xa - Online in the Cloud

PROGRAM:

NAME

xa - 6502/R65C02/65816 cross-assembler

SYNOPSIS

xa [OPTION]... FILE

DESCRIPTION

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.

OPTIONS

-v Verbose output.

-x Use old filename behaviour (overrides -o, -e and -l). This option is now
deprecated.

-C No CMOS opcodes (default is to allow R65C02 opcodes)

-W No 65816 opcodes (default).

-w Allow 65816 opcodes.

-B Show lines with block open/close (see PSEUDO-OPS).

-c 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.

-r Add cross-reference list to labellist (requires -l).

-M Allow colons to appear in comments; for MASM compatibility. This does not affect
colon interpretation elsewhere.

-R Start assembler in relocating mode.

-Llabel
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.

-G Suppress list of exported globals.

-DDEF=TEXT
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).

-p? 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'~' ).

--help Show summary of options.

--version
Show version of program.

ASSEMBLER SYNTAX

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,

123 decimal value

$234 hexadecimal value

&123 octal

%010110 binary

* 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.

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 handing, 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)

- subtraction (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,

lda label2+1

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,

lda #<label2

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

or

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

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.

.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.

.( 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.

PREPROCESSOR

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
xa 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.

#endif Closes a conditional block.

#else 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:

#iflused label
#ifldef label
#echo label already defined, library function label cannot be inserted
#else
label /* your code */
#endif
#endif

ENVIRONMENT

xa utilises the following environment variables, if they exist:

XAINPUT
Include file path; components should be separated by `,'.

XAOUTPUT
Output file path.

NOTES'N'BUGS

The R65C02 instructions ina (often rendered inc a) and dea (dec a) must be rendered as
bare inc and dec instructions respectively.

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.

Use xa online using onworks.net services