Services vs Directives vs System Calls

[ricardo-e-silva at criticalsoftware.com (ricardo)]

Hi,


In the RTEMS POSIX API User's Guide for RTEMS 4.6.6 the API functions
are separated in three groups (services, directives or system calls).
Some managers have services (like mutex manager), others have directives
(like semaphore manager) and only one has system calls (timer manager)


In RTEMS C User?s Guide (for classical api), we only find directives
under each manager. 

Anyone can explain me what is the difference between this three types of
API functions? 


Thanks,

Ricardo