| Layer | Characteristics |
|---|---|
| Language RTL's | Language specific. One for C, one for Fortran etc.. Using the conventions of the language. |
| VMS RTL | LIB$, MTH$ etc. routines. Callable from all languages. Used by the language RTL's. Typical Fortran calling convention. |
| System services | SYS$ routines. Used by VMS RTL. Direct OS interface. Interface geared towards assembler and Bliss. |