Language-based extensible systems such as Java use type safety to
provide memory safety in a single address space. Memory safety alone,
however, is not sufficient to protect different applications from each
other.  Such systems must support a "process model" that enables the
control and management of computational resources.  In particular,
language-based extensible systems must support resource control
mechanisms analogous to those in standard operating systems.  They must
support the separation of processes and limit their use of resources,
but still support safe and efficient interprocess communication. We
demonstrate how this challenge can be addressed in Java operating
systems.  First, we describe the technical issues that arise when
implementing a process model in Java.  In particular, we lay out the
design choices for managing resources.  Second, we describe the
solutions that we are exploring in two complementary projects, "Alta"
and "GVM."  GVM is similar to a traditional monolithic kernel, whereas
Alta closely models the Fluke operating system.  Features of our
prototypes include flexible control of processor time using CPU
inheritance scheduling, per-process memory controls, fair allocation
of network bandwidth, and execution directly on hardware using the
OSKit. Finally, we compare our prototypes with other language-based
operating systems and explore the tradeoffs between the various
designs.