xa [OPTION]... FILE
xa is a multi-pass cross-assembler for the 8-bit processors in the 6502 series (such as the 6502, 65C02, 6504, 6507, 6510, 7501, 8500, 8501 and 8502), the Rockwell R65C02, and the 16-bit 65816 processor. For a description of syntax, see Assembler Syntax further in this manual page.
No CMOS opcodes (default is to allow R65C02 opcodes).
No 65816 opcodes (default).
Allow 65816 opcodes.
Show lines with block open/close (see Pseudo-Ops).
Produce o65 object files instead of executable files (no linking performed); files may contain undefined references.
- -o filename
Set output filename. The default is a.o65; use the special filename - to output to standard output.
- -e filename
Set errorlog filename, default is none.
- -l filename
Set labellist filename, default is none. This is the symbol table and can be used by disassemblers such as dxa(1) to reconstruct source.
Add cross-reference list to labellist (requires -l).
Allow colons to appear in comments; for MASM compatibility. This does not affect colon interpretation elsewhere.
Start assembler in relocating mode.
Defines label as an absolute (but undefined) label even when linking.
- -b? addr
Set segment base for segment ? to address addr. ? should be t, d, b or z for text, data, bss or zero segments, respectively.
- -A addr
Make text segment start at an address such that when the file starts at address addr, relocation is not necessary. Overrides -bt; other segments still have to be taken care of with -b.
Suppress list of exported globals.
Define a preprocessor macro on the command line (see Preprocessor).
- -I dir
Add directory dir to the include path (before XAINPUT; see Environment).
- -O charset
Define the output charset for character strings. Currently supported are ASCII (default), PETSCII (Commodore ASCII), PETSCREEN (Commodore screen codes) and HIGH (set high bit on all characters).
Set the alternative preprocessor character to ?. This is useful when you wish to use cpp(1) and the built-in preprocessor at the same time (see Preprocessor). Characters may need to be quoted for your shell (example: -p'~' ).
Show summary of options.
Show version of program.
The following options are deprecated and will be removed in 2.4 and later versions:
Use old filename behaviour (overrides -o, -e and -l).
Allow preprocessor substitution within strings (this is now disallowed for better cpp(1) compatibility).
An introduction to 6502 assembly language programming and mnemonics is beyond the scope of this manual page. We invite you to investigate any number of the excellent books on the subject; one useful title is "Machine Language For Beginners" by Richard Mansfield (COMPUTE!), covering the Atari, Commodore and Apple 8-bit systems, and is widely available on the used market.
xa supports both the standard NMOS 6502 opcodes as well as the Rockwell CMOS opcodes used in the 65C02 (R65C02). With the -w option, xa will also accept opcodes for the 65816. NMOS 6502 undocumented opcodes are intentionally not supported, and should be entered manually using the .byte pseudo-op (see Pseudo-Ops). Due to conflicts between the R65C02 and 65816 instruction sets and undocumented instructions on the NMOS 6502, their use is discouraged.
In general, xa accepts the more-or-less standard 6502 assembler format as popularised by MASM and TurboAssembler. Values and addresses can be expressed either as literals, or as expressions; to wit,
current value of the program counter
The ASCII value of any quoted character is inserted directly into the program text (example: "A" inserts the byte "A" into the output stream); see also the Pseudo-Ops section. This is affected by the currently selected character set, if any.
Labels define locations within the program text, just as in other multi-pass assemblers. A label is defined by anything that is not an opcode; for example, a line such as
label1 lda #0
defines label1 to be the current location of the program counter (thus the address of the LDA opcode). A label can be explicitly defined by assigning it the value of an expression, such as
label2 = $d000
which defines label2 to be the address $d000, namely, the start of the VIC-II register block on Commodore 64 computers. The program counter * is considered to be a special kind of label, and can be assigned to with statements such as
* = $c000
which sets the program counter to decimal location 49152. With the exception of the program counter, labels cannot be assigned multiple times. To explicitly declare redefinition of a label, place a - (dash) before it, e.g.,
-label2 = $d020
which sets label2 to the Commodore 64 border colour register. The scope of a label is affected by the block it resides within (see Pseudo-Ops for block instructions). A label may also be hard-specified with the -L command line option.
