Operating Systems Part M10 Introduction to OS, SW Interrupts, Supervisory Call. Florina Ciorba 16/23 October 2015

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1 Operating Systems Part M10 Introduction to OS, SW Interrupts, Supervisory Call Florina Ciorba 16/23 October 2015

2 Today s Overview Introduction to Operating Systems, earlier Operating Systems see 5 th Operating System Concepts (OSC) slides ( Dinosaur Book ) Additions OS Gallery, Multics, JAVA-OS, Plan9, Minix 3.0 Protected CPU commands, CPU modes, system calls see 5 th OSC slides ( Dinosaur Book ) , 2.19, 3.14 Software interrupts, exceptions, traps, and the supervisory calls (SVC) Operating Systems Concepts 3

(not necessarily small size) Representatives: MACH,

3 Monolithic Kernels vs Microkernels Microkernels: OS services treated as user processes Claim: Modularity (not necessarily small size) Representatives: MACH, CHORUS, Amoeba, Minix, QNX, and others Operating Systems Concepts 4

4 Early Operating Systems Operating Systems Concepts, 5 th Edition Slide set M10-b Slides Operating Systems Concepts 5

5 Module 1: Introduction What is an operating system? Simple Batch Systems Multiprogramming Batched Systems Time-Sharing Systems Personal-Computer Systems Parallel Systems Distributed Systems Real-Time Systems Operating System Concepts 1.1 Silberschatz and Galvin c 1998 What is an Operating System? A program that acts as an intermediary between a user of a computer and the computer hardware. Operating system goals: Execute user programs and make solving user problems easier. Make the computer system convenient to use. Use the computer hardware in an efficient manner. Operating System Concepts 1.2 Silberschatz and Galvin c 1998

6 Computer System Components 1. Hardware provides basic computing resources (CPU, memory, I/O devices). 2. Operating system controls and coordinates the use of the hardware among the various application programs for the various users. 3. Applications programs define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs). 4. Users (people, machines, other computers). Operating System Concepts 1.3 Silberschatz and Galvin c 1998 Abstract View of System Components user 1 user 2 user 3... user n compiler assembler text editor... database system system and application programs operating system computer hardware Operating System Concepts 1.4 Silberschatz and Galvin c 1998

7 Operating System Definitions Resource allocator manages and allocates resources. Control program controls the execution of user programs and operation of I/O devices. Kernel the one program running at all times (all else being application programs). Operating System Concepts 1.5 Silberschatz and Galvin c 1998 Early Systems Bare machine (early 1950s) Structure Large machines run from console Single user system Programmer/User as operator Paper tape or punched cards Early Software Assemblers, compilers Linkers, loaders Libraries of common subroutines Device drivers Secure Inefficient use of expensive resources Low CPU utilization Significant amount of setup time Operating System Concepts 1.6 Silberschatz and Galvin c 1998

8 Simple Batch Systems Hire an operator User operator Add a card reader Reduce setup time by batching similar jobs Automatic job sequencing automatically transfers control from one job to another. First rudimentary operating system. Resident monitor initial control in monitor control transfers to job when job completes control transfers back to monitor Operating System Concepts 1.7 Silberschatz and Galvin c 1998 Simple Batch Systems (Cont.) Problems 1. How does the monitor know about the nature of the job (e.g., Fortran versus Assembly) or which program to execute? 2. How does the monitor distinguish (a) job from job? (b) data from program? Solution Introduce control cards Operating System Concepts 1.8 Silberschatz and Galvin c 1998

9 Control Cards Special cards that tell the resident monitor which programs to run. $JOB $FTN $RUN $DATA $END Special characters distinguish control cards from data or program cards: $ in column 1 // in column 1 and in column 1 Operating System Concepts 1.9 Silberschatz and Galvin c 1998 Control Cards (Cont.) Parts of resident monitor Control card interpreter responsible for reading and carrying out instructions on the cards. Loader loads systems programs and applications programs into memory. Device drivers know special characteristics and properties for each of the system s I/O devices. Problem: Slow Performance I/O and CPU could not overlap; card reader very slow. Solution: Off-line operation speed up computation by loading jobs into memory from tapes and card reading and line printing done off-line. Operating System Concepts 1.10 Silberschatz and Galvin c 1998

