Concept - Access Control Function

An access attempt triggers (s.18):

1. A subject $S_0$ issues a request of type $\alpha$ for object $X$.
2. The system (OS or access control interface module) generates a message $(S_0,\alpha ,X)$ to the controller for $X$.
3. The controller checks the access matrix $A$ to see if $\alpha \in A[S_0,X]$. If yes, access is allowed; if not, access is denied and a protection violation occurs.

Figure (s.19): requests such as $(S_i,\text{read},F)$ and $(S_j,\text{wakeup},P)$ go to mechanisms for the file system, memory addressing hardware, process manager, terminal and device manager and instruction decoding hardware; grant/delete requests go to the access matrix monitor, which reads and writes the access matrix.

(Added context) Quiz link: “every access request should be checked against an access control system” is the complete mediation principle from your Chapter 1 notes (Quiz A part (m)); this function is that idea in action.

Related: Concept - Access Matrix, Concept - Access Control. Appears in 04 Access Control - Chunk 01 (s.1-28).