Info Catalog
as: Equ
as: Pseudo Ops
as: Eqv
7.28 '.equiv SYMBOL, EXPRESSION'
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The '.equiv' directive is like '.equ' and '.set', except that the
assembler will signal an error if SYMBOL is already defined. Note a
symbol which has been referenced but not actually defined is considered
to be undefined.
Except for the contents of the error message, this is roughly
equivalent to
.ifdef SYM
.err
.endif
.equ SYM,VAL
plus it protects the symbol from later redefinition.