What does the initial mīm of مَقْدَمٌ justify?

Noun Patterns: Jāmid, Maṣdar, Mushtaq and 50 Basic-Triliteral Maṣdar Entries

• It is necessarily a passive participle
• It is an imperative
• It is a basic biliteral
• It is a mīm-pattern word, whose function depends on context