Operating System Organization

1. Operating Systems: A Modern Perspective, Chapter 3

2. Operating SystemOrganization 3 Operating Systems: A Modern Perspective, Chapter 3

3. Goals • Learn how an operating system is organized • Understand the responsibilities of an OS • efficiency • security • correctness • Learn about the parts of an OS (the managers) • device manager • memory manager • process manager • file manager • Learn about OS design approaches • microkernels • Understand some themes of OS implementation • modes • system calls/traps • Examine the organization of some modern OSes Operating Systems: A Modern Perspective, Chapter 3

4. Purpose of an OS Coordinate Use of the Abstractions Processes The Abstractions Create the Abstractions Operating Systems: A Modern Perspective, Chapter 3

5. OS Requirements 1. Provide resource abstractions • Process abstraction of CPU/memory use • Address space • Thread abstraction of CPU within address space • Resource abstraction • “Anything a process can request that can block the process if it is unavailable” • NT uses “object abstraction” to reference resources • File abstraction of secondary storage use Operating Systems: A Modern Perspective, Chapter 3

6. DOS -- Resource Abstraction Only Program Program Libraries Program OS Services ROM Routines Processor(s) Main Memory Devices Operating Systems: A Modern Perspective, Chapter 3

7. OS Requirements (cont) 1. Provide resource abstractions 2. Manage resource sharing • Time/space-multiplexing • Exclusive use of a resource • Isolation • Managed sharing Operating Systems: A Modern Perspective, Chapter 3

8. Process Process Process Program Program Program State State State Abstraction & Sharing Libraries • OS Services • Abstraction • Manage sharing ROM Routines Processor(s) Main Memory Devices Operating Systems: A Modern Perspective, Chapter 3

9. OS Design Constraints • Performance • Protection and security • Correctness • Maintainability • Commercial factors • Standards and open systems Operating Systems: A Modern Perspective, Chapter 3

10. Performance • The OS is an overhead function should not use too much of machine’s resources • does not provide a solution • rather provides an environment in which a solution is implemented. • Minimum functionality is to implement abstractions • Additional function must be traded off against performance • DOS: one process • UNIX: low level file system, not graphical environment Operating Systems: A Modern Perspective, Chapter 3

11. Protection & Security • Multiprogramming resource sharing • Processes need exclusive control over resources • OS must ensure processes do not interfere with other processes • OS must allow specific sharing of resources • Security policy: Sharing strategy chosen by computer’s owner • Protection mechanism: Tool to implement a family of security policies • OS enforces a policy • processes must be prevented from changing the policy Operating Systems: A Modern Perspective, Chapter 3

12. Correctness & Maintainability • Security depends on correct operation of software trusted vs untrusted software • kernel code must be trusted • application programs, system software, etc. are untrusted • must have barriers between trusted software (kernel) and untrusted software (user programs) Operating Systems: A Modern Perspective, Chapter 3

13. Correctness & Maintainability • Maintainability relates to ability of software to be changed • If either is sufficiently important, can limit the function of the OS • Guiding a manned spaceship • Managing a nuclear reactor Operating Systems: A Modern Perspective, Chapter 3

14. Traditional OS Functions • To provide abstractions and manage those abstractions, split OS into 4 areas: • Device management • process, thread, and resource management • Memory management • File management Operating Systems: A Modern Perspective, Chapter 3

15. Device Management • Device Management is the way that an OS handles generic devices • disks, tapes, terminals, printers treated in same general manner • Processor and RAM handled separately Operating Systems: A Modern Perspective, Chapter 3

16. Device Management • Two parts to a device manager • device-dependent part (device driver) • implement unique aspects of device • e.g., device driver for keyboard must sense keystrokes • independent part • defines a general software environment in which the device driver operates • includes system call interface • includes mechanism to forward calls to correct driver • independent part is small part of device manager Operating Systems: A Modern Perspective, Chapter 3

17. Device Management • Two parts to a device manager • makes it easier to add a device to a computer • independent part can work with wide range of devices • does not have to be rewritten for each device • e.g., one independent part for all postscript printers Operating Systems: A Modern Perspective, Chapter 3

