[OS] Operating System Structure 2

Operating System Structure Part 2

Operating System Structure

MS-DOS

• written to provide the most functionality in the least space
• not divided into modules
• its interfaces and levels of functionality are not well separated
• allow applications to access HW access
• no user- and kernel- mode separation

• Single-tasking, single memory space
• Shell (command interpreter) invoked when system booted
• Simple method to run program (loads program into memory, overwriting all include small part of CI)
• Program exit -> the remaining CI reloads shell from disk

Layered Approach

• The operating system is divided into a number of layers
• The bottom layer is the hardware, the highest layer is the user interace
• Layers are selected such that each layer uses functions (operations) and services of only lower-level layers

Traditional UNIX System Structure (Monolithic)

• consists of everything below the system call interface and above the physial hardware
• provides a large number of functions for one level
  • file system, CPU scheduling, memory management, I/O, …
• Pros : Performance
• Cons
  • Difficult to maintain and upgrade due to interdependencies in the code
  • Security / reliability
    • Any component of OS can ruin entire system
    • Entire OS code runs in a single memory space

Microkernel System Structure

• moves out as much as possible from the kernel to user-space services
• communication takes place between user modules using message passing
• Pros
  • Easier to extend (Modular structure, easy to add services)
  • Easier to port
  • More reliable, secure
• Cons
  • Performance overhead of user space to kernel space communication
• Example
  • The Mach
  • L3 / L4
    • seL4 (mathematically verfied software kernel)

Modern Operating System Structure

• Hybrid!!
• Pros of Monolithic + Pros of Microkernel