5.4 Interrupts and Exceptions

5.4 Interrupts and Exceptions

1. Interrupts and exceptions are operating system events that divert the processor to code outside the normal flow of execution

   • Exceptions = Synchronous. Which means they only occur at specific times in response to a specific event or action
     • Generally regarded as error conditions
       • Can be reproduced
         • Generated by hardware and software
   • Interrupts are asynchronous and can occur anytime
     • Typically generated by Input / Output devices
       • Interrupts are TRAPPED by the OS
         • Cause the state of the running process to be saved and a new process invoked

2. SVC - Supervisor Call Interrupt

   • Generated when a process running in user mode requests and OS service that runs in supervisor mode
     • Once this happens a new process is started and runs in supervisor mode and has unlimited access to system resources
       • Important to have a way of restricting the access of this process because the user mode process could get access to services via the supervisor process it would not normally be able to access
         • Controlling this is done via gates and the Requester Privilege Field in the segment selector

3. IVT Interrupt Vector Table

   • Resides in main memory
   • Contains interrupt ID and Pointers to the interrupt handler
     • The Interrupt or Exception handler is the program that is excecuted in response to each I/E
       • The handler is invoked by the OS as a new process
   • IVT contains info that dictate the behaviour of a computer which makes it an ideal target for attack
     • If you change entries in the IVT you can get the OS to execute malicious code

4. Boot Sector Viruses

   • Bootstrap loaded loads OS from disk into main memory at startup
     • Bootstrap loader fits into a number of chained memory sectors
       • Boot Sector viruses change the links in the chain causing viruses to execute as well as the loader
         • See brain virus pg 18