COMP3891

Operating Systems Overview

Role of an Operating System

• Software layer on top of hardware that manages all parts of the system.
• Controls all computer resources.
• Provides base upon which application programs can be written.

Abstract (or Extended) Machine

• Extends basic hardware with added functionality.
• Provides high-level abstractions.
  • Easier to program with.
  • Common core for all applications.
• Hides hardware and implementation details.
  • More portable code.

Resource Manager

• Responsible for allocating resources to users and processes.
• Must ensure:
  • No starvation (process is not able to acquire desired resource to progress its execution)
  • Allocation of resources according to some policy.
  • System is overall efficient.

Structure of an Operating System

Privileged Component

Kernel

• Portion of operating system running in privileged mode.
• Usually resident in memory while computer is running.
• Contains fundamental functionality:
  • Whatever is required to implement other services.
  • Whatever is required to provide security.
• Contains most-frequently used functions.

Operating System is Privileged

• Applications should not be able to interfere or bypass the OS.
  • OS can enforced the “extended machine”.
  • OS can enforce resource allocation policies.
  • OS can prevent applications from interfering with each other.

Interaction Between Application Programs and the OS

• OS runs in Kernel Mode while application programs run in User Mode.
• OS interacts via load and store instructions to all memory, CPU and device registers.
• Applications interact with themselves and via functions call to library procedures.
• Applications can interact with OS via a system call to request a service. E.g. accessing a hard disk drive.

Note on System Libraries

Systems libraries are libraries of support functions (procedures, subroutines).

• Subset of library functions are system calls
  • strcmp(), memcpy() are pure library functions.
    • Only manipulate memory within the application or perform computations.
  • open(), close(), read(), write() are system calls.
    • Cross user-kernel boundary.
    • Implementation focused on passing request to OS and returning result to application.

Aside: Privilege-less OS

• Some Embedded OSs have no privileged component.
  • E.g. PalmOS, Mac OS 9, RTEMS.
  • These OSs can implement standard OS functionality, but cannot enforce it.
    • All software runs together.
    • No isolation.
    • One fault potentially brings down entire system.