These are safe, low-privilege ways to call kernel functions.
I don't know the difference between the two.
Thank you for contacting Intel Embedded Community.
The information that help you can be found in sections 4.8.2 and 4.8.3, on pages 4-18 through 4-20 of the Intel(R) 64 and IA-32 Architectures Software Developer’s Manual Volume 3A: System Programming Guide, Part 1 document # 253668. This document can be found at:
Also, please review the following websites that have more details that may clarify: