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<html><head><title>6502 binary relocation format</title>
<link rev=made href="mailto:a.fachat@physik.tu-chemnitz.de">
</head>
<body bgcolor="#ffffff">
<h1>6502 binary relocation format</h1>
<h2>V1.3 as of 31 mar 2005</h2>
<h3>(c) André Fachat (afachat@gmx.de)</h3>
<hr>
<h3>Changes from V1.2</h3>
<p>
These changes have been lingering in my inbox for quite a while, only now did I find the
time to update the document.
<ol>
<li>New Operating System ID for <a href="#change13a">CC65 generic and opencbm</a></li>
<li>New mode bit for <a href="#change13b">simple address schema</a></li>
<li>A note about <a href="#change13c">exporting an undefined reference</a></li>
<li>An <a href="#change13d">example</a> for late binding</li>
<li>An <a href="#change13e">improved explanation</a> of the relocation table entries</li>
<li>Additional notes about character encodings and name lengths for <a href="#change13g">undefined
references</a> and <a href="#change13f">exported labels</a>.</li>
<li>A new header bit to <a href="#change13h">chain</a> o65 files.</li>
<li>New header bits to <a href="#change13i">detect</a> 65xx CPU types.</li>
<li>A New header bit to <a href="#change13j">signal bss zeroing</a>.</li>
</ol>
</p>
<h3>Changes from V1.1</h3>
<p>
The <a href="#change12a">order</a> for saving the undefined reference and the low byte of a
high byte relocation entry has changed. This makes the OS/A65 lib6502
implementation easier.
</p>
<hr>
<p>
<h3>0) Preface</h3>
<p>
With some new 6502/C64/C128 operating systems comes the need for a new
binary format. In multitasking operating systems like Lunix, SMOS, or
OS/A65, a binary file cannot be loaded to a fixed location that might
already be used by another program. Therefore it must be possible to
relocate the program to an arbitrary address at least at load time.
In addition to that, more specific information might be stored in a
binary executable file, like interrupt vectors for example.
<p>
This text gives a good solution to this problem for the 6502 CPU and an
assembler source format to use this format in a general manner. The file
format can even be used as an object file format, i.e. a format a linker
can use as an input file. It is also usable as a 65816 file format.
Instead of zeropage addressing modes, the 65816 has direct addressing
modes, that add the contents of the direct register to the zeropage
address in the opcode.
<p>
<h3>1) 6502/65816 specifics</h3>
<p>
The 6502 has the special feature of a 'zeropage', i.e. a very limited
memory address range used for special addressing modes. So the format
should not only provide a means to relocate absolute addresses but also
zeropage addresses. The 65816 replaces zeropage addressing with direct
addressing modes.
<p>
The stack space is also very limited. A binary format has to provide a
measure of how much stack space is needed for the application.
<p>
Such limits should be defined as 2 byte values, even if the 6502 only has
a range of 8 address bits for zeropage and stack. But the 65816 behaves
differently, it has a 16 bit stack pointer for example. For further
expandability, a 32 bit format should be provided, although the 16 bit
format suffices for the 65816 already.
<p>
Another problem is, that an address can be 'split', i.e. you can just use
the high byte or the low byte separately in an opcode. This gives need
to a special relocation table format, that can cope with half-address
references. The 65816 can even have three byte addresses, i.e. address
in a segment and segment number.
<p>
<h3>2) binary format</h3>
<h4>2.1) General</h4>
<p>
The file differs from the known Commodore file formats, in that a lot
more information is stored in the file. First the data is structured
in separate segments to allow different handling of text (program code),
data (like tables) and bss (uninitialized data).
<p>
Also tables are included to allow late binding, i.e. linking the
file with other files at load time, and relocation, i.e. executing
the file at different addresses in 6502 address space.
<p>
<h4>2.2) Segments</h4>
<p>
As already used in other formats, the assembler uses three different
segment types, i.e. text (the actual program code), data (initialized
variables), and bss (uninitialized variables).
To have these different segments seems to be 'overdesigned', but they
actually make memory handling easier in more complex operating systems
or systems with virtual addresses (OS/A65, for example).
<p>
The text segment is defined to be read-only memory. This doesn't allow
self-modifying code in this segment, but allows memory sharing in virtual
memory architectures. The data segment actually is like the text segment,
only it is allocated writable. This segment might not be shared between
different processes. The contents of these two segments are loaded from
the file. The bss segment is uninitialized data, i.e. upon program start,
it is not defined - and not loaded from the file. This area is read-write
and can be used during program execution. It is also not shared between
processes. In addition to these segments, the 6502 format also includes a
zeropage segment type, to allow zeropage variables to be relocated. This
zeropage segment is like a bss segment, in that only the length, but
not the data is saved. For the 65816 the zeropage segment changes its
meaning to a bank zero segment.
<p>
The different segments hold different type of data and can be located
anywhere in memory (except zero segment, which has to be in the zeropage
resp. bank zero). The program must therefore not assume anything about
the relative addresses between different segments.
<p>
<h4>2.3) Relocation</h4>
<p>
In general, there are three ways to handle the relocation problem so far:
<pre>
- Tables: have a relocation table for a text segment
if the relocation table is put in front of code
you have to save the table in a side-storage
if table is behind, you still cannot relocate 'on the fly'.
- Deassembling: go through the code, deassemble it and change all absolute
addresses. Problem: needs to know or have hints about where some
data is in the code.
- Relocation info in the code: here each address is preceeded with an
'escape' code and is relocated when loading. But this disallows block
oriented transfer from storage media to memory.
</pre>
This binary format uses the first method, with the table after the
code/data. This way block oriented transfer for the text/data segment can
be used. And while reading the relocation tables bytewise, the relocation
can be done without the need to save the table somewhere.
<p>
<h4>2.4) External References & Exported Globals</h4>
<p>
As this file format should not only be used as an executable format, but
also as object file format, it must provide a way to define references
- references exported from this object and labels referenced in this
object. The external references list (also called 'undefined list') lists
the addresses where labels not defined in this object are referenced.
The exported globals list lists the addresses that are available for
other objects. The labels are named by null-terminated ASCII strings.
<p>
Even an executable file can have non-empty globals and externals lists,
but only if the operating system allows this. In this case, so called
'late binding' is used to link the object with some global libraries
at link time.
<p>
<h4>2.5) File extension</h4>
<p>
The proposed standard extension for the described format is ".o65" when
used as an object file.
<p>
<h4>2.6) Format description</h4>
<p>
The binary format is the following:
<pre>
(
header
text segment
data segment
external references list
relocation table for text segment
relocation table for data segment
exported globals list
)
</pre>
The description of the parts follows:
<p>
<h5>2.6.1) Header</h5>
<p>
The header contains the minimum needed data in a fixed struct.
The rest of the necessary information is put into the header options.
[Note: .word is a 16 bit value, low byte first, .byt is a simple byte.
.long is a 32 bit value, low byte first. .size is a 16 or 32 bit
value according to .word and .long, depending on the size bit in the
mode field ]
<p>
This is the fixed struct:
<pre>
(
.byt $01,$00 ; non-C64 marker
.byt $6f, $36, $35 ; "o65" MAGIC number!
.byt 0 ; version
.word mode ; mode word
.size tbase ; address to which text is assembled to
; originally
.size tlen ; length of text segment
.size dbase ; originating address for data segment
.size dlen ; length of data segment
.size bbase ; originating address for bss segment
.size blen ; length of bss segment
.size zbase ; originating address for zero segment
.size zlen ; length of zero segment
.size stack ; minimum needed stack size, 0= not known.
; the OS should add reasonable values for
; interrupt handling before allocating
; stack space
)
</pre>
The mode word currently has these defined bits:
<a name="change13b"></a>
<pre>
mode.15 : CPU 0= 6502 1= 65816
mode.14 : reloc 0= bytewise... 1= page(256byte)wise relocation
allowed
mode.13 : size 0= size=16 bit, 1= size=32 bit
mode.12 : obj 0= executable 1= object file
mode.11 : simple 0= (ignored) 1= simple file addresses
mode.10 : chain 0= (ignored) 1= another file follows this one
mode.9 : bsszero 0= (ignored) 1= the bss segment must be zeroed out for this file
mode.4-7 : CPU2 0000 = 6502 core (no undocumented opcodes)
0001 = 65C02 /w some bugfix, no illegal opcodes
0010 = 65SC02 (enhanced 65C02), some new opcodes
0011 = 65CE02 some 16bit ops/branches, Z register is modifiable
0100 = NMOS 6502 (including undocumented opcodes)
0101 = 65816 in 6502 emulation mode
011x = reserved
1xxx = reserved
^^^^
|||+- Bit 4
||+-- Bit 5
|+--- Bit 6
+---- Bit 7
mode.0-1: align 0= byte align,
1= word (i.e. 2 byte) align
2= long (4 byte) align
3= block (256 byte) align
</pre>
The <code>CPU</code> bit tells the loader for which CPU the file was made. This has
implications on the zero segment, for example. Also a system can check
if the program will run at all (on a 6502 that is).