10 Off-Line Operation card reader printer satellite processor system tapes main computer Operating System Concepts 1.11 Silberschatz and Galvin c 1998 Off-Line Operation (Cont.) Advantage main computer not constrained by the speed of the card readers and line printers, but only by the speed of faster magnetic tape units. No changes need to be made to the application programs to change from direct to off-line I/O operation. Real gain possibility of using multiple reader-to-tape and tape-to-printer systems for one CPU. Operating System Concepts 1.12 Silberschatz and Galvin c 1998

11 Spooling disk I/O CPU on-line card reader line printer Overlap I/O of one job with computation of another job. While executing one job, the OS: reads next job from card reader into a storage area on the disk (job queue). outputs printout of previous job from disk to printer. Job pool data structure that allows the OS to select which job to run next in order to increase CPU utilization. Operating System Concepts 1.13 Silberschatz and Galvin c 1998 Multiprogrammed Batch Systems Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them. OS u1 u2 u3 u4 u1 L read () L+1 u2 CPU OS SIO scheduler I/O M block scheduler R R+1 interrupt scheduler Operating System Concepts 1.14 Silberschatz and Galvin c 1998

12 OS Features Needed for Multiprogramming I/O routine supplied by the system. Memory management the system must allocate the memory to several jobs. CPU scheduling the system must choose among several jobs ready to run. Allocation of devices. Operating System Concepts 1.15 Silberschatz and Galvin c 1998 Time-Sharing Systems Interactive Computing The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory). A job is swapped in and out of memory to the disk. On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next control statement not from a card reader, but rather from the user s keyboard. On-line file system must be available for users to access data and code. Operating System Concepts 1.16 Silberschatz and Galvin c 1998

13 Personal-Computer Systems Personal computers computer system dedicated to a single user. I/O devices keyboards, mice, display screens, small printers. User convenience and responsiveness. Can adopt technology developed for larger operating systems; often individuals have sole use of computer and do not need advanced CPU utilization or protection features. Operating System Concepts 1.17 Silberschatz and Galvin c 1998 Parallel Systems Multiprocessor systems with more than one CPU in close communication. Tightly coupled system processors share memory and a clock; communication usually takes place through the shared memory. Advantages of parallel systems: Increased throughput Economical Increased reliability graceful degradation fail-soft systems Operating System Concepts 1.18 Silberschatz and Galvin c 1998

14 Parallel Systems (Cont.) Symmetric multiprocessing Each processor runs an identical copy of the operating system. Many processes can run at once without performance deterioration. Asymmetric multiprocessing Each processor is assigned a specific task; master processor schedules and allocates work to slave processors. More common in extremely large systems. Operating System Concepts 1.19 Silberschatz and Galvin c 1998 Distributed Systems Distribute the computation among several physical processors. Loosely coupled system each processor has its own local memory; processors communicate with one another through various communication lines, such as high-speed buses or telephone lines. Advantages of distributed systems: Resource sharing Computation speed up load sharing Reliability Communication Operating System Concepts 1.20 Silberschatz and Galvin c 1998

15 Real-Time Systems Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems. Well-defined fixed-time constraints. Hard real-time system. Secondary storage limited or absent; data stored in short-term memory, or read-only memory (ROM). Conflicts with time-sharing systems; not supported by general-purpose operating systems. Soft real-time system. Limited utility in industrial control or robotics. Useful in applications (multimedia, virtual reality) requiring advanced operating-system features. Operating System Concepts 1.21 Silberschatz and Galvin c 1998

16 Online Documentation on Operating Systems Examples: Multics Important project for historical reference, influence on UNIX - Concept of protection rings (see next slide) JavaOS (see slide after the next) Java VM can directly run on hardware => Additional services are made available without underlying OS Operating Systems Concepts 17

17 Multics Protection Rings Privileged system core Rings: decreasing privilege order Processes in outer rings call services from inner rings Operating Systems Concepts 18

JavaOS Structure Remark: JavaOS (released 1996) is discontinued since 2006. There are other projects, e.g.

