Operating Systems Part of E1.9 - Principles of Computers and Software Engineering. Lecture 2: Operating system structures

Transcription

1 Operating Systems Part of E1.9 - Principles of Computers and Software Engineering Lecture 2: Operating system structures

2 Objectives To answer the following questions: What services does an OS provide? What are the major components of an OS system? How do programs use these components? How are these components organised? To introduce the concepts of: System calls Process Microkernel Privileged modes of operation Hardware support for protection capabilities E1.9 - Operating systems (Lect 2) 2

3 Services OS provides environment for the execution of programs; it provides certain services to programs and to users of these programs Program execution; protection I/O operations File system manipulation Communications Error detection Resource allocation; accounting We will now look into these in more detail: E1.9 - Operating systems (Lect 2) 3

4 E1.9 - Operating systems (Lect 2) 4

5 Services (1) Program execution & protection Load program into memory What if there isn t enough memory? Run program Provide resources requested Protect from other running programs End program execution, normally or abnormally (indicating error) Provision for initiation of programs by users. E1.9 - Operating systems (Lect 2) 5

6 Services (2) I/O operations Users cannot access I/O devices directly (for efficiency and protection reasons) OS must provide an interface to I/O devices: Ability to issue commands to the devices (e.g. read/write) Ability to handle interrupts Ability to handle errors To the extend possible, same interface for all devices (device independence) I/O scheduling Serving application requests for I/O in the order received not always the best choice. Buffering, caching, spooling E1.9 - Operating systems (Lect 2) 6

7 Services (3) File system manipulation Create/delete files Read/Write to files Hierarchical organisation (directories). Support for mounting file systems (e.g removable hard disks, CDROM) E1.9 - Operating systems (Lect 2) 7

8 Services (4) Communications Exchange information between processes Processes can be local or in remote machines Implementation of communications facilities: Message Passing vs Shared Memory Communication through shared memory Process 1 Shared memory Process 1 Communication through Message passing Process 2 Process 2 Operating System Operating System E1.9 - Operating systems (Lect 2) 8

9 Services (5) Error detection OS needs to be aware of errors CPU errors (e.g. illegal operations) Memory errors (e.g. attempt to access protected memory locations) I/O errors (e.g connection failure) OS must take appropriate action to avoid catastrophic failure Error handlers Termination of erroneous programs E1.9 - Operating systems (Lect 2) 9

10 Services (6) Resource allocation & accounting Resource allocation: Of particular interest in the multiuser & multitasking cases Scheduling issues: optimization of resource use, fairness Accounting: CPU time (e.g. in supercomputing facilities), storage quotas E1.9 - Operating systems (Lect 2) 10

11 OS Components To provide these services, OS have the following components: Process management Memory management File management Communication & Networking management Command interpreter system E1.9 - Operating systems (Lect 2) 11

12 OS Components (1) Process management (1) Process: A program in execution A process consists of: Instructions (the program) Not modified during execution Data - modified explicitly by the running process. Stack & context modified implicitly by the process Process attributes modified by the OS E1.9 - Operating systems (Lect 2) 12

13 OS Components (1) Process management (2) Process management component deals with the following activities: Creating and deleting user and system processes Suspending and resuming processes Providing process synchronization mechanisms Providing process communication mechanisms Providing mechanisms for deadlock prevention and detection E1.9 - Operating systems (Lect 2) 13

14 OS Components (1) Process management (3) E1.9 - Operating systems (Lect 2) 14

15 OS Components (2) Memory management (1) Memory management component deals with the following activities: Allocating and deallocating memory space as needed Logical- vs physical-address space Keeping track of which parts of memory are currently used and by which process Deciding which processes are to be loaded into memory when memory space becomes available Note: this is not the same as executing the processes E1.9 - Operating systems (Lect 2) 15