The <code>reloc</code> bit defines if an object file can be relocated bytewise,
or if it must be page-aligned. A page has 256 bytes. The restriction to
pagewise relocation simplifies the relocation table and also allows
simpler compilers/assemblers.
The <code>size</code> bit determines the size of the segment base address and length
entries. Currently the 16 bit size (size bit = 0) works for 6502 and
65816 CPUs.
<p>
The <code>obj</code> bit distinguishes between object files and executables.
An object file is used as assembler output that can be linked with
other object files to build an executable or an object library.
</p><p>
The <code>simple</code> bit signals the loader that the load addresses have a specific form.
This form fulfills the following conditions:
<pre>
dbase = tbase + tlen
bbase = dbase + dlen
</pre>
This condition ensures that the loader can actually load the text and data segments in
one block, and can then use the same base address for the relocation of all three, the
text, data and bss segments. The <code>simple</code> mode bit is optional, in that when it is set the conditions
must be fulfilled, but if not set the conditions may or may not be fulfilled.
</p><p><a name="change13h"></a>
The <code>chain</code> bit signals the loader that after the current o65 "file"
there is another "file" appended to the actual file on disk. This way "multi-o65" files can be
built. An "o65" file in a multi-o65 file is here now called "section".
Chaining allows the following scenarios:
<ol>
<li>Init code in a separate segment - the chain contains a first o65 section with the code to run
the program, and a second o65 with initialization code that can be thrown away after init.
As the init code may just as any program need zero-, data- and bss segments, a full o65
file structure is provided in the section.
<li>
Larger systems have mapped memory. The chain bit allows to
provide different sections to be loaded in different memory mappings in a single file.
<li>Fat binaries: A single file could hold different o65 sections, one for each
different type of CPU. The loader could ignore the parts that do not fit the CPU
that it is running on.
</ol>
The loader may support binding undefined
references in a later section to global labels exported from an earlier section.
Otherwise the operating system should provide calls to access the separate
sections, e.g. when they are loaded into different memory mappings.
The next o65 section starts again with the header (including non-C64 marker and magic number),
so sections with different characteristics may be chained.
The last section must have <code>chain=0</code>.
The <code>chain</code> bit is optional, if it is set and a loader does not support it, the file
may be rejected right away.
It is recognized that for these purposes the loader must have a means of identifying
different sections and their purposes. Currently there is no simple way except using the
order of the sections in the file. A more complicated way would be to use optional headers
in each section.
</p><p><a name="change13j"></a>
The <code>bsszero</code> bit tells the loader that the executable to be loaded requires the bss segment to be zeroed out. If it is not set, then the code must not assume any special value in the bss segment (which is the default behaviour for o65 version 1.2 and below). A loader that does not support zeroing out the bss segment must reject a file with this bit set.
</p><p><a name="change13i"></a>
The <code>CPU2</code> bits determine the type of 6502 CPU. <code>6502 core</code> means that
only the originally documented 6502 opcodes are used. In addition the <code>NMOS 6502</code>
signals that in addition to the code, some undocumented opcodes of the NMOS version are
used. The other values indicate other versions of 6502 CPUs. Please see the appendix for an additional note.
</p><p>
The two <code>align</code> bits give the address boundary the segments can be
placed. Even the 6502 needs this, as, for example, "jmp ($xxFF)" is
broken. The align bits are valid for all of the segments.
[Note: if reloc=1, then align should be 3. But if align=3, reloc need
not be 1, because reloc switches to a simpler version of the relocation
table. The reloc bit might be obsoleted in newer versions of this
format. Though it should be set, if necessary.]