Redefining a label does not change previously assembled code that used the earlier value. Therefore, because the program counter is a special type of label, changing the program counter to a lower value does not reorder code assembled previously and changing it to a higher value does not issue padding to put subsequent code at the new location. This is intentional behaviour to facilitate generating relocatable and position-independent code, but can differ from other assemblers which use this behaviour for linking. However, it is possible to use pseudo-ops to simulate other assemblers' behaviour and use xa as a linker; see Pseudo-Ops and Linking.
For those instructions where the accumulator is the implied argument (such as asl and lsr; inc and dec on R65C02; etc.), the idiom of explicitly specifying the accumulator with a is unnecessary as the proper form will be selected if there is no explicit argument. In fact, for consistency with label handling, if there is a label named a, this will actually generate code referencing that label as a memory location and not the accumulator. Otherwise, the assembler will complain.
Labels and opcodes may take expressions as their arguments to allow computed values, and may themselves reference other labels and/or the program counter. An expression such as lab1+1 (which operates on the current value of label lab1 and increments it by one) may use the following operands, given from highest to lowest priority:
multiplication (priority 10)
integer division (priority 10)
addition (priority 9)
shift left (8)
shift right (8)
- >= =>
greater than or equal to (7)
greater than (7)
- <= =<
less than or equal to (7)
less than (7)
equal to (6)
- <> ><
does not equal (6)
bitwise AND (5)
bitwise XOR (4)
bitwise OR (3)
logical AND (2)
logical OR (1)
Parentheses are valid. When redefining a label, combining arithmetic or bitwise operators with the = (equals) operator such as += and so on are valid, e.g.,
-redeflabel += (label12/4)
Normally, xa attempts to ascertain the value of the operand and (when referring to a memory location) use zero page, 16-bit or (for 65816) 24-bit addressing where appropriate and where supported by the particular opcode. This generates smaller and faster code, and is almost always preferable.
Nevertheless, you can use these prefix operators to force a particular rendering of the operand. Those that generate an eight bit result can also be used in 8-bit addressing modes, such as immediate and zero page.
low byte of expression, e.g., lda #<vector
high byte of expression
in situations where the expression could be understood as either an absolute or zero page value, do not attempt to optimize to a zero page argument for those opcodes that support it (i.e., keep as 16 bit word)
render as 24-bit quantity for 65816 (must specify -w command-line option). This is required to specify any 24-bit quantity!
force further optimization, even if the length of the instruction cannot be reliably determined (see NOTES'N'BUGS)
Expressions can occur as arguments to opcodes or within the preprocessor (see Preprocessor for syntax). For example,
takes the value at label2+1 (using our previous label's value, this would be $d021), and will be assembled as $ad $21 $d0 to disk. Similarly,
will take the lowest 8 bits of label2 (i.e., $20), and assign them to the accumulator (assembling the instruction as $a9 $20 to disk).
Comments are specified with a semicolon (;), such as
;this is a comment
They can also be specified in the C language style, using /* */ and // which are understood at the Preprocessor level (q.v.).
Normally, the colon (:) separates statements, such as
label4 lda #0:sta $d020
label2: lda #2
(note the use of a colon for specifying a label, similar to some other assemblers, which xa also understands with or without the colon). This also applies to semicolon comments, such that
; a comment:lda #0
is understood as a comment followed by an opcode. To defeat this, use the -M command line option to allow colons within comments. This does not apply to /* */ and // comments, which are dealt with at the preprocessor level (q.v.).
Pseudo-ops are false opcodes used by the assembler to denote meta- or inlined commands. Like most assemblers, xa has a rich set.
- .byt value1,value2,value3,...
