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><A
NAME="AEN483"
>3.7. Macros</A
></H1
><P
>LWASM is a macro assembler. A macro is simply a name that stands in for a
series of instructions. Once a macro is defined, it is used like any other
assembler directive. Defining a macro can be considered equivalent to adding
additional assembler directives.</P
><P
>Macros may accept parameters. These parameters are referenced within a
macro by the a backslash ("\") followed by a digit 1 through 9 for the first
through ninth parameters. They may also be referenced by enclosing the
decimal parameter number in braces ("{num}"). The special expansion "\*"
translates to the exact parameter string, including all parameters, passed
to the macro. These parameter references are replaced with the verbatim text
of the parameter passed to the macro. A reference to a non-existent
parameter will be replaced by an empty string. Macro parameters are expanded
everywhere on each source line. That means the parameter to a macro could be
used as a symbol or it could even appear in a comment or could cause an
entire source line to be commented out when the macro is expanded. </P
><P
>Parameters passed to a macro are separated by commas and the parameter list
is terminated by any whitespace. This means that neither a comma nor whitespace
may be included in a macro parameter.</P
><P
>Macro expansion is done recursively. That is, within a macro, macros are
expanded. This can lead to infinite loops in macro expansion. If the assembler
hangs for a long time while assembling a file that uses macros, this may be
the reason.</P
><P
>Each macro expansion receives its own local symbol context which is not
inherited by any macros called by it nor is it inherited from the context
the macro was instantiated in. That means it is possible to use local symbols
within macros without having them collide with symbols in other macros or
outside the macro itself. However, this also means that using a local symbol
as a parameter to a macro, while legal, will not do what it would seem to do
as it will result in looking up the local symbol in the macro's symbol context
rather than the enclosing context where it came from, likely yielding either
an undefined symbol error or bizarre assembly results.</P
><P
>Note that there is no way to define a macro as local to a symbol context. All
macros are part of the global macro namespace. However, macros have a separate
namespace from symbols so it is possible to have a symbol with the same name
as a macro.</P
><P
>Macros are defined only during the first pass. Macro expansion also
only occurs during the first pass. On the second pass, the macro
definition is simply ignored. Macros must be defined before they are used.</P
><P
>The following directives are used when defining macros.</P
><P
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><DT
><CODE
CLASS="PARAMETER"
>macroname</CODE
> MACRO [NOEXPAND]</DT
><DD
><P
>This directive is used to being the definition of a macro called
<CODE
CLASS="PARAMETER"
>macroname</CODE
>. If <CODE
CLASS="PARAMETER"
>macroname</CODE
> already
exists, it is considered an error. Attempting to define a macro within a
macro is undefined. It may work and it may not so the behaviour should not
be relied upon.</P
><P
>If NOEXPAND is specified, the macro will not be expanded in a program
listing. Instead, all bytes emitted by all instructions within the macro
will appear to be emitted on the line where the macro is invoked, starting
at the address of the line of the invokation. If the macro uses ORG or other
directives that define symbols or change the assembly address, these things
will also be hidden (except in the symbol table) and the output bytes will
appear with incorrect address attribution. Thus, NOEXPAND should only be
used for macros that do not mess with the assembly address or otherwise
define symbols that should be visible.</P
></DD
><DT
>ENDM</DT
><DD
><P
>This directive indicates the end of the macro currently being defined. It
causes the assembler to resume interpreting source lines as normal.</P
></DD
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author	William Astle <lost@l-w.ca>
date	Sat, 11 Aug 2012 15:18:58 -0600
parents	2cc599f1bebf
children