<p>
All unused bits in the mode field must be zero.
<p>
Note that the header size is 26 if the size bit is zero and 44 if the
size bit is one.
<p>
The fixed sized struct is immediately followed by a list of header options.
Each header option consists of a single byte total length, a type byte
and some data bytes if needed. A single length byte of $00 ends the
header option list.
<pre>
(
{ ; optional options, more than one allowed
.byt olen ; overall length (including length and type
; byte
.byt otype ; option type
[ .byt option_bytes ]
}
.byt $00 ; end of options marker (i.e. option len=0)
)
</pre>
The header options currently defined/proposed are:
<a name="change13a"></a>
<pre>
- Filename:
type=0; len=strlen(filename_in_ascii)+3; content="filename_in_ascii",0
The string contains the name of the object.
- Operating System Header
type=1; len=?
the first data byte is the OS type:
1 OSA/65 header supplement
2 Lunix header supplement
3 CC65 generic module (new in v1.3)
4 opencbm floppy modules (new in v1.3)
[others to follow?]
the following data contains OS specific information.
A suggested data byte is the OS version as second byte.
- Assemblerprogram:
type=2; len=strlen(ass)+3; content="ass",0
The string contains the name of the assembler resp. linker that produced
this file/object.
For example (syntax see below)
.fopt 2, "xa 2.1.1g",0
becomes
0c 02 78 61 20 32 2e 31 2e 31 67 00
in the file.
- Author:
type=3; len=strlen(author)+3; content="author",0
The string contains the author of the file.
- Creation data:
type=4; len=strlen(date)+3; content="date_string",0
The string contains the creation date in format like:
"Sat Dec 21 14:00:23 MET 1996", where we have the day, Month, date,
time, timezone and year. See output of `date`...
</pre>
<h5>2.6.2) text and data segments</h5>
<p>
The text and data segments are just the assembled code.
The only difference between text and data segments is the read/write mode
of the two segments. Therefore, to be compliant to this file format,
self-modifying code goes into the data segment.
<p>
<h5>2.6.3) Undefined references list</h5>
<p>
The next list is an ASCII list of labels that are referenced in this file
but not defined. The lists is preceeded with the number of undefined labels
(16 or 32 bits, according to the mode.size bit).
<pre>
undef_list: number_of_undefined_labels.s
"undefined_label1",0
"undefined_label2",0
...
</pre>
<p><a name="change13g"></a>
The character encoding and length of the names of the undefined labels should be
appropriate for the target platform, that may define additional constraints.
The encoding must allow zero-terminated byte arrays
as string representations. To allow short loading times, the names should not be
exceedingly long.
</p>
<h5>2.6.4) Relocation tables</h5>
<p>
The relocation tables are the same format for the two segments, text and
data. In general a relocation entry consists of the offset from the
previous relocation address to the next one, the type of the relocation
and additional info. Relocation not only defines the relocation when
moving object code to a different address, but also filling in the
undefined references.
<p>
Each table starts at relocation address = segment base address -1.
I.e. if the segment base address is $1000 for example, the first entry
has an offset computed from base address-1 = $0fff.
The offset to the next relocation address is the first byte of each
entry. If the offset is larger than 254 (i.e. 255 or above), than a
255 is set as offset byte, the offset is decremented by 254 (note the
difference) and the entry is started again.
<pre>
{ [255,...,255,] offset of next relocation (b), typebyte|segmentID [, low_byte] }+
</pre>
where typebyte has the bits 5, 6 and 7 and is one of
<pre>
WORD $80 2 byte address
HIGH $40 high byte of an address
LOW $20 low byte of an address
SEGADR $c0 3 byte address (65816)
SEG $a0 segment byte of 3 byte address
</pre>
The segmentID stands for the segment the reference points to:
<pre>
0 undefined
1 absolute value
2 text segment
3 data segment
4 bss segment
5 zero segment
</pre>
(Of course the absolute value will never appear in a relocation table,
but this value is necessary for the exported list)
<p>
If the type is HIGH, the low byte of the value is stored behind the
relocation table entry, if bytewise relocation is allowed (header mode
field bit 14). If only pagewise relocation is allowed, then only HIGH
relocation entries can occur, and the low byte is implicitely set zero
(i.e. it is _not_ saved in the relocation table).
