3.9. Local Labels

NASM gives special treatment to symbols beginning with a period. A label beginning with a single period is treated as a local label, which means that it is associated with the previous non-local label. So, for example:

label1  ; some code
.loop   ; some more code
        jne .loop
label2  ; some code
.loop   ; some more code
        jne .loop

In the above code fragment, each JNE instruction jumps to the line immediately before it, because the two definitions of .loop are kept separate by virtue of each being associated with the previous non-local label.

NASM goes one step further, in allowing access to local labels from other parts of the code. This is achieved by means of defining a local label in terms of the previous non-local label: the first definition of .loop above is really defining a symbol called label1.loop, and the second defines a symbol called label2.loop. So, if you really needed to, you could write

label3  ; some more code
        ; and some more
        jmp label1.loop

Sometimes it is useful - in a macro, for instance - to be able to define a label which can be referenced from anywhere but which doesn’t interfere with the normal local-label mechanism. Such a label can’t be non-local because it would interfere with subsequent definitions of, and references to, local labels; and it can’t be local because the macro that defined it wouldn’t know the label’s full name. NASM therefore introduces a third type of label, which is probably only useful in macro definitions: if a label begins with the special prefix ..@, then it does nothing to the local label mechanism. So you could code

label1: ; a non-local label
.local: ; this is really label1.local
..@foo: ; this is a special symbol
label2: ; another non-local label
.local: ; this is really label2.local
        jmp ..@foo              ; this will jump three lines up

NASM has the capacity to define other special symbols beginning with a double period: for example, ..start is used to specify the entry point in the obj output format.