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(nasm.info)Section 4.9.3

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4.9.3. `%local' Directive
-------------------------

   The `%local' directive is used to simplify the use of local temporary
stack variables allocated in a stack frame. Automatic local variables
in C are an example of this kind of variable. The `%local' directive is
most useful when used with the `%stacksize' (see *Note Section 4.9.2::
and is also compatible with the `%arg' directive (see *Note Section
4.9.1::). It allows simplified reference to variables on the stack which
have been allocated typically by using the `ENTER' instruction (see
*Note Section B.4.65:: for a description of that instruction). An
example of its use is the following:

     silly_swap:
     
         %push mycontext             ; save the current context
         %stacksize small            ; tell NASM to use bp
         %assign %$localsize 0       ; see text for explanation
         %local old_ax:word, old_dx:word
     
             enter   %$localsize,0   ; see text for explanation
             mov     [old_ax],ax     ; swap ax & bx
             mov     [old_dx],dx     ; and swap dx & cx
             mov     ax,bx
             mov     dx,cx
             mov     bx,[old_ax]
             mov     cx,[old_dx]
             leave                   ; restore old bp
             ret                     ;
     
         %pop                        ; restore original context

   The `%$localsize' variable is used internally by the `%local'
directive and _must_ be defined within the current context before the
`%local' directive may be used. Failure to do so will result in one
expression syntax error for each `%local' variable declared. It then
may be used in the construction of an appropriately sized ENTER
instruction as shown in the example.