Questions
Q: How is a Hydra procedure different from the procedures we are familiar with in a typical language and runtime environment?
Overview
- Developed for C.mmp, the Carnegie-Mellon Multi-Mini-Processor (16 PDP-11s, shared 32 MB core).
- Represents an early attempt at a kernel-based OS—a minimal core of universal mechanisms from which different operating environments could be built.
- Primary design theme: separate mechanism from policy → the kernel provides primitives for protection, object management, and process control; higher-level systems define their own policies.
- No built-in notion of priority, fairness or preemption
- kernel gives generic tools
Context and Goals
Hardware environment
- C.mmp architecture:
- Up to 16 independent PDP-11 processors.
- Shared memory via a crossbar switch.
- Each CPU → private memory, I/O devices, local disk.
- Relocation hardware → virtual → physical mapping.
- Global clock → unique name generation + time base.
Introduction
Goals:
- To provide an environment for effective utilization of the hardware resources
- To facilitate the construction of such environments
Design Philosophy
General mindset:
- “universal applicability” - extensible/modular
- “absolute reliability” - reliable primitives Collection of basic programs required in OS called Kernel or nucleus
- Specifically:
- Multiprocessor environment
- Separation of mechanism and policy - for flexibility
- Integration of the design with implementation methodology - separation of interface and logic
- Rejection of strict hierarchical layering for flexibility
- Protection
- Reliability
- separates mechanism and policy - allows multiple OSes to coexist
What does an OS is or does? according to paper Two views:
- an operating system defines an “abstract machine” by providing facilities, or resources, which are more convenient than those provided by the “bare” hardware
- an operating system allocates (hard-ware) resources in such a way to most effectively utilize them Both put emphasis on the central role of resources both physical and abstract
The mechanisms provided by the HYDRA kernel are all intended to support the abstracted notion of a resource(incarnations of a resource are called objects) - EVERYTHING IS AN OBJECT
- Object = encapsulated resource (state + behavior)
- Mechanisms like creation and representation of new types of resources and operations defined on them
- as well as protected access to instances of one or more resources within controlled execution domains
- controlled passing of both control and resources between execution domains
Three key object types provided by the kernel:
- procedure
- LNS
- Process Used to create and manipulate an execution environment
Execution Environment and Core Primitives
Hydra’s execution is built around three object types:
| Object Type | Purpose |
|---|---|
| Procedure | Code + static capabilities (list of objects it may access). Acts like a protected subroutine. |
| LNS (Local Name Space) | Environment object created when a procedure is invoked → holds actual capabilities and parameter bindings. |
| Process | Dynamic execution thread = stack of LNSs (created by procedure calls). - Each procedure call creates its own LNS with its own parameter bindings and capabilities - Stack of LNSs is like a call stack but for both execution state and access rights |