Complex sentence: Originally implemented was highly tuned Intel i386-specific assembly language code the API has seen extensive development in a number of directions, both in achieving a higher grade of platform independence and also in improving security, isolation, and robustness.
Split into simpler sentences: Originally implemented was highly tuned Intel i386-specific assembly language code. The API has seen extensive development in a number of directions, both in achieving a higher grade of platform independence and also in improving security, isolation, and robustness.