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1. 

   AYESHA MUMTAZ on February 8, 2019 at 5:07 pm

   Paper- 1 28-02-2016 10:30 AM
   Q.2.Describe the main roles of a conflict resolution module?
   Conflict Resolution
   A mechanism that allows different device drivers to reserve hardware resources and to protect those resources from accidental use by
   another driver.
   The conflict resolution module aims to:
   – Prevent modules from clashing over access to hardware resources
   – Prevent auto probes from interfering with existing device drivers
   – Resolve conflicts with multiple drivers trying to access the same hardware
   Q.3.Consider a paging system with a page table stored in memory,
   i) If a memory reference takes 200 nano seconds, how long does a paged reference takes (reference is retrieved from page
   table in memory and then program data is retrieved from memory) Assume all pages are in memory.
   Answer: 400 nanoseconds. 200 ns to access the page table plus 200 ns to access the word in memory.
   ii) If we add a TLB and 75% of all page table reference are found in TLB hit ratio is 75%,what effective memory reference
   time or average(Assume that finding a table entry in TLB takes 10 nanoseconds, if the entry is there)Assume all pages
   are in memory.
   Answer: 75% * TLB hit-time + 25% * TLB miss-time = 75% * 200ns + 25% * 400ns = 250ns
   Q4.consider the following I/O scenarios on a single user PC
   a). A disk drive containing user files.
   b). A graphic card with direct bus connection, accessible through memory mapped I/O.
   For each of these I/O scenarios would you design the OS to use buffering, Spooling, caching or a combination? Would you use
   parallel I/O or interrupt _driven I/O.
   Answer:
   a) A disk drive containing user files
   Buffering can be used to hold data while in transit from user space to he disk, and versa. Caching can be used to hold
   disk-resident data for improved performance.
   Spooling is not necessary because disks are shared-access dev
   Mandatory Access Control
   In mandatory access control (MAC), the system (and not the users) specifies which subjects can access specific data objects.
   The MAC model is based on security labels. Subjects are given a security clearance (secret, top secret, confidential, etc.),
   and data objects are given a security classification (secret, top secret, confidential, etc.). The clearance and classification
   data are stored in the security labels, which are bound to the specific subjects and objects.
   When the system is making an access control decision, it tries to match the clearance of the subject with the classification of
   the object. For example, if a user has a security clearance of secret, and he requests a data object with a security
   classification of top secret, then the user will be denied access because his clearance is lower than the classification of the
   object.
   The MAC model is usually used in environments where confidentiality is of utmost importance, such a military institution.
   Examples of the MAC-based commercial systems are SE Linux and Trusted Solaris.
   Q.1 Design principles of security
   Solution: • Isolation
    Separate processes execute in separate memory space
    Process can only manipulate allocated pages
   • Authentication
    Who can access the system. Involves proving identities to the system
   • Access control
    When can process create or access a file?
    Create or read/write to socket?
    Make a specific system call?
   • Protection problem
    Ensure that each object is accessed correctly and only by those processes that
   are allowed to do so
   • Comparison between different operating systems
    Compare protection models: which model supports least privilege most
   effectively?
    Which system best enforces its protection model?
   Q.2 disk scheduling methods
   FCFS (do nothing)
   o Reasonable when load is low
   o Long waiting times for long request queues
   SSTF (shortest seek time first)
   o Minimize arm movement (seek time), maximize request rate
   o Favors middle blocks
   SCAN (elevator)
   o Service requests in one direction until done, then reverse
   C-SCAN
   o Like SCAN, but only go in one direction (typewriter)