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><H1
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><A
NAME="AEN584"
>3.10. Assembler Modes and Pragmas</A
></H1
><P
>There are a number of options that affect the way assembly is performed.
Some of these options can only be specified on the command line because
they determine something absolute about the assembly process. These include
such things as the output target. Other things may be switchable during
the assembly process. These are known as pragmas and are, by definition,
not portable between assemblers.</P
><P
>LWASM supports a number of pragmas that affect code generation or
otherwise affect the behaviour of the assembler. These may be specified by
way of a command line option or by assembler directives. The directives
are as follows.</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>PRAGMA <CODE
CLASS="PARAMETER"
>pragma[,...]</CODE
></DT
><DD
><P
>Specifies that the assembler should bring into force all <CODE
CLASS="PARAMETER"
>pragma</CODE
>s
specified. Any unrecognized pragma will cause an assembly error. The new
pragmas will take effect immediately. This directive should be used when
the program will assemble incorrectly if the pragma is ignored or not supported.</P
></DD
><DT
>*PRAGMA <CODE
CLASS="PARAMETER"
>pragma[,...]</CODE
></DT
><DD
><P
>This is identical to the PRAGMA directive except no error will occur with
unrecognized or unsupported pragmas. This directive, by virtue of starting
with a comment character, will also be ignored by assemblers that do not
support this directive. Use this variation if the pragma is not required
for correct functioning of the code.</P
></DD
><DT
>*PRAGMAPUSH <CODE
CLASS="PARAMETER"
>pragma[,...]</CODE
></DT
><DD
><P
>This directive saves the current state of the specified pragma(s) for later retrieval. See discussion below for more information.</P
><P
>This directive will not throw any errors for any reason.</P
></DD
><DT
>*PRAGMAPOP <CODE
CLASS="PARAMETER"
>pragma[,...]</CODE
></DT
><DD
><P
>This directive restores the previously saved state of the specified pragma(s). See discussion below for more information.</P
><P
>This directive will not throw any errors for any reason.</P
></DD
></DL
></DIV
><P
>Each pragma supported has a positive version and a negative version.
The positive version enables the pragma while the negative version disables
it. The negatitve version is simply the positive version with "no" prefixed
to it. For instance, "pragma" vs. "nopragma". Only the positive version is
listed below.</P
><P
>Pragmas are not case sensitive.</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
>index0tonone</DT
><DD
><P
>When in force, this pragma enables an optimization affecting indexed addressing
modes. When the offset expression in an indexed mode evaluates to zero but is
not explicity written as 0, this will replace the operand with the equivalent
no offset mode, thus creating slightly faster code. Because of the advantages
of this optimization, it is enabled by default.</P
></DD
><DT
>cescapes</DT
><DD
><P
>This pragma will cause strings in the FCC, FCS, and FCN pseudo operations to
have C-style escape sequences interpreted. The one departure from the official
spec is that unrecognized escape sequences will return either the character
immediately following the backslash or some undefined value. Do not rely
on the behaviour of undefined escape sequences.</P
></DD
><DT
>importundefexport</DT
><DD
><P
>This pragma is only valid for targets that support external references. When
in force, it will cause the EXPORT directive to act as IMPORT if the symbol
to be exported is not defined.  This is provided for compatibility with the
output of gcc6809 and should not be used in hand written code.  Because of
the confusion this pragma can cause, it is disabled by default.</P
></DD
><DT
>undefextern</DT
><DD
><P
>This pragma is only valid for targets that support external references. When in
force, if the assembler sees an undefined symbol on the second pass, it will
automatically define it as an external symbol. This automatic definition will
apply for the remainder of the assembly process, even if the pragma is
subsequently turned off. Because this behaviour would be potentially surprising,
this pragma defaults to off.</P
><P
>The primary use for this pragma is for projects that share a large number of
symbols between source files. In such cases, it is impractical to enumerate
all the external references in every source file. This allows the assembler
and linker to do the heavy lifting while not preventing a particular source
module from defining a local symbol of the same name as an external symbol
if it does not need the external symbol. (This pragma will not cause an
automatic external definition if there is already a locally defined symbol.)</P
><P
>This pragma will often be specified on the command line for large projects.
