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(nasm.info)Section 4.7

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4.7. The Context Stack
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   Having labels that are local to a macro definition is sometimes not
quite powerful enough: sometimes you want to be able to share labels
between several macro calls. An example might be a `REPEAT' ... `UNTIL'
loop, in which the expansion of the `REPEAT' macro would need to be
able to refer to a label which the `UNTIL' macro had defined. However,
for such a macro you would also want to be able to nest these loops.

   NASM provides this level of power by means of a _context stack_. The
preprocessor maintains a stack of _contexts_, each of which is
characterised by a name. You add a new context to the stack using the
`%push' directive, and remove one using `%pop'. You can define labels
that are local to a particular context on the stack.

Section 4.7.1

`%push' and `%pop': Creating and Removing Contexts

Section 4.7.2

Context-Local Labels

Section 4.7.3

Context-Local Single-Line Macros

Section 4.7.4

`%repl': Renaming a Context

Section 4.7.5

Example Use of the Context Stack: Block IFs