<p>
If the type is SEG, then the two lower bytes of the three byte segment
address are stored behind the entry in the relocation table, lower byte
first.
<p>
<!---
If the segment is "undefined", then the whole relocation entry is followed
by a two (mode.size=0) or four (mode.size=1) byte value index in the
undefined references list.
--->
<a name="change12a"></a>
If the segment is "undefined", the typebyte is immediately followed
by the two (mode size=0) or four (mode size=1) byte value index
in the undefined references list. If it is a high byte relocation,
the low byte is saved behind the index value. The index value
determines the undefined reference, which must be looked up by the
loader.
<p>
The value taken from the relocation address in the segment, together with
the low byte from the relocation table (if HIGH entry) form the address
used if the segment would be used unrelocated. To relocate the segment,
the difference between the relocated segment base address and the segment
base address from the file is then added to the above address. The result
is again saved in the segment.
<p>
A zero offset byte ends the relocation table. The first offset is computed
from the segment base address-1, to avoid a 0 value in the first entry.
<p>
Note that direct addressing modes do not generate entries in the
relocation table. instead it is assumed that the 65816 direct register
holds the correct value (i.e. zero segment base address) when running
this program.
<p>
Example (for file contents see appendix B.1):
<p>
Segment Base address in file (header.tbase) is $1000.
The start address of the text segment after relocation is real.tbase = $1234.
<p>
Now the first (unrelocated) address at which a relocation should take
place is here:
<pre>
$1222 A9 23 lda #&gt;vector
</pre>
<!--
This generates the offset: $1222-($1000-1) = $223. This is larger than
254 ($fe), so the first byte is 255 ($ff). The offset is decremented
by $fe, and gives $125. This again is larger than $fe, so the next byte
is $ff again. After substracting $fe again, we have $27. But this is
the address of the opcode. To get the address of the address byte, we
have to add 1 to get $28, which becomes the third byte.
--><a name="change13e"></a>
To compute the relocation table entry, we have to identify the address
that must be relocated. This is not the opcode address $1222, but the
address of the parameter to the offset, i.e. $1223. The first relocation
table entry offset is calculated from the start of the segment minus one, i.e.
$0fff in this case. The offset to be stored in the relocation table
therefore is $1223-$0fff=$224. This is larger than $fe, therefore the
first byte in the relocation table entry is $ff, and the offset is
decremented by $fe, which results in $126. This again is larger than
$fe, so the next byte in the relocation table entry is $ff again and the
offset is decremented by $fe, resulting in $28. This offset becomes the
next byte in the relocation table entry.
The offset for the next relocation table entry is then computed from $1223,
because this is the last relocation address.
<p>
Now we reference the high byte of an address, lets say vector=$23d0 (not
relocated), in the text segment. Therefore the relocation type becomes
'HIGH | text_segmentID = $42', which is the next byte. Because we are
referencing a high byte of an address, the low byte of the unrelocated
address is saved behind the typebyte in the relocation entry. This byte
is missing when referencing a low byte or address.
<p>
The relocation table entry is now:
<pre>
$ff, $ff, $28, $42, $d0.
</pre>
When actually doing the relocation, the relocation pointer is initialized
to real.tbase-1 = $1233 (this value correlates to the unrelocated text segment
start minus one, $0fff). Then we add the offset of $224 from the first
relocation table entry, which brings
us to $1457, where the parameter byte of the opcode is after loading
the file to $1234. We now have to compute the new address, where <code>vector</code>
is after relocation. So we take the unrelocated low byte from the
relocation table ($d0) and the high byte from $1457 ($23).
<pre>
vector_file = ($23 &lt;&lt; 8) + $d0 = $23d0
</pre>
To this value we add
the difference between the address the program is assembled to and the
real load address:
<pre>
vector_relocated = vector_file + (real.tbase - header.tbase)
= $23d0 + ($1234 - $1000)
= $23d0 + $234
= $2604
</pre>
From this value the high byte is then written back to the address $1457.
Had we not saved the low byte in the relocation table, and only added
the high bytes, we would have missed the carry bit that increments
the high byte in this case!
<p>
Had "vector" now been an undefined reference, and "vector" would be
the second label in the undefined references list, we would get the
following relocation table entry (assuming mode.size=0):
<pre>
$ff, $ff, $28, $40, $02, $00, $00
</pre>
The value computed with the above formula for vector_file is now added
to the address the label "vector" now really has (This must of course
be looked up into an external table or list).
Had the opcode been "LDA #&gt;vector+$567", then the low byte in the relocation
table would be $67, while the high byte in the opcode would be $05.
This value would result in vector_file and the real address of "vector"
would be added before wrting back the high byte to the opcode.
<p>
<h5>2.6.5) exported globals list</h5>
<p>
The global list is a list of names, together with the target segment
and the offset in the segment for each name. It is preceeded with the
number of exported labels. This allows the loader to allocate a table
large enough, if needed. The number of labels and the offset value
are 16 bit or 32 bit values according to the size bit in the header mode
field. The segmentID is a byte value and the same as in the relocation
table entry (see section 2.6.3).
<pre>
number_of_exported_labels.s
"global_label_name_in_asc1",0, segmentID.b, value.s
...
</pre>
<a name="change13c"></a>
<p>Note: an undefined reference can not be exported. Doing this would lead
to circular references for example when linking multiple object files,
therefor it is not allowed.
</p><p><a name="change13f"></a>
The character encoding and length of the names of the undefined labels should be
appropriate for the target platform, that may define additional constraints.
The encoding must allow zero-terminated byte arrays
as string representations. To allow short loading times, the names should not be
exceedingly long.
</p>
<h3>3) assembler source format</h3>
<p>
The assembler source format is a suggestion only. It will be implemented
in xa65, a cross assembler for 6502 CPUs running on Unix/Atari ST/Amiga
as a reference platform.
<p>
The assembler provides a way to embed absolute address code in relocatable
code. This is needed when code should be copied to a specific location
known at assemble time.
There also is a way to make a file 'romable'. You can give the start
address of the _file_ in ROM, and the assembler automatically sets
the text segment start address to where the code will be in the ROM.
Of course, the other segments must be taken care of with -b? command
line parameter, that set the segment start address.
<p>
<h4>3.1) embed absolute code in relocatable files</h4>
<p>
When the assembler is started in relocatable mode, everything is put into
a .o65 relocatable file. All address references generate relocation table
entries. If a "*= value" pseudo opcode is encountered,
then the assembler switches to absolute mode. The following opcodes don't
generate relocation table entries. If a "*=" without a value is read,
then the assembler switches back to relocatable mode. The relocation
program counter is increased with the length of the absolute part and
the absolute code is embedded between the relocatable parts.
<p>
<h4>3.2) embed relocatable code in absolute files</h4>
<p>
This is dropped - too complicated. Should better be done with some
objdump or linker programs or so.
<p>
<h4>3.2) Header options</h4>
<p>
Before any opcode (after starting in relocatable mode, or after a .reloc
opcode), a header option can be set by:
<pre>
.fopt byte1, byte2, ...
</pre>
The header option length is automatically set by the assembler.
An example for an file author entry:
<pre>
.fopt 3, "Andre Fachat",0
</pre>
The 3 is the type byte for the author header option. The last zero ends
the name. The assembler can be configured to automatically include an
assembler header option into a file header.
<p>
<h4>3.3) allocation of data segment/zeropage segment address space</h4>
<p>
The assembler switches between the different segments by the means of
".text", ".data", ".bss" and ".zero" pseudo opcodes. After starting in
relocatable mode, the assembler is in the text segment.
<p>
The text segment contains the program code. Data holds the initialized data,
while bss and zero segments contain uninitialized data for normal/zeropage
address space.
Everything that is between one of these segment opcodes and the next segment
opcode gets into the corresponding segment, i.e. labels, assembled code etc.
The text and data segments are saved in the file, while for the bss and
zero segments only the length is saved in the file.
<p>
The assembler should issue a warning when a direct addressing mode
is used without a zero segment address and vice versa for 65816 CPUs.
<p>
<h4>3.4) referencing data/bss/zeropage addresses</h4>
<p>
One problem with the 6502 is, that it cannot load an address within one
step or assembler opcode. So an address is loaded with standard byte
opcodes, like "lda #&lt;label". But how do we decide, whether "label"
is an address or not, and what do we if we get something like
"lda #zp_label + 12 * label2"?
