Microkernel
- 1. Seminar on MICROKERNEL BY TUSHAR.N.TEKADE (ME IT 9114) SCTR’s PUNE INSTITUTE OF COMPUTER TECHNOLOGY, PUNE
- 2. Computer has 2 modes of operation……. • Kernel Mode • User Mode • The OS is the most fundamental piece of software & runs in kernel mode(supervisor mode). • In this mode, it has complete access to all the h/w and execute any instruction the machine is capable of. • Everything running in Kernel Mode is clearly a part of OS ,but some program running outside it are arguably also part of it or closely associated with it.
- 3. • Distinction between OS mode and user mode is that if a user does not like a particular e-mail reader , he is free to get different one or write his own. • Whereas he is not free to write his own clock interrupt handler, a part of OS & protected by h/w against attempts by user to modify it. • There are however programs which run in user mode but help the OS or perform privileged functions. Ex. Changing Password program- not part of OS and not runs in kernel mode.
- 4. SYSTEM STRUCTURE • Design and Implementation of OS not “solvable”, but some approaches have proven successful • Internal structure of different Operating Systems can vary widely • Start by defining goals and specifications • 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
- 5. • Approach is to partition the task into small components rather than a monolithic structure. • Each of these modules should be well-defined portions of the system , with proper inputs, outputs and functions. • All the modules (process , main memory, file, i/o system, secondary storage, networking , command-interpreter management ) are interconnected and melded into a kernel.
- 6. SIMPLE STRUCTURE • MS- DOS an example of such system, originally designed and implemented to provide most functionality in least space(limited h/w). • Not divided in modules • It has some structure but its interfaces and functionalities not well separated.
- 8. UNIX • OS initially limited by h/w functionality. • 2 separable parts - System Programs - 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
- 10. MONOLITHIC STRUCTURE • In this approach the entire operating system runs as a single program in kernel mode. • The operating system is written as a collection of procedures, linked together into a single large executable binary program. • An unwieldy and difficult to understand system.
- 12. The main disadvantages of monolithic structures are: The dependencies between system components - a bug in a device driver might crash the entire system. Large kernels can become very difficult to maintain.
- 13. LAYERED APPROACH • It’s a way of Modularization of system, in which OS is broken into number of layers or levels. • Each built on top of lower layers, bottom layer(layer 0) is the h/w & the highest layer (layer N) is user interface. • OS layer is an implementation of an abstract object that is encapsulation of data & of the operations that manipulate that data. • It contains data structures and routines that can be invoked by higher level layers & it can invoke operations on lower layer.
- 15. Advantages: Modularity Simplifies debugging and system verification. Ease in error solving. Each layer hides the existence of certain data structures, operations and h/w from higher layers. Difficulties: Careful definition of each layer. They tend to be less efficient than other type(due to overhead added by each layer to system call). THE system built at the Technische Hogeschool Eindhoven in the Netherlands by E. W. Dijkstra (1968) and his students
- 16. MICROKERNELS • With layered approach the designers have a choice where to draw the kernel-user boundary. • Traditionally all the layers were in the kernel. • In fact, a strong base can be made for putting as little as possible in kernel mode because bugs in kernel can bring down the system instantly. • In contrast user processes can be set up to have less power so that a bug there may not be fatal.
- 17. • The basic idea behind the microkernel design is to achieve high reliability by splitting the OS up into smaller but well defined modules , only one mode THE MICROKERNEL runs in the kernel mode& the rest run as relatively less power ordinary user processes. • Another function is to provide communication facility between client program and various services running. BENEFITS: Ease of extending the OS All new services added to user space, thus no need to modify kernel , and when needed changes are few because microkernel is smaller kernel.
- 18. BENEFITS: OS is more easier to port from one h/w design to another. More security & reliability since most services are running as user rather than kernel processes. If a service fails the rest of OS remains untouched, which is in contrast with monolithic system.
- 19. • Tru64 UNIX(formerly Digital UNIX) provides a UNIX interface to the user , uses microkernel , implemented with Mach kernel.(which maps UNIX system calls into messages to appropriate user-level servies.) • The Apple MacOS Server OS is also based on Mach Kernel. • QNX is RTOS based on Microkernel design, providing services for message passing and process scheduling.
- 20. MINIX 3 • MINIX 3 is a POSIX conformant, open source microkernel system freely available . • It is only about a 3200 lines of C & 800 lines of assembler for very low level functions such as catching interrupts & switching process. • The C code manages & schedules processes, handles inter process communication & offers a set of about 35 kernel calls to allow the rest of OS to do its work. • These calls does: looking handlers for interrupts, moving data between address spaces , installing new memory maps for newly created processes.
- 22. • Lowest layer in user mode: Device Drivers Do not have physical access to i/o port space & cannot issue i/o command directly. • Above lowest layer : Server All servers like file server which manages file system i.e. create , destroy and manages process. One interesting server is “reincarnation server” which checks if other server & drivers are working properly and if found faulty corrects it…so a self-healing system. • User Program obtain OS services by sending short messages to the servers asking for the POSIX system calls • Each layer has exactly the power do its work and nothing more , limiting the damage a baggy component can do.
- 24. • Idea of Minimal Kernel is to put the mechanism for doing something in the kernel but not the policy. • Ex. Scheduling Problem Mechanism: look for highest priority process & run it. Policy: assigning priorities to process , can be done by user mode process. Thus policy and mechanism can be decoupled and kernel can be made smaller. “Microkernel (also known as μ-kernel or Samuel kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS)”