Specifies a string of bytes to be directly placed into the assembled object. The arguments may be expressions. Any number of bytes can be specified.
- .asc "text1" ,"text2",...
Specifies a character string which will be inserted into the assembled object. Strings are understood according to the currently specified character set; for example, if ASCII is specified, they will be rendered as ASCII, and if PETSCII is specified, they will be translated into the equivalent Commodore ASCII equivalent. Other non-standard ASCIIs such as ATASCII for Atari computers should use the ASCII equivalent characters; graphic and control characters should be specified explicitly using .byt for the precise character you want. Note that when specifying the argument of an opcode, .asc is not necessary; the quoted character can simply be inserted (e.g., lda #"A" ), and is also affected by the current character set. Any number of character strings can be specified.
.byt and .asc are synonymous, so you can mix things such as .byt $43, 22, "a character string" and get the expected result. The string is subject to the current character set, but the remaining bytes are inserted wtihout modification.
- .aasc "text1" ,"text2",...
Specifies a character string that is always rendered in true ASCII regardless of the current character set. Like .asc, it is synonymous with .byt.
- .word value1,value2,value3...
Specifies a string of 16-bit words to be placed into the assembled object in 6502 little-endian format (that is, low-byte/high-byte). The arguments may be expressions. Any number of words can be specified.
- .dsb length,fillbyte
Specifies a data block; a total of length repetitions of fillbyte will be inserted into the assembled object. For example, .dsb 5,$10 will insert five bytes, each being 16 decimal, into the object. The arguments may be expressions. See Linking for how to use this pseudo-op to link multiple objects.
- .bin offset,length,"filename"
Inlines a binary file without further interpretation specified by filename from offset offset to length length. This allows you to insert data such as a previously assembled object file or an image or other binary data structure, inlined directly into this file's object. If length is zero, then the length of filename, minus the offset, is used instead. The arguments may be expressions. See Linking for how to use this pseudo-op to link multiple objects.
Opens a new block for scoping. Within a block, all labels defined are local to that block and any sub-blocks, and go out of scope as soon as the enclosing block is closed (i.e., lexically scoped). All labels defined outside of the block are still visible within it. To explicitly declare a global label within a block, precede the label with + or precede it with & to declare it within the previous level only (or globally if you are only one level deep). Sixteen levels of scoping are permitted.
Closes a block.
- .as .al .xs .xl
Only relevant in 65816 mode (with the -w option specified). These pseudo-ops set what size accumulator and X/Y-register should be used for future instructions; .as and .xs set 8-bit operands for the accumulator and index registers, respectively, and .al and .xl set 16-bit operands. These pseudo-ops on purpose do not automatically issue sep and rep instructions to set the specified width in the CPU; set the processor bits as you need, or consider constructing a macro. .al and .xl generate errors if -w is not specified.
The following pseudo-ops apply primarily to relocatable .o65 objects. A full discussion of the relocatable format is beyond the scope of this manpage, as it is currently a format in flux. Documentation on the proposed v1.2 format is in doc/fileformat.txt within the xa installation directory.
- .text .data .bss .zero
These pseudo-ops switch between the different segments, .text being the actual code section, .data being the data segment, .bss being uninitialized label space for allocation and .zero being uninitialized zero page space for allocation. In .bss and .zero, only labels are evaluated. These pseudo-ops are valid in relative and absolute modes.
- .align value
Aligns the current segment to a byte boundary (2, 4 or 256) as specified by value (and places it in the header when relative mode is enabled). Other values generate an error.
- .fopt type,value1,value2,value3,...
Acts like .byt/.asc except that the values are embedded into the object file as file options. The argument type is used to specify the file option being referenced. A table of these options is in the relocatable o65 file format description. The remainder of the options are interpreted as values to insert. Any number of values may be specified, and may also be strings.
xa implements a preprocessor very similar to that of the C-language preprocessor cpp(1) and many oddiments apply to both. For example, as in C, the use of /* */ for comment delimiters is also supported in xa, and so are comments using the double slash //. The preprocessor also supports continuation lines, i.e., lines ending with a backslash (\); the following line is then appended to it as if there were no dividing newline. This too is handled at the preprocessor level.
For reasons of memory and complexity, the full breadth of the cpp(1) syntax is not fully supported. In particular, macro definitions may not be forward-defined (i.e., a macro definition can only reference a previously defined macro definition), except for macro functions, where recursive evaluation is supported; e.g., to #define WW AA , AA must have already been defined. Certain other directives are not supported, nor are most standard pre-defined macros, and there are other limits on evaluation and line length. Because the maintainers of xa recognize that some files will require more complicated preparsing than the built-in preprocessor can supply, the preprocessor will accept cpp(1)-style line/filename/flags output. When these lines are seen in the input file, xa will treat them as cc would, except that flags are ignored. xa does not accept files on standard input for parsing reasons, so you should dump your cpp(1) output to an intermediate temporary file, such as
cc -E test.s > test.xa
No special arguments need to be passed to xa; the presence of cpp(1) output is detected automatically.
Note that passing your file through cpp(1) may interfere with xa's own preprocessor directives. In this case, to mask directives from cpp(1), use the -p option to specify an alternative character instead of #, such as the tilde (e.g., -p'~' ). With this option and argument specified, then instead of #include, for example, you can also use ~include, in addition to #include (which will also still be accepted by the xa preprocessor, assuming any survive cpp(1)). Any character can be used, although frankly pathologic choices may lead to amusing and frustrating glitches during parsing. You can also use this option to defer preprocessor directives that cpp(1) may interpret too early until the file actually gets to xa itself for processing.
The following preprocessor directives are supported.
- #include "filename"
Inserts the contents of file filename at this position. If the file is not found, it is searched using paths specified by the -I command line option or the environment variable XAINPUT (q.v.). When inserted, the file will also be parsed for preprocessor directives.
- #echo comment
Inserts comment comment into the errorlog file, specified with the -e command line option.
- #print expression
Computes the value of expression expression and prints it into the errorlog file.
- #define DEFINE text
Equates macro DEFINE with text text such that wherever DEFINE appears in the assembly source, text is substituted in its place (just like cpp(1) would do). In addition, #define can specify macro functions like cpp(1) such that a directive like #define mult(a,b) ((a)*(b)) would generate the expected result wherever an expression of the form mult(a,b) appears in the source. This can also be specified on the command line with the -D option. The arguments of a macro function may be recursively evaluated, unlike other #defines; the preprocessor will attempt to re-evaluate any argument refencing another preprocessor definition up to ten times before complaining.
The following directives are conditionals. If the conditional is not satisfied, then the source code between the directive and its terminating #endif are expunged and not assembled. Up to fifteen levels of nesting are supported.
Closes a conditional block.
Implements alternate path for a conditional block.
- #ifdef DEFINE
True only if macro DEFINE is defined.
- #ifndef DEFINE
The opposite; true only if macro DEFINE has not been previously defined.
- #if expression
True if expression expression evaluates to non-zero. expression may reference other macros.
- #iflused label
True if label label has been used (but not necessarily instantiated with a value). This works on labels, not macros!
- #ifldef label
True if label label is defined and assigned with a value. This works on labels, not macros!
Unclosed conditional blocks at the end of included files generate warnings; unclosed conditional blocks at the end of assembly generate an error.
#iflused and #ifldef are useful for building up a library based on labels. For example, you might use something like this in your library's code:
#echo label already defined, library function label cannot be inserted
label /* your code */
xa is oriented towards generating sequential binaries. Code is strictly emitted in order even if the program counter is set to a lower location than previously assembled code, and padding is not automatically emitted if the program counter is set to a higher location. Changing the program location only changes new labels for code that is subsequently emitted; previous emitted code remains unchanged. Fortunately, for many object files these conventions have no effect on their generation.
