1. © 2004, D. J. Foreman 1 O/S Organization

2. © 2004, D. J. Foreman 2 Topics  Basic functions of an OS ■ Dev mgmt ■ Process & resource mgmt ■ Memory mgmt ■ File mgmt ■ Functional organization  General implementation methodologies ■ Performance ■ Trusted software  UNIX & Windows NT organization

3. © 2004, D. J. Foreman 3 Design Constraints  Performance  Security  Correctness  Maintainability  Cost and marketability  Standards  Usability

4. © 2004, D. J. Foreman 4 Performance  O/S adds overhead (reduces available CPU)  Added functionality changes performance of the system, BUT ■ Reduces overall load on system by concentrating services ■ Makes life easier for implementers ■ Allows for concurrency to leverage overlapping I/O with CPU

5. © 2004, D. J. Foreman 5 Security  Multiprogramming & sharing lead to breakdown of security & protection  O/S must manage access  Security policy: Strategy chosen by computer’s owner  Security mechanism: Tools to implement a family of security policies

6. © 2004, D. J. Foreman 6 Correctness & Maintainability  Correct operation->greater security  S/W must be upgradeable  Importance of either may lead to reduction of services

7. © 2004, D. J. Foreman 7 Device Mgmt-1  The O/S owns all devices  O/S provides access via the abstractions  At least 2 API's exist for this in most languages ■ High level access Get/Put – blocks/streams of data Insertion & Extraction (" >") ■ Low level access Read/Write/Seek – accesses individual bytes/records

8. © 2004, D. J. Foreman 8 Device Mgmt-2  Two areas of device mgmt: ■ Physical device independence ■ Physical device dependence  In modern systems, application NEVER accesses the device directly ■ Changing a pixel on a display ■ Reading specific sectors ■ Accessing a directory block ■ Turning on/off the "numlock" light

9. © 2004, D. J. Foreman 9 Device Mgmt-3  O/S code accessed via the API perform these operations for the application  Provides integrity  Provides security  Provides commonality ■ Consistent program action ■ Consistent error handling

10. © 2004, D. J. Foreman 10 Modes of Execution  Processor modes ■ Supervisor or Kernel mode ■ User mode  Supervisor or Kernel mode Can execute all machine instructions Can reference all memory locations  User mode Can only execute a subset of instructions Can only reference a subset of memory locations

11. © 2004, D. J. Foreman 11 Modes-2  Mode determined by ONE bit  Special instructions set mode-bit to Kernel mode  When ON, all instructions are valid  When OFF, 'privileged' instructions cause a trap to the Kernel  Kernel code can change the bit and return control to user code – method depends on instruction set

12. © 2004, D. J. Foreman 12 Modes-3  Mechanisms for getting into Kernel space ■ Call to function that issues a "trap" or "supervisor call" instruction ■ "Send" message to the Kernel Effectively issues a "trap" ■ Interrupts H/W sets mode bit to 1 Next inst is in kernel at interrupt handler code – No "call" or "send" required