Index Problem

I want to query this...
--> SELECT * FROM bbs ORDER BY ref desc, step ASC LIMIT 12;

this query doesn't refer the index that made by this query.
--> CREATE INDEX idx_bbs ON bbs (ref, step);

but, i change the query that "ref desc" to "ref asc".
then query refer the index, and i can see a result very fast. :-(

so, i want to view an result that one column ordered by ascending, and oterh column ordered by descending using index.
what do i do?
how make an index?

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