System calls in modern operating systems give programmers the ability
to access and manipulate system-level objects. Example objects are
files and processes, and example system calls are read, write, and
fork. Thus, the operating comes with a predefined set of system calls
that can be used to interact with a predefined set of objects. An
important point is that users cannot create new objects without
rebuilding the entire system.

The Hydra CALL mechanism is also used to manipulate system-level
objects. The distinction with respect to modern system calls is that
the CALL mechanism is very general -- it can be used to run any
user-defined procedure. Since Hydra allows users to define their own
system-level objects dynamically, it is necessary for them to be able
to write their own procedures to interact with those objects. Also,
modern os system calls generally only interact with a single object
when invoked because they are at such a low level. Very few objects in
Hydra are at such a low level. Instead, created objects generally
build off of existing objects, making it necessary for CALLed
procedures to be able to interact with other objects frequently.

The protection schemes employed are also worthy of note. Modern
operating systems predefine the access rights to various objects,
which correspond to the system calls a user is allowed to run on those
objects. In Hydra, the access rights correspond to the user-defined
procedures a user is allowed to CALL on the object. So the two
protection schemes are similar in function, but different in terms of
flexibility, with Hydra allowing for more flexible protection.