However, some applications may require generating an object file built from several previously generated components, and/or submodules which may need to be present at specific memory locations. With a minor amount of additional specification, it is possible to use xa for this purpose as well.
The first means of doing so uses the o65 format to make relocatable objects that in turn can be linked by ldo65(1) (q.v.).
The second means involves either assembled code, or insertion of previously built object or data files with .bin, using .dsb pseudo-ops with computed expression arguments to insert any necessary padding between them, in the sequential order they are to reside in memory. Consider this example:
* = $1000
; this is your code at $1000
; this label marks the end of code
; DON'T PUT A NEW .word HERE!
* = $2000
.dsb (*-endofpart1), 0
; yes, set it again
* = $2000
; this is your code at $2000
This example, written for Commodore microcomputers using a 16-bit starting address, has two "modules" in it: one block of code at $1000 (4096), indicated by the code between labels part1 and endofpart1, and a second block at $2000 (8192) starting at label part2.
The padding is computed by the .dsb pseudo-op between the two modules. Note that the program counter is set to the new address and then a computed expression inserts the proper number of fill bytes from the end of the assembled code in part 1 up to the new program counter address. Since this itself advances the program counter, the program counter is reset again, and assembly continues.
When the object this source file generates is loaded, there will be an rts instruction at address 4096 and another at address 8192, with null bytes between them.
Should one of these areas need to contain a pre-built file, instead of assembly code, simply use a .bin pseudo-op to load whatever portions of the file are required into the output. The computation of addresses and number of necessary fill bytes is done in the same fashion.
Although this example used the program counter itself to compute the difference between addresses, you can use any label for this purpose, keeping in mind that only the program counter determines where relative addresses within assembled code are resolved.
xa utilises the following environment variables, if they exist:
Include file path; components should be separated by `,'.
Output file path.
The R65C02 instructions ina (often rendered inc a) and dea (dec a) must be rendered as bare inc and dec instructions respectively.
The 65816 instructions mvn and mvp use two eight bit parameters, the only instructions in the entire instruction set to do so. Older versions of xa took a single 16-bit absolute value. Since 2.3.7, the standard syntax is now accepted and the old syntax is deprecated (a warning will be generated).
Forward-defined labels -- that is, labels that are defined after the current instruction is processed -- cannot be optimized into zero page instructions even if the label does end up being defined as a zero page location, because the assembler does not know the value of the label in advance during the first pass when the length of an instruction is computed. On the second pass, a warning will be issued when an instruction that could have been optimized can't be because of this limitation. (Obviously, this does not apply to branching or jumping instructions because they're not optimizable anyhow, and those instructions that can only take an 8-bit parameter will always be casted to an 8-bit quantity.) If the label cannot otherwise be defined ahead of the instruction, the backtick prefix ` may be used to force further optimization no matter where the label is defined as long as the instruction supports it. Indiscriminately forcing the issue can be fraught with peril, however, and is not recommended; to discourage this, the assembler will complain about its use in addressing mode situations where no ambiguity exists, such as indirect indexed, branching and so on.
Also, as a further consequence of the way optimization is managed, we repeat that all 24-bit quantities and labels that reference a 24-bit quantity in 65816 mode, anteriorly declared or otherwise, MUST be prepended with the @ prefix. Otherwise, the assembler will attempt to optimize to 16 bits, which may be undesirable.
The following options and modes will be REMOVED in 2.4 and later versions of xa:
the original mvn $xxxx syntax
file65(1), ldo65(1), printcbm(1), reloc65(1), uncpk(1), dxa(1)
This manual page was written by David Weinehall <firstname.lastname@example.org>, Andre Fachat <email@example.com> and Cameron Kaiser <firstname.lastname@example.org>. Original xa package (C)1989-1997 Andre Fachat. Additional changes (C)1989-2023 Andre Fachat, Jolse Maginnis, David Weinehall, Cameron Kaiser. The official maintainer is Cameron Kaiser.
30 Years of Xa
file65(1), ldo65(1), printcbm(1), reloc65(1), uncpk(1).