Computer Science 322
Operating Systems

• Announcements
  • Reminder: our second exam is scheduled for April 28-30.
  • Practice exam out
• Disk Caching
  • disks are slow, we want to use caching to speed up
  • caches may be in main memory, disk controller, disk drive
  • many Unix systems use: buffer cache
  • this introduces potential inconsistencies, need consistency checkers like fsck, scandisk
• RAID - a better description is on Wikipedia
• Mostly skipping: Hierarchical Storage
  • basic idea: use tapes or other large/cheap/removeable media as "swap space"
  • just another level of the memory hierarchy, really
• Protection
  • A main function of the OS: ensure that each object (hardware or software) is accessed correctly and only by those allowed access
    • memory protection
    • file protection
    • CPU protection
  • Protection domains: a set of access rights to objects
  • Processes execute in a domain, rights determined by this domain
  • Unix: domain is defined by UID and GID
    • process can change UID/GID
    • superuser (UID=0) can become any other UID/GID
    • regular users must run setuid/setgid programs
  • Other protection mechanisms are described in SG&G and we will not discuss in any detail in class
    • protection matrix
    • access control lists
    • capabilities

Due at the start of class, Monday, April 26.

Please submit answers to these questions either as a hard copy (typeset or handwritten are OK) or by email to jteresco AT mtholyoke.edu by the start of class. We will discuss these questions at the start of class, so no late submissions are accepted.

Submit a one-page summary of the important ideas behind the Unix Buffer Cache, based on the supplemental readings from the handout from Bach (1986).

• setuid