<p>
The assembler is now intelligent enough to evaluate such expressions
and check for:
<pre>
- no address label : ok, absolute
- one address label, only add to label : ok, relocate
- difference between two addresses : If addresses in same segment, compute
diff and set absolute, otherwise bail
- everything else : warning
</pre>
This way there is no change in syntax. Address labels are distinguished
by using the "label:" syntax, as opposed to "label = value".
Also, if the assembler is capable of doing so, an address label may be
defined by "label opcode", i.e. without a colon.
<p>
<h4>3.5) aligning code</h4>
<p>
The 6502 has the problem that some opcodes (e.g. "JMP ($xxFF)" are
broken, if the address given is at some (odd) address. But when loading
a relocatable file, one cannot know if an address will be odd or even.
Therefore there is a new opcode,
<pre>
.align 2
</pre>
that aligns the next address at the given address boundary. Valid
values are 2, 4, and 256. For the 6502 the opcode may insert NOP
operations ($EA opcodes) until the alignment is reached. In addition
the header align bits must be set appropriately.
<p>
<h3>4) Additional Notes</h3>
<h4>4.1 Clearance</h4>
<p>
This file is surely not the optimum and could be improved. Also the
header option "assigned numbers" should be added here.
<p>
For this reason the author, Andr&eacute; Fachat, will function as a
clearing point, where problems can be discussed and numbers can be assigned.
<h4>4.2 Character Sets</h4>
<h2>Appendix</h2>
<h3>A) Additional note</h3>
<h4>A.1) "inofficially" supported CPUs.</h4>
As this format has already been used for other CPUs than the 6502 or 65816,
there are CPU codes that are reserved for these CPUs. Please note that these
codes are derived from the current use of the file format and not any
preference of the author.
<pre>
mode.4-7 : CPU2 0000 = 6502 core (no undocumented opcodes)
0001 = 65C02 /w some bugfix, no illegal opcodes
0010 = 65SC02 (enhanced 65C02), some new opcodes
0011 = 65CE02 some 16bit ops/branches, Z register is modifiable
0100 = NMOS 6502 (including undocumented opcodes)
0101 = 65816 in 6502 emulation mode
011x = reserved
1000 = 6809
1010 = Z80
1101 = 8086
1110 = 80286
^^^^
|||+- Bit 4
||+-- Bit 5
|+--- Bit 6
+---- Bit 7
</pre>
<h3>B) Late binding</h3>
<p>
<a name="change13d"></a>
Late binding means that during the assembler run the values of some
variables are not known. Instead these variable values are filled in
when the program file is loaded into the system.
</p><p>
As an example let's discuss an example for a program that needs to
access some hardware at the expansion port of the C64. The hardware
is located either at IO1 ($de00) or IO2 ($df00) depending on some
hardware switch. To allow to use only one executable for the program,
it uses a variable "IOPORT" that is not defined in the program itself,
but set by the o65 loader using late binding.
</p>
<p>
The program accesses the io port is using the variable:
<pre>
lda IOPORT
</pre>
When assembling this one line program, the assembler is told to accept the
variable IOPORT as undefined. In <code>xa</code> this is done
using the <code>-L</code> option:
<pre>
xa -R -LIOPORT -o program.o65 program.a65
</pre>
Then <code>program.o65</code> contains relocatable code with an
undefined reference named <code>IOPORT</code>. Every time the
code uses this variable, the relocation table contains an entry
with a reference to the label in the undefined reference table.
</p>
<p>The resulting file looks like:
<pre>
00000000 01 00 6f 36 35 00 00 00 00 10 03 00 00 04 00 00 |..o65...........|
00000010 00 40 00 00 04 00 00 00 00 00 00 ad 00 00 01 00 |.@..............|
00000020 49 4f 50 4f 52 54 00 02 80 00 00 00 00 00 00 |IOPORT.........|
</pre>
The first six bytes are the magic number. After that the mode bits
(bytes seven and eight) are all zero. The text segment starts at $1000
and has a length of 3. The data segment starts at $0400 with a length of zero,
and the bss segment starts at $4000 with a length of zero too.
The zerospace segment starts at $0004, but also with a length of zero.
