Operating Systems

Goals

• Abstract away hardware knowledge so that programs don't need to worry about them
• can evolve by replacing interals while conforming to the interface
• manage resources available on the computer

Pieces

Kernel

• runs in main memory
• contains most frequently used functions

Monitor

• Runs in Kernel mode, allows access to protected memory and can run privileged functions
• Controls sequence of events
• Batch programs together
• program returns control when finished

Hardware features

• Memory protection (disallowing an area to be altered)
• Timer (prevents a job from taking up all the time)
• privileged instructions (certain intructions can only be run by the monitor)

Time sharing systems

• Multiple users on terminals, users should see feedback immediately

Processes

• Program in execution
• instance of program running
• entity that can be assigned to and executed on a processor
• A single unit of activity characterized by
  • a single sequential thread of execution
  • a current state
  • an associated set of system resources
• Contains three components:
  • executable program
  • associated data needed for program
  • execution context of program (everything the OS needs to manage the process)
• Common problems with designing system software:
  • has to juggle multiple programs, threads, interrupts, IO, shared resources
  • improper sync
  • failed mutual exclusion
  • nondeterministic operation
  • deadlocks

Memory management

• process isolation
• automatic allocation and management
• support swapping program code
• protection and access control
• long term storage
• virtual memory
  • allows programmers to address memory logically without worrying about physical memory
  • virtual address: page number plus offset in page
  • real address: physical address

Info Protection

• Availability: protects against interruption (e.g. RAID, dual power supply)
• Confidentiality: permissions (e.g. chmod)
• data integrity: protect data from unauthorized modification
• authenticity: proper verification of identity of users (e.g. phishing, XSS)

Scheduling

• Fairness
• Differential responsiveness
  • discriminate among different classes of jobs
  • QoS
• Efficiency: maximize throughput, minimize response time

Modern Operating Systems

• Microkernel architecture (e.g. QNX)
  • assigns only a few essential functions to the kernel
    • address spaces
    • interpricess communication (IPC)
    • basic scheduling
  • everything else in user space
• Multithreading
  • Process divided into concurrent threads
  • Thread: dispatchable unit of work, can be interrupted
  • Process is a collection of threads
• Symmetric multiprocessing
  • multiple processors
  • share memory and IO
  • can all do the same things
• Distributed
  • illusion of single main memory
• asymmetric multiprocessing
  • moderate success of cell processor
  • Big-Little architectures
• Faults
  • permanent, temporary, transient/intermittent
  • redundancy (spacial, temporal, info)
  • process isolation, concurrency control, VMs, checkpoints/rollback
• Multicore
  • concurrency: reentrant routines, proper kernel structure management
  • scheduling: user-level, kernel level
  • sync: IO management from single or multiple cores
  • memory management: spacial separation of memory
  • reliability and fault tolerance: new types of faults