes;
}
Let's have a closer look at this SEGMENTS section. We specify that the
CODE segment goes into ROM1 (the one at $A000). The readonly data
goes into ROM2. Read/write data will be loaded into ROM2 but is run
in RAM2. That means that all references to labels in the DATA
segment are relocated to be in RAM2, but the segment is written to
ROM2. All your startup code has to do is, to copy the data from it's
location in ROM2 to the final location in RAM2.
So, how do you know, where the data is located? This is the second point,
where you get help from the linker. Remember the "define" attribute?
Since we have set this attribute to true, the linker will define three
external symbols for the data segment that may be accessed from your code:
__DATA_LOAD__ This is set to the address where the segment
is loaded, in this case, it is an address in
ROM2.
__DATA_RUN__ This is set to the run address of the segment,
in this case, it is an address in RAM2.
__DATA_SIZE__ This is set to the segment size.
So, what your startup code must do, is to copy __DATA_SIZE__ bytes from
__DATA_LOAD__ to __DATA_RUN__ before any other routines are called.
All references to labels in the DATA segment are relocated to RAM2
by the linker, so things will work properly.
There are some other attributes not covered above. Before starting the reference section, I will discuss the remaining things here.
You may request symbols definitions also for memory areas. This may be useful for things like a software stack, or an i/o area.
MEMORY {
STACK: start = $C000, size = $1000, define = yes;
}
This will define three external symbols that may be used in your code:
__STACK_START__ This is set to the start of the memory
area, $C000 in this example.
__STACK_SIZE__ The size of the area, here $1000.
__STACK_LAST__ This is NOT the same as START+SIZE.
Instead, it it defined as the first
address that is not used by data. If we
don't define any segments for this area,
the value will be the same as START.
A memory section may also have a type. Valid types are
ro for readonly memory
rw for read/write memory.
The linker will assure, that no segment marked as read/write or bss is put into a memory area that is marked as readonly.
Unused memory in a memory area may be filled. Use the "fill = yes"
attribute to request this. The default value to fill unused space is zero. If
you don't like this, you may specify a byte value that is used to fill these
areas with the "fillval" attribute. This value is also used to fill unfilled
areas generated by the assemblers .ALIGN and .RES directives.
The symbol %S may be used to access the default start address (that is,
the one defined in the
FEATURES section, or the
value given on the command line with the
-S
option).
Segments may be aligned to some memory boundary. Specify "align = num" to
request this feature. Num must be a power of two. To align all segments on a
page boundary, use
SEGMENTS {
CODE: load = ROM1, type = ro, align = $100;
RODATA: load = ROM2, type = ro, align = $100;
DATA: load = ROM2, run = RAM2, type = rw, define = yes,
align = $100;
BSS: load = RAM2, type = bss, define = yes, align = $100;
}
If an alignment is requested, the linker will add enough space to the output
file, so that the new segment starts at an address that is dividable by the
given number without a remainder. All addresses are adjusted accordingly. To
fill the unused space, bytes of zero are used, or, if the memory area has a
"fillval" attribute, that value. Alignment is always needed, if you have
used the .ALIGN command in the assembler. The alignment of a segment
must be equal or greater than the alignment used in the .ALIGN command.
The linker will check that, and issue a warning, if the alignment of a segment
is lower than the alignment requested in an .ALIGN command of one of the
modules making up this segment.
For a given segment you may also specify a fixed offset into a memory area or
a fixed start address. Use this if you want the code to run at a specific
address (a prominent case is the interrupt vector table which must go at
address $FFFA). Only one of ALIGN or OFFSET or START may be
specified. If the directive creates empty space, it will be filled with zero,
of with the value specified with the "fillval" attribute if one is given.
The linker will warn you if it is not possible to put the code at the
specified offset (this may happen if other segments in this area are too
large). Here's an example:
SEGMENTS {
VECTORS: load = ROM2, type = ro, start = $FFFA;
}
or (for the segment definitions from above)
SEGMENTS {
VECTORS: load = ROM2, type = ro, offset = $1FFA;
}
The "align", "start" and "offset" attributes change placement
of the segment in the run memory area, because this is what is usually
desired. If load and run memory areas are equal (which is the case if only the
load memory area has been specified), the attributes will also work. There is
also an "align_load" attribute that may be used to align the start of the
segment in the load memory area, in case different load and run areas have
been specified. There are no special attributes to set start or offset for
just the load memory area.