18. Device Management Device-Independent Part Device-Dependent Part Device-Dependent Part Device-Dependent Part … Device Device Device Operating Systems: A Modern Perspective, Chapter 3

19. Process, Thread, and Resource Management • Process and Thread are basic units of computation • Resources are elements of the computing environment needed by a process so that its threads can execute. • The process manager implements the abstract machine: • the hardware processor resource • the thread resource • the memory resource (shared with memory manager) • physical resources • abstract resources such as messages Operating Systems: A Modern Perspective, Chapter 3

20. Process, Thread, and Resource Management • Process, Thread, and Resource Manager (called the “process manager”) responsibilities: • Enforce isolation of resource access among processes • Allows sharing of resources among processes Operating Systems: A Modern Perspective, Chapter 3

21. Process, Thread, and Resource Management Thread Abstraction Process Abstraction Generic Resource Manager Multiprogramming Other … Primary Memory Abstract Resources Processor Operating Systems: A Modern Perspective, Chapter 3

22. Memory Management • Memory manager cooperates with the process manager to administer the allocation and use of primary memory • Allocates memory to competing processes according to specified policy • enforces resource isolation (memory protection) • allows sharing of memory • implements virtual memory (RAM + disk) • modern OS allow threads on one machine to access memory on another machine over a network. Operating Systems: A Modern Perspective, Chapter 3

23. Memory Management Isolation & Sharing Virtual Memory Block Allocation Process Manager Primary Memory Storage Devices Operating Systems: A Modern Perspective, Chapter 3

24. File Management • File manager implements the file abstraction on persistent storage devices like disks • different file abstractions provided • byte streams, records, etc. • modern OS allows file system to be distributed a network of machines • process can read/write to files stored on local system or other machines on the network Operating Systems: A Modern Perspective, Chapter 3

25. OS implementation mechanisms • Processor modes. • Mode bit to distinguish between user and system • Kernels. • critical part of the OS. • designed as trusted software module. • Method of invoking system service. • how user processes request services from OS • System calls or message passing to a system process Operating Systems: A Modern Perspective, Chapter 3

26. Processor Modes • Mode bit: Supervisor or User mode • Supervisor mode • Can execute all machine instructions • Can reference all memory locations • trusted code (kernel) operates in this mode • instructions in this mode called supervisor, privileged, or protected instructions • examples: I/O • user program cannot perform • must call kernel to perform I/O Operating Systems: A Modern Perspective, Chapter 3

27. Processor Modes • Mode bit: Supervisor or User mode • User mode • Can only execute a subset of instructions • Can only reference a subset of memory locations Operating Systems: A Modern Perspective, Chapter 3

28. Mode Bit Example • Resource access only done via supervisor mode • processor uses a hardware register to identify an object that is only accessible to the running process. • When process A uses the processor, the register points to A’s object. • When process B uses the processor, register points to B’s object. • Register can only be changed with a privileged instruction. Operating Systems: A Modern Perspective, Chapter 3

29. Exclusive Access to a Resource Processor Process A Process B A’s Protected Object Supervisor Program Operating Systems: A Modern Perspective, Chapter 3

30. Processor Modes • use mode bit to define areas of memory used when processor is in supervisor mode • when mode bit is in supervisor mode (kernel software executing) can access all memory • when mode bit is in user mode (application software executing) can access only user memory space Operating Systems: A Modern Perspective, Chapter 3

31. Supervisor and User Memory User Space User Process Supervisor Process Supervisor Space Operating Systems: A Modern Perspective, Chapter 3

32. Kernels • The part of the OS critical to correct operation (trusted software) • Executes in supervisor mode • The trap instruction is used to switch from user to supervisor mode, entering the OS Operating Systems: A Modern Perspective, Chapter 3

33. trap instruction • Must ensure that when processor is in supervisor mode will only execute OS code • Mode bit is set by the user mode hardware instruction (called a trap) • changes mode bit to supervisor mode • simultaneously jumps to OS code • similar to a hardware interrupt • Trap calls are expensive Operating Systems: A Modern Perspective, Chapter 3