The minimum stack size needed is zero too. The list of header options
starts at file offset $001a. As the first byte is zero, there is no header
option. After this follows the text segment containing the bytes
$ad $00 $00. If the data segment size would not be zero, the data segment
would come here.
Then the undefined references list follows. The first two bytes (at file offset
$001e) state that there is a single undefined reference, and the name of the
undefined reference "IOPORT" followed by the ending zero byte is stored after
this number. Then the relocation table follows. The first byte in the relocation
table is $02. As the relocation table offset starts at <code>tbase-1</code> this
means that the first relocation position is at the second byte (offset 1) in the
text segment. The type byte $80 defines that it is an undefined, absolute
reference. The next two bytes define the index in the undefined reference table,
in this case $0000, which means that the reference <code>IOPORT</code> is
referenced. The next byte is zero, signalling the end of the text segment
relocation table. At file offset $002b the relocation table for the data
segment starts. As the first byte is zero, there is no relocation entry
for the data segment (obviously, as the data segment is empty). After this
relocation table the number of exported globals follows. This is zero, as
there is no exported global variable.</p>
<p>
When loading the file, the loader must know in advance what value
IOPORT should be assigned. This is not further discussed here.
When the loader loads the file, and recognizes the name
<code>IOPORT</code> in the undefined references table, it remembers
the index of this name in the table. Then, when the relocation table
contains a reference to the undefined label with the index value for
<code>IOPORT</code>, the value for that variable is then used
in the relocation.
</p>
<p>
If, for example the loader knows that <code>IOPORT=$de00</code>, then the
text segment is relocated to
<pre>
$ad $00 $de
</pre>
If the source is changed for example to
<pre>
lda IOPORT+1
</pre>
one byte in the the file changes:
<pre>
00000010 00 40 00 00 04 00 00 00 00 00 00 ad 01 00 01 00 |.@..............|
^^
</pre>
If the file is then relocated, the loader adds the value in the opcode in the
text segment ($0001 in this case) to the value resuling from the reference resolution
(<code>IOPORT</code> in this case). The resulting code then becomes:
<pre>
$ad $01 $de
</pre>
</p>
<h3>C) File examples</h3>
<h4>C.1 Example from section 2.6.4</h4>
<p>The example source file is
<pre>
.text
.dsb $222,$aa
lda #>vector
.dsb $23d0-$1224,$55
vector = *;
</pre>
Using the command
<pre>
xa -R -o test2.o65 test2.a65
</pre>
results in this file
<pre>
00000000 01 00 6f 36 35 00 00 00 00 10 d0 13 00 04 00 00 |..o65...........|
00000010 00 40 00 00 04 00 00 00 00 00 00 aa aa aa aa aa |.@..............|
00000020 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa |................|
*
00000230 aa aa aa aa aa aa aa aa aa aa aa aa aa a9 23 55 |..............#U|
00000240 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 |UUUUUUUUUUUUUUUU|
*
000013e0 55 55 55 55 55 55 55 55 55 55 55 00 00 ff ff 28 |UUUUUUUUUUU....(|
000013f0 42 d0 00 00 01 00 76 65 63 74 6f 72 00 82 d0 23 |B.....vector...#|
00001400
</pre>
After relocating the file with
<pre>
ld65 -bt 4660 test2.o65
</pre>
to the new address $1234 (using <code>ld65</code> from the <code>xa</code>
package), the resulting file is:
<pre>
00000000 01 00 6f 36 35 00 00 00 34 12 d0 13 00 10 00 00 |..o65...4.......|
00000010 00 40 00 00 02 00 00 00 00 00 00 aa aa aa aa aa |.@..............|
00000020 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa |................|
*
00000230 aa aa aa aa aa aa aa aa aa aa aa aa aa a9 26 55 |..............&U|
00000240 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 55 |UUUUUUUUUUUUUUUU|
*
000013e0 55 55 55 55 55 55 55 55 55 55 55 00 00 ff ff 28 |UUUUUUUUUUU....(|
000013f0 42 04 00 00 01 00 76 65 63 74 6f 72 00 82 d0 23 |B.....vector...#|
</pre>
which confirms the addresses computed above.
<h4>B.x more examples</h4>
<p>
(to be done with reference assembler)
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