1. Operating Systems Course: Operating Systems Instructor: Umar Kalim
NUST Institute of Information Technology, Pakistan

2. Agenda
Revisiting Operating System Structures

3. Operating System Design and Implementation
Design and Implementation of OS not “solvable”, but some approaches have proven successful
Internal structure of different Operating Systems can vary widely
Affected by choice of hardware, type of system
User goals and System goals
User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast
System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

4. Operating System Design and Implementation (Cont.)
Important principle to separate
Policy: What will be done? Mechanism: How to do it?
Mechanisms determine how to do something, policies decide what will be done
The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later
Implementation
Using higher level languages
Can be easily recompiled for different platforms
Assembly language
Optimum implementation
Hybrid

5. Processor support for dual mode
Course: Operating Systems
Instructor: Umar Kalim
NUST Institute of Information Technology, Pakistan

6. Simple Structure
MS-DOS – written to provide the most functionality in the least space
Not divided into modules
Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

7. MS-DOS Layer Structure

8. Layered Approach
The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.
With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers
Current trend
Less layers, more functionality per layer

9. Layered Approach
Debugging is fairly easy as it can be implemented in layers.
Difficulty in careful definition of layers, because each layer can only use layers beneath it.
Another problem is that layered approach tends to be a little slow at times because each process has to go through subsequent layers and each layer might add extra calculations or traps.
Advantage is modularity!
But tend to be less efficient than other types
User program executes an IO operation, it executes a system call that is trapped to the IO layer
IO layer calls the memory management layer
Memory management layer calls the CPU scheduling layer
Then passed to hardware
Overhead at each layer !

10. Layered Operating System

11. UNIX
UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts
System programs [backup slides]
The kernel
Consists of everything below the system-call interface and above the physical hardware
Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

12. UNIX System Structure

13. Microkernel System Structure
Kernels grew in size with increased functionality in the OS.
This give birth to the idea of Micro Kernel
Remove unnecessary programs from the kernel into the user space.
Micro kernels typically provide minimal process and memory management, in addition to communication facility
Communication takes place between user modules using message passing.
Idea of having Clients and Servers within one system.
If a program wants to access a file it does not interact directly with kernel but kernel is just used to pass the messages indirectly.

14. Microkernel System Structure
Benefits:
Easier to extend a microkernel
Easier to port the operating system to new architectures
More reliable (less code is running in kernel mode)
More secure
Detriments:
Performance overhead of user space to kernel space communication
Example: Tru64 UNIX, QNX

15. Modules Most modern operating systems implement kernel modules
Uses object-oriented approach
Each core component is separate
Each talks to the others over known interfaces
Each is loadable as needed within the kernel
Overall, similar to layers but with more flexible

16. Solaris Modular Approach

17. Mac OS X Structure Hybrid structure Mach
Memory management, inter-process communication, scheduling
BSD
BSD command line interface, support for networking, and file systems etc
Applications can interact with Mach & BSD facilities directly

18. Windows NT Architecture (Simplified View)
In modern operating systems, applications are kept separate from the operating system itself.
The operating system code runs in a privileged processor mode known as kernel-mode and has access to system data and hardware.
Applications run in a non-privileged processor mode known as user mode and have limited access to system data and hardware through a set of tightly controlled application programming interfaces (APIs).

19. Windows NT (Case Study)
Windows NT is a multithreaded microkernel-based operating system.
Keeping the base operating system as small and as tight as possible was one of the primary design goals of Windows NT.
To do this, Microsoft kept in the base operating system only those functions that could not reasonably be performed elsewhere.
Functionality pushed out of the kernel was put in six non-privileged servers known as protected subsystems.
The protected subsystems provide the traditional operating system support to applications through a feature-rich set of APIs.

