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Who is the author of the Maamar Mordechai quoted often in the Mishna Berurah.
There are quite a number of sefarim on HebrewBooks with that title and the old prints make it hard to scan through them to find a given quote.
His name is Rabbi Mordechai Karmi. You can read the English and Hebrew Wikipedia articles on him here. The Hebrew article notes (correctly) that he is often cited by the Mishnah Berurah.