Complex sentence: Starting with AmigaOS 4.0, AmigaDOS abandoned its legacy with BCPL, and starting from AmigaOS 4.1 it has been rewritten with full 64-bit support.
Split into simpler sentences: Starting with AmigaOS 4.0, AmigaDOS abandoned its legacy with BCPL. Starting from AmigaOS 4.1, AmigaDOS has been rewritten with full 64-bit support.