Access metrics mechanism.

Lists of Questions Answers and Short Study Notes

  • (1) Explain Access metrics mechanism.


Question -1  Explain Access metrics mechanism.

Model of protection can be viewed abstractly as a matrix called an access matrix.

  • Row of access matrix represents domains and a column represents objects.
  • Each entry in the matrix consists of set of access rights.
  • The entry access (i, j) defines set of operations that a process executing in domain Di, can invite an object Oj.
  • The access matrix scheme provides us with the mechanism for specifying a variety of policies.
  • We must ensure that process executing in domain D can access only those objects specified in row P as allows by the access matrix.
  • Access matrix also provides the mechanism of defining and implementing control for both static and dynamic association between process and domains.
  • We are executing an operation on an object.
  • We can control these changes by including the access matrix as an object.
  • Operations on domains and the access matrix are not in themselves important.
  • Important is that they illustrate the ability of the access matrix model to allow the implementation and control of dynamic protection requirements.
  • New objects and new domains can be created dynamically and included in access matrix model.
  • We need to consider only the operations that are possible on these new objects (domains and access matrix) and decide how we want processes to be able to execute these operations.
  • Allowing controlled change to the contents of the access matrix entries requires three additional operations: copy, owner and control.
  • The ability to copy an access right from one domain (row) of the access matrix to another is denoted by asterisk (*) appended to the access right.



  • For example as shown in figure (A) a process executing in domain D2 can copy the read operation into any entry associated with file F2.
  • Hence the access matrix of figure (A) can be modified into figure (B).
  • We also need mechanism to allow addition of owner rights and removal of some rights.
  • The ownership right controls these operations.


  • For example figure (C) domain D1 is the owner of F1 and thus can add and delete any valid right in column F1.
  • Similarly Domain D2 is the owner of F2 and F3 and thus can add and remove any valid right within these two columns.
  • Thus the access matrix figure (C) can be modified to the access matrix shown in figure (D).
  • The copy and owner rights allow a process to change the entries in a column. A mechanism is needed to change the entries in row.
  • If access (i, j) includes the control right then a process executing in domain Di can remove any access right from row j.
  • These operations on the domains and the access matrix are not in themselves particularly model to allow implementation and control of dynamic protection requirements.