20. Windows NT (Case Study)
This design results in a very stable base operating system. Enhancements occur at the protected subsystem level.
New protected subsystems can be added without modifying either the base operating system or the other existing protected subsystems

21. Questions? Course: Operating Systems Instructor: Umar Kalim
NUST Institute of Information Technology, Pakistan

22. Virtual Machines
A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware
A virtual machine provides an interface identical to the underlying bare hardware
The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

23. Virtual Machines (Cont.)
The resources of the physical computer are shared to create the virtual machines
CPU scheduling can create the appearance that users have their own processor
Spooling and a file system can provide virtual card readers and virtual line printers
A normal user time-sharing terminal serves as the virtual machine operator’s console

24. Virtual Machines (Cont.)
(a) Non virtual machine (b) virtual machine
Non-virtual Machine
Virtual Machine

25. Virtual Machines (Cont.)
The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.
A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.
The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

26. VMware Architecture

27. The Java Virtual Machine

28. Operating System Generation
Operating systems are designed to run on any of a class of machines; the system must be configured for each specific computer site
SYSGEN program obtains information concerning the specific configuration of the hardware system
Booting – starting a computer by loading the kernel
Bootstrap program – code stored in ROM that is able to locate the kernel, load it into memory, and start its execution

29. System Boot
Operating system must be made available to hardware so hardware can start it
Small piece of code – bootstrap loader, locates the kernel, loads it into memory, and starts it
Sometimes two-step process where boot block at fixed location loads bootstrap loader
When power initialized on system, execution starts at a fixed memory location
Firmware used to hold initial boot code
Boot sequence

30. Questions? Recommended Reading: OSRC
M. F. Kaashoek, et. al.,
"Application Performance and Flexibility on Exokernel Systems “
G. Hamilton and P. Kougiouris,
" The Spring Nucleus: A Microkernel for Objects ,“
Questions?
Course: Operating Systems
Instructor: Umar Kalim
NUST Institute of Information Technology, Pakistan

31. System Programs
System programs provide a convenient environment for program development and execution. The can be divided into:
File manipulation
Status information
File modification
Programming language support
Program loading and execution
Communications
Application programs
Most users’ view of the operation system is defined by system programs, not the actual system calls

32. System Programs
Provide a convenient environment for program development and execution
Some of them are simply user interfaces to system calls; others are considerably more complex
File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories
Status information
Some ask the system for info - date, time, amount of available memory, disk space, number of users
Others provide detailed performance, logging, and debugging information
Typically, these programs format and print the output to the terminal or other output devices
Some systems implement a registry - used to store and retrieve configuration information

33. Boot Sequence
All computers running Windows XP Professional share the same startup sequence:
Power-on self test (POST) phase
Initial startup phase
Boot loader phase
Detect and configure hardware phase
Kernel loading phase
Logon phase

34. Power On Self Test (POST)
As soon as you turn on a computer, its central processing unit (CPU) begins to carry out the programming instructions contained in the basic input/output system (BIOS).
The BIOS, which is a type of firmware, contains the processor dependent code that starts the computer regardless of the operating system installed.
The first set of startup instructions is the power-on self test (POST). The POST is responsible for the following system and diagnostic functions:
Performs initial hardware checks, such as determining the amount of memory present.
Verifies that the devices needed to start an operating system, such as a hard disk, are present.
Retrieves system configuration settings from non-volatile complementary metal-oxide semiconductor (CMOS) memory, which is located on the motherboard.

35. booting process … Power on self-test (POST).
Find boot device and load boot record (handled by the system BIOS). The master boot record (MBR) reads the partition table, determines the location of the boot record, and then jumps to that sector and begins execution there.
Windows NT bootstrap routine finds and loads NTLDR.
NTLDR switches processor to 32-bit flat memory mode.
NTLDR reads the BOOT.INI and displays operating system choices.
User selects an operating system (Windows NT in this example).
NTLDR loads NTDETECT.COM.
NTDETECT.COM builds the hardware list. It then passes the list and control back to NTLDR.
NTLDR loads NTOSKRNL.EXE.
NTOSKRNL.EXE loads and initializes Windows NT using information from the Registry to load and initialize drivers and other system settings.