Using as: Macro

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### 7.58 `.macro`

The commands `.macro` and `.endm` allow you to define macros that generate assembly output. For example, this definition specifies a macro `sum` that puts a sequence of numbers into memory:

            .macro  sum from=0, to=5
            .long   \from
            .if     \to-\from
            sum     "(\from+1)",\to
            .endif
            .endm

With that definition, ‘`SUM 0,5`’ is equivalent to this assembly input:

            .long   0
            .long   1
            .long   2
            .long   3
            .long   4
            .long   5

`.macro macname`
`.macro macname macargs …`

  Begin the definition of a macro called macname. If your macro definition requires arguments, specify their names after the macro name, separated by commas or spaces. You can qualify the macro argument to indicate whether all invocations must specify a non-blank value (through ‘`:`req``’), or whether it takes all of the remaining arguments (through ‘`:`vararg``’). You can supply a default value for any macro argument by following the name with ‘`=deflt`’. You cannot define two macros with the same macname unless it has been subject to the `.purgem` directive (see Purgem) between the two definitions. For example, these are all valid `.macro` statements:

  `.macro comm`

    Begin the definition of a macro called `comm`, which takes no arguments.

  `.macro plus1 p, p1`
  `.macro plus1 p p1`

    Either statement begins the definition of a macro called `plus1`, which takes two arguments; within the macro definition, write ‘`\p`’ or ‘`\p1`’ to evaluate the arguments.

  `.macro reserve_str p1=0 p2`

    Begin the definition of a macro called `reserve_str`, with two arguments. The first argument has a default value, but not the second. After the definition is complete, you can call the macro either as ‘`reserve_str a,b`’ (with ‘`\p1`’ evaluating to a and ‘`\p2`’ evaluating to b), or as ‘`reserve_str ,b`’ (with ‘`\p1`’ evaluating as the default, in this case ‘`0`’, and ‘`\p2`’ evaluating to b).

  `.macro m p1:req, p2=0, p3:vararg`

    Begin the definition of a macro called `m`, with at least three arguments. The first argument must always have a value specified, but not the second, which instead has a default value. The third formal will get assigned all remaining arguments specified at invocation time.

    When you call a macro, you can specify the argument values either by position, or by keyword. For example, ‘`sum 9,17`’ is equivalent to ‘`sum to=17, from=9`’.

  Note that since each of the macargs can be an identifier exactly as any other one permitted by the target architecture, there may be occasional problems if the target hand-crafts special meanings to certain characters when they occur in a special position. For example, if the colon (`:`) is generally permitted to be part of a symbol name, but the architecture specific code special-cases it when occurring as the final character of a symbol (to denote a label), then the macro parameter replacement code will have no way of knowing that and consider the whole construct (including the colon) an identifier, and check only this identifier for being the subject to parameter substitution. So for example this macro definition:

      	.macro label l
      \l:
      	.endm

  might not work as expected. Invoking ‘`label foo`’ might not create a label called ‘`foo`’ but instead just insert the text ‘`\l:`’ into the assembler source, probably generating an error about an unrecognised identifier.

  Similarly problems might occur with the period character (‘`.`’) which is often allowed inside opcode names (and hence identifier names). So for example constructing a macro to build an opcode from a base name and a length specifier like this:

      	.macro opcode base length
              \base.\length
      	.endm

  and invoking it as ‘`opcode store l`’ will not create a ‘`store.l`’ instruction but instead generate some kind of error as the assembler tries to interpret the text ‘`\base.\length`’.

  There are several possible ways around this problem:

  `Insert white space`

    If it is possible to use white space characters then this is the simplest solution. eg:

        	.macro label l
        \l :
        	.endm

  `Use ‘`\()`’`

    The string ‘`\()`’ can be used to separate the end of a macro argument from the following text. eg:

        	.macro opcode base length
                \base\().\length
        	.endm

  `Use the alternate macro syntax mode`

    In the alternative macro syntax mode the ampersand character (‘`&`’) can be used as a separator. eg:

        	.altmacro
        	.macro label l
        l&:
        	.endm

  Note: this problem of correctly identifying string parameters to pseudo ops also applies to the identifiers used in `.irp` (see Irp) and `.irpc` (see Irpc) as well.

`.endm`

  Mark the end of a macro definition.

`.exitm`

  Exit early from the current macro definition.

`\@`

  `as` maintains a counter of how many macros it has executed in this pseudo-variable; you can copy that number to your output with ‘`\@`’, but only within a macro definition.

`LOCAL name [ , … ]`

  Warning: `LOCAL` is only available if you select “alternate macro syntax” with ‘`--alternate`’ or `.altmacro`. See `.altmacro`.

Next: MRI, Previous: Long, Up: Pseudo Ops [Contents][Index]