16 OS Components (2) Memory management (2) What if process needs more memory than what is available? In early computers, such process is out of luck Modern OS provide support for virtual memory Operating system keeps part of the address space in memory Rest is kept in secondary storage (e.g. hard disk) Pieces are shuffled back and forth between them as needed NB! Choice of memory management algorithms depends heavily on the availability of hardware support MMU (memory management unit) Base (or relocation) and limit registers E1.9 - Operating systems (Lect 2) 16

17 OS Components (3) File management One of the most visible components of the OS Information can be stored in a variety of physical media (e.g hard disks, tapes, CDRWs) Must provide uniform logical view of information storage: abstracts from the physical properties of its storages devices to define a logical storage unit, the file. Deals with the following activities: Creating and deleting files and directories Supporting primitives for manipulating files and directories Mapping of files onto secondary storage E1.9 - Operating systems (Lect 2) 17

18 OS Components (3) File management Example disk drive [Silberschatz et al 2001] E1.9 - Operating systems (Lect 2) 18

19 OS Components (4) Communication and Networking management Handles requests for information transfer and synchronization between processes In distributed systems, processes might reside in remote systems Provides access to remote machines: remote login (telnet, ssh), remote file transfer (ftp) WWW s innovation: HTTP (hypertext transfer protocol) did not require user to log in before she is allowed to use a resource of that machine simply ask the info from the server and the information (text, graphics, links) is sent back. Should your WWW browser be considered part of your OS? (1998 USA DoJ vs Microsoft) E1.9 - Operating systems (Lect 2) 19

20 OS Components (5) Command Interpreter system The interface between the OS and the user A special program that reads user s next command statement and executes it Known as the command-line interpreter or shell. OS frequently differentiated in the area of the shell: User-friendly command interpreters through the use of graphical user interfaces (GUI) e.g Windows, MacOS Keyboard-based shells (MS-DOS, Unix shells) Command statements can deal with process creation (e.g. run a new program), file system management (copy or delete files), and other OS services. E1.9 - Operating systems (Lect 2) 20

21 System calls (1) OS provides an interface for programs to access these components through system calls Generally available as assembly-language instructions Certain systems allow system calls to be made directly from a higher-level language. Examples: Microsoft s Win32 API (Application Program Interface) POSIX standard E1.9 - Operating systems (Lect 2) 21

22 System calls (2) Examples of Systems calls in Unix and Win32 [Tanenbaum, 2001] E1.9 - Operating systems (Lect 2) 22

23 System calls (2) System calls interface assists protection Distinction between two separate modes of operation in the computer: user mode and privileged mode (also known as kernel mode, supervisor mode, system mode) User programs run in user mode and request services from the OS Service calls are handled by the OS which runs in privileged mode Distinction between the two relies on hardware support In more detail: E1.9 - Operating systems (Lect 2) 23

24 Hardware support for OS(1) Dual mode operation (1) Hardware provides a mode bit to indicate current mode of operation Certain machine instructions can be executed only in privileged mode (designated as privileged instructions, e.g all low level I/O instructions) Switching between the two: At boot time, hardware starts in privileged mode OS is loaded and runs in privileged mode OS start user processes they run in user mode When an (software or hardware) interrupt occurs, mode bit is switched to privileged mode and control is returned to the OS E1.9 - Operating systems (Lect 2) 24

25 Hardware support for OS(1) Dual mode operation (2) If an attempt is made to execute a privileged instruction in user mode, hardware does not execute the instruction but traps it to the OS If user wants to complete a task requiring privileged instructions, user program must use a system call. Example to illustrate all the different stages: User program wants to open a file E1.9 - Operating systems (Lect 2) 25

26 Example illustrating use of system calls for privileged operations (1) Pascal code:.. Var.. Begin datafile: TextFile; Assignfile(datafile, data.txt ); Reset(datafile); End; ARM Assembler:.. SWI_Open EQU 0x66 Readonly EQU 0.. ENTRY Start ADR r0, Datafile MOV r1, #Readonly SWI SWI_Open.. Datafile = data.txt, 0 END E1.9 - Operating systems (Lect 2) 26

