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

2. 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

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

4. 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

5. 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

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

8. 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

9. 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

10. 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

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
© 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
© 2004, D. J. Foreman

13. 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