5.4.2 Equiv Directive

The .equiv symbol, expression directive operates like the .equ directive, except that the assembler will signal an error if symbol is already defined. Note that a symbol which has been referenced but not actually defined is considered to be undefined.

Except for the contents of the error message, this directive is roughly equivalent to:

.ifdef SYM
.err
.endif
.equ SYM,VAL