27 Example (2) SWI SWI_Open: Program counter and other registers are saved Mode bit switched to privileged. Trap to the OS system calls handling routine, with code 0x66 (or use an interrupt vector ) Handling routine gets the code and branches to relevant OS routine (open-file) E1.9 - Operating systems (Lect 2) 27

28 Example (3) SWI SWI_OpenFile [continued]: OS open file routine retrieves information from stack (e.g. r0 and r1 registers) and checks validity of arguments. Executes privileged instructions for handling the opening of a file File handle or error code returned to a register. System call complete interrupt serviced Mode bit switched back to user mode Context read back, PC incremented (Execution of user s program resumed). E1.9 - Operating systems (Lect 2) 28

29 Hardware support for OS(2) Memory protection Need the ability to determine the range of legal memory addresses for a process. An attempt to access outside a process range generates error and traps to the OS Provision of registers: Base register: smaller legal physical memory address Limit register: size of the range Protection accomplished by the hardware comparing every address generated in user mode with the registers Prevents users from (accidentally or deliberately) modifying the code or data structures of OS or other users. Instructions altering these registers are privileged. Memory management revisited in later lectures E1.9 - Operating systems (Lect 2) 29

30 Hardware support for OS(3) Timers What if user program gets stuck (e.g. infinite loop) and never returns control to the OS? OS never gets the chance to regain control (to switch to another process for example) Use of timers; interrupt after a specified (fixed or variable) period OS regains control regularly Instructions that modify the operation of the timers are privileged. Timers are used to implement time sharing. E1.9 - Operating systems (Lect 2) 30

31 OS organisation (1) Monolithic system No structure Any part can call any other part of the OS (and often does) Layered systems Lower levels have the most basic of functionality Each layer constructed using only functionality of level below. Advantages: modularity Disadvantages: reduced efficiency E1.9 - Operating systems (Lect 2) 31

32 OS organisation (2) Virtual machines (1) Implement a layer above the hardware level that provides several virtual machines. Each virtual machine exact copy of the bare hardware (incl. Kernel/user mode, I/O, interrupts, etc) OS organisation: (a) Non-virtual machine (b) Virtual machine [Silberschatz et al, 2001] E1.9 - Operating systems (Lect 2) 32

33 OS organisation (2) Virtual machines (2) IBM s VM operating system is prime example With VM organisation you can have several OS running on same machine! Java Virtual Machine Java class loader Class verifier Java interpreter Architecture-neutral E1.9 - Operating systems (Lect 2) 33

34 OS organisation (3) Client-server model (1) Move most OS functionality in user mode processes, leaving only a minimal microkernel Microkernel passes messages (requests) from a client-process (user program) to a server-process Separating policy decisions from mechanisms Client-server model [Tanenbaum, 2001] E1.9 - Operating systems (Lect 2) 34

35 OS organisation (3) Client-server model (2) Advantages: Ease of extending OS: new services added to user space; microkernel is not modified. More security and reliability if a service fails, the rest of the OS remains untouched. Easier to port to different architectures Can be easily adapted to distributed environments Little consensus on what should be included in the microkernel Typically: minimal process and memory management, and a communication facility. E1.9 - Operating systems (Lect 2) 35

36 Summary Operating systems: Provide services for programmers and users, through the use of system calls. Operating system is aided by hardware protection mechanisms Microkernel mechanisms include process and memory management, and offer several advantages over monolithic systems. Next lecture: Process management E1.9 - Operating systems (Lect 2) 36

37 Recommended Reading Read chapter 3 of Silberschatz s book Read chapter 1, sections 1.5-end of Tanenbaum s book Read chapter 2, sections 2.3 and 2.4 of Stalling s book E1.9 - Operating systems (Lect 2) 37