Operating Systems: Structures and Implementation - Fall 2004 (CMSC 412) - Title - Prof. Mi, Study notes of Operating Systems

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CMSC 412

Fall 2004

Operating Systems Structures

Announcements

• Project #
  • Posted. Due next Tuesday.
• Reading
  • Chapter 2-
  • Chapter 5 (next time)

OS Implementation

• What language should I build my OS in? - High- vs. low-level languages
• How should I structure it to manage complexity? - Balance reliability, maintainability, and performance

OS Structure: Simple

• Any part of the system may use the functionality of the rest of the system - MS-DOS (user programs can call low level I/O routines)

OS Structure: Hybrid

• Layering has problems on its own
  • May be difficult to communicate information easily
  • May be inefficient
• Hybrid
  • Use layered, modular approach, but permit “back doors” to improve information flow at the cost of abstraction

Modules

• Most modern operating systems implement kernel modules - Uses object-oriented approach - Each core component is separate - Each talks to the others over known interfaces - Each is loadable as needed within the kernel
• Hybrid approach: similar to layers but with more flexible

Policy vs. Mechanism

• Policy - what to do
  • users should not be able to read other users files
• Mechanism- how to accomplish the goal
  • file protection properties are checked on open system call
• Want to be able to change policy without having to change mechanism - change default file protection
• Extreme examples of each:
  • micro-kernel OS - all mechanism, no policy
  • MacOS - policy and mechanism bound together

Microkernel System Structure

• Goal: make the kernel as small as possible.
• Communication takes place between user modules using message passing via the kernel.
• Benefits:
  • easier to extend a microkernel
  • easier to port the OS to new architectures
  • more reliable and secure (less code running in kernel mode)
• Drawback:
  • More overhead for operation

Virtual Machines (Cont.)

(a) Nonvirtual machine (b) virtual machine Non-virtual Machine (^) Virtual Machine

VMware Architecture