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

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

Design Constraints Performance Security Correctness Maintainability Cost and "sell-ability" Standards Usability © 2004, D. J. Foreman

Performance O/S adds overhead (reduces available CPU) Added functionality reduces performance of the O/S, BUT Reduces overall load on system by concentrating services Makes life easier for implementors © 2004, D. J. Foreman

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

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

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

Device Mgmt-2 Two levels 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 the structure of a directory Turning on/off the "numlock" light © 2004, D. J. Foreman

Device Mgmt-3 O/S programs accessed via the API perform these functions for the app Provides integrity Provides security Two ways to perform I/O Synchronous Asynchronous © 2004, D. J. Foreman

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

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 © 2004, D. J. Foreman

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 © 2004, D. J. Foreman

Modes-4 system call example fork (My_fork_loc); { ● ● trap (FORK, *My_fork_loc); } My_fork_loc:…; K_fork(loc) { ● ● start_process( loc); mode=0; return; } Trap table *K_fork Kernel space K_fork is entry # "FORK" © 2004, D. J. Foreman