34. trap instruction • Two techniques of requesting kernel service • System call • Message passing Operating Systems: A Modern Perspective, Chapter 3

35. trap instruction • System Call • Use trap instruction • parameter indicates the service • actually a reference into the trap table • OS provides a library of “stub’ functions with names that are same as the system call Operating Systems: A Modern Perspective, Chapter 3

36. send(…, A, …); receive(…, B, …); send/receive receive(…A, …); … send(…, B, …); Procedure Call and Message Passing Operating Systems call(…); trap return; Operating Systems: A Modern Perspective, Chapter 3

37. System Call example • fork( ) system call has a stub function named fork( ) in the library • has the correct number and type of parameters • Each stub function contains a trap instruction to actual OS function • trap switches to supervisor mode • then branches via a jump table (called the trap table) to OS code • return from trap restores user mode Operating Systems: A Modern Perspective, Chapter 3

38. System Call Using the trap Instruction … fork(); … Trap Table Kernel fork() { … trap N_SYS_FORK() … } sys_fork() sys_fork() { /* system function */ … return; } Operating Systems: A Modern Perspective, Chapter 3

39. Message Passing • User process constructs a message A that requests a service • Uses an OS send( ) system call to pass message to kernel • send( ) function delivers message to a kernel process that implements the service • kernel process must be started or resumed before can receive message • start/resume sets mode to supervisor • send( ) effectively executes a trap • When kernel is finished, returns result • user process waits for result with a message receive( ) Operating Systems: A Modern Perspective, Chapter 3

40. send(…, A, …); receive(…, B, …); send/receive receive(…A, …); … send(…, B, …); Procedure Call and Message Passing Operating Systems call(…); trap return; Operating Systems: A Modern Perspective, Chapter 3

41. Comparison • System call interface more efficient • message requires implicit trap, message formation, message copying • system call just requires trap Operating Systems: A Modern Perspective, Chapter 3

42. OS process • In system call model there is not a specific OS process • user process switches to supervisor mode then back • never create an explicit OS process • Exceptions: daemons • examples: printer daemon, network daemon • operate in user mode • exist to implement OS-like functionality Operating Systems: A Modern Perspective, Chapter 3

43. A Thread Performing a System Call User Space Kernel Space Thread fork(); sys_fork() { } Operating Systems: A Modern Perspective, Chapter 3

44. OS organization • Four functions of an OS • Process, Thread, Resource Manager • Memory Manager • File Manager • Device Manager • Each maintains data structures • other managers must read and write these DS Operating Systems: A Modern Perspective, Chapter 3

45. File Manager Memory Manager Device Manager Basic Operating System Organization Process, Thread & Resource Manager Processor(s) Main Memory Devices Operating Systems: A Modern Perspective, Chapter 3

46. OS organization • Examples: • Memory manager controls RAM • For VM, MM must coordinate with PM when it schedules processor (when to load into memory) • Must also coordinate with FM (to store on disk) • File manager controls disks • More efficient to buffer, I.e., FM reads from a storage device before it is requested • must coordinate with MM Operating Systems: A Modern Perspective, Chapter 3

47. OS organization • Examples: • File manager controls disks • Must coordinate with DM to write to disk • Device manager • interrupts require coordination with PM Operating Systems: A Modern Perspective, Chapter 3

48. Design Criteria • Software engineering suggests that each module should be separate • Efficiency suggests that there be no modules • UNIX: the 4 modules are combined into a single monolithic software module (the kernel) • functions in one module can directly call functions in another module • problem: changing a line in the PM can break code in the MM, etc. Operating Systems: A Modern Perspective, Chapter 3

49. Design Criteria • Microkernel approach. • only implement necessary code in a microkernel • thread scheduling, hardware device management, fundamental protection mechanisms • rest of OS implemented as user function • remainder of process, memory, file, and device management functions Operating Systems: A Modern Perspective, Chapter 3

50. Design Criteria • Microkernel approach. • problem: user space OS functions have to make may system calls • system calls are very inefficient • can microkernels be made fast enough so that extra performance cost is insignificant? • answer is still being researched Operating Systems: A Modern Perspective, Chapter 3