However, depending on the specific dynamics of the project, it may be sufficient
for one or two files to use this pragma internally.</P
></DD
><DT
>dollarlocal</DT
><DD
><P
>When set, a "$" in a symbol makes it local. When not set, "$" does not
cause a symbol to be local.  It is set by default except when using the OS9
target.</P
></DD
><DT
>dollarnotlocal</DT
><DD
><P
> This is the same as the "dollarlocal" pragma except its sense is
reversed.  That is, "dollarlocal" and "nodollarnotlocal" are equivalent and
"nodollarlocal" and "dollarnotlocal" are equivalent.  </P
></DD
><DT
>pcaspcr</DT
><DD
><P
> Normally, LWASM makes a distinction between PC and PCR in program
counter relative addressing. In particular, the use of PC means an absolute
offset from PC while PCR causes the assembler to calculate the offset to the
specified operand and use that as the offset from PC. By setting this
pragma, you can have PC treated the same as PCR. </P
></DD
><DT
>shadow</DT
><DD
><P
>When this pragma is in effect, it becomes possible to define a macro
that matches an internal operation code. Thus, it makes it possible to
redefine either CPU instructions or pseudo operations. Because this feature
is of dubious utility, it is disabled by default.</P
></DD
><DT
>nolist</DT
><DD
><P
>Lines where this pragma is in effect will not appear in the assembly
listing.  Also, any symbols defined under this pragma will not show up in
the symbol list.  This is most useful in include files to avoid spamming the
assembly listing with dozens, hundreds, or thousands of irrelevant
symbols.</P
></DD
><DT
>autobranchlength</DT
><DD
><P
>One of the perennial annoyances for 6809 programmers is that the
mneumonics for the short and long branch instructions are different (bxx vs.
lbxx), which is at odds with the rest of the instruction set.  This pragma
is a solution to those annoying byte overflow errors that short branch
instructions tend to aquire.</P
><P
>When this pragma is in effect, which is not the default, whenever any
relative branch instruction is used, its size will be automatically
determined based on the actual distance to the destination.  In other words,
one can write code with long or short branches everywhere and the assembler
will choose a size for the branch.</P
><P
>Also, while this pragma is in effect, the &#62; and &#60; symbols can be used
to force the branch size, analogous to their use for other instructions with
&#60; forcing 8 bit offsets and &#62; forcing 16 bit offets.</P
><P
>Because this pragma leads to source that is incompatible with other
assemblers, it is strongly recommended that it be invoked using the PRAGMA
directive within the source code rather than on the command line or via the
*PRAGMA directive.  This way, an error will be raised if someone tries to
* assemble the code under a different assembler.</P
></DD
></DL
></DIV
><P
>As a convenience, each input file has a pragma state stack. This
allows, through the use of *PRAGMAPUSH and *PRAGMAPOP, a file to change a
pragma state and then restore it to the precise state it had previously.
If, at the end of an input file, all pragma states have not been popped,
they will be removed from the stack.  Thus, it is critical to employ
*PRAGMAPOP correctly. Because each input file has its own pragma stack,
using *PRAGMAPUSH in one file and *PRAGMAPOP in another file will not
work.</P
><P
>Pragma stacks are more useful in include files, in particular in
conjunction with the nolist pragma.  One can push the state of the nolist
pragma, engage the nolist pragma, and then pop the state of the nolist
pragma at the end of the include file.  This will cause the entire include
file to operate under the nolist pragma.  However, if the file is included
while nolist is already engaged, it will not undo that state.</P
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author	William Astle <lost@l-w.ca>
date	Sun, 10 Jun 2012 14:14:43 -0600
parents	1e0a0e6cd918
children