To suppress the warning, the linker issues if it encounters a segment that is
not found in any of the input files, use "optional=yes" as additional
segment attribute. Be careful when using this attribute, because a missing
segment may be a sign of a problem, and if you're suppressing the warning,
there is no one left to tell you about it.
The FILES section is used to support other formats than straight binary
(which is the default, so binary output files do not need an explicit entry
in the FILES section).
The FILES section lists output files and as only attribute the format of
each output file. Assigning binary format to the default output file would
look like this:
FILES {
%O: format = bin;
}
The only other available output format is the o65 format specified by Andre Fachat (see the 6502 binary relocation format specification). It is defined like this:
FILES {
%O: format = o65;
}
The necessary o65 attributes are defined in a special section labeled
FORMAT.
The FORMAT section is used to describe file formats. The default (binary)
format has currently no attributes, so, while it may be listed in this
section, the attribute list is empty. The second supported format,
o65,
has several attributes that may be defined here.
FORMATS {
o65: os = lunix, version = 0, type = small,
import = LUNIXKERNEL,
export = _main;
}
In addition to the MEMORY and SEGMENTS sections described above, the
linker has features that may be enabled by an additional section labeled
FEATURES.
CONDES is used to tell the linker to emit module constructor/destructor
tables.
FEATURES {
CONDES: segment = RODATA,
type = constructor,
label = __CONSTRUCTOR_TABLE__,
count = __CONSTRUCTOR_COUNT__;
}
The CONDES feature has several attributes:
segmentThis attribute tells the linker into which segment the table should be placed. If the segment does not exist, it is created.
typeDescribes the type of the routines to place in the table. Type may be one of
the predefined types constructor, destructor, interruptor, or
a numeric value between 0 and 6.
labelThis specifies the label to use for the table. The label points to the start of the table in memory and may be used from within user written code.
countThis is an optional attribute. If specified, an additional symbol is defined by the linker using the given name. The value of this symbol is the number of entries (not bytes) in the table. While this attribute is optional, it is often useful to define it.
orderOptional attribute that takes one of the keywords increasing or
decreasing as an argument. Specifies the sorting order of the entries
within the table. The default is increasing, which means that the
entries are sorted with increasing priority (the first entry has the lowest
priority). "Priority" is the priority specified when declaring a symbol as
.CONDES with the assembler, higher values mean higher priority. You may
change this behaviour by specifying decreasing as the argument, the
order of entries is reversed in this case.
Please note that the order of entries with equal priority is undefined.
Without specifying the CONDES feature, the linker will not create any
tables, even if there are condes entries in the object files.
For more information see the .CONDES command in the
ca65 manual.
STARTADDRESS is used to set the default value for the start address,
which can be referenced by the %S symbol. The builtin default for the
linker is $200.
FEATURES {
# Default start address is $1000
STARTADDRESS: default = $1000;
}
Please note that order is important: The default start address must be defined
before the %S symbol is used in the config file. This does usually
mean, that the FEATURES section has to go to the top of the config file.
The configuration file may also be used to define symbols used in the link
stage. The mandatory attribute for a symbol is its value. A second, boolean
attribute named weak is available. If a symbol is marked as weak, it may
be overridden by defining a symbol of the same name from the command line. The
default for symbols is that they're strong, which means that an attempt to
define a symbol with the same name from the command line will lead to an
error.
The following example defines the stack size for an application, but allows
the programmer to override the value by specifying --define
__STACKSIZE__=xxx on the command line.
SYMBOLS {
# Define the stack size for the application
__STACKSIZE__: value = $800, weak = yes;
}
The builtin configurations are part of the linker source. They can be retrieved
with --dump-config and don't have a special format. So if you need a
special configuration, it's a good idea to start with the builtin configuration
for your system. In a first step, just replace -t target by -C
configfile. Then go on and modify the config file to suit your needs.
Several machine specific binary packages are distributed together with secondary
configurations (in the cfg directory). These configurations can be used with
-C configfile too.