18 JavaOS Structure Remark: JavaOS (released 1996) is discontinued since There are other projects, e.g., to write a full OS in Java (JNode, JOS, JX), but as far as I know, they have no relevance and little interest. Operating Systems Concepts 19

A couple of other (randomly selected?) OS Plan9: http://plan9.bell-labs.

19 A couple of other (randomly selected?) OS Plan9: - Since 1990 designed as the successor of UNIX - Open source since Interesting concepts (local namespaces, everything is a single file ) - UNIX good enough despite its age (few urgent reasons to change) Minix3: - A research project since Famous exchange between Prof Andrew Tanenbaum and Linus Torwalds - Current version 3.3 (September 2014) Each has a kernel size between 15-25,000 lines of code. This ends the introduction to OS. Now transition to implementation. Operating Systems Concepts 20

20 Protected CPU Commands, CPU Modes, System Calls Operating Systems Concepts, 5 th Edition Slide set M10-c Slides , 2.19, and 3.14 Operating Systems Concepts 21

21 Hardware Protection Dual-Mode Operation I/O Protection Memory Protection CPU Protection Operating System Concepts 2.11 Silberschatz and Galvin c 1998 Dual-Mode Operation Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly. Provide hardware support to differentiate between at least two modes of operations. 1. User mode execution done on behalf of a user. 2. Monitor mode (also supervisor mode or system mode) execution done on behalf of operating system. Operating System Concepts 2.12 Silberschatz and Galvin c 1998

22 Dual-Mode Operation (Cont.) Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1). When an interrupt or fault occurs hardware switches to monitor mode interrupt/fault monitor user set user mode Privileged instructions can be issued only in monitor mode. Operating System Concepts 2.13 Silberschatz and Galvin c 1998 I/O Protection All I/O instructions are privileged instructions. Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part of its execution, stores a new address in the interrupt vector). Operating System Concepts 2.14 Silberschatz and Galvin c 1998

23 Memory Protection Must provide memory protection at least for the interrupt vector and the interrupt service routines. In order to have memory protection, add two registers that determine the range of legal addresses a program may access: base register holds the smallest legal physical memory address. limit register contains the size of the range. Memory outside the defined range is protected. Operating System Concepts 2.15 Silberschatz and Galvin c 1998 Example of Memory Protection 0 monitor job job 2 base register job 3 limit register job 4 Operating System Concepts 2.16 Silberschatz and Galvin c 1998

24 Protection Hardware base base + limit CPU address yes yes < no no trap to operating system monitor addressing error memory When executing in monitor mode, the operating system has unrestricted access to both monitor and users memory. The load instructions for the base and limit registers are privileged instructions. Operating System Concepts 2.17 Silberschatz and Galvin c 1998 General-System Architecture Given that I/O instructions are privileged, how does the user program perform I/O? System call the method used by a process to request action by the operating system. Usually takes the form of a trap to a specific location in the interrupt vector. Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to monitor mode. The monitor verifies that the parameters are correct and legal, executes the request, and returns control to the instruction following the system call. Operating System Concepts 2.19 Silberschatz and Galvin c 1998

25 System Calls System calls provide the interface between a running program and the operating system. Generally available as assembly-language instructions. Languages defined to replace assembly language for systems programming allow system calls to be made directly (e.g., C, Bliss, PL/360). Three general methods are used to pass parameters between a running program and the operating system: Pass parameters in registers. Store the parameters in a table in memory, and the table address is passed as a parameter in a register. Push (store) the parameters onto the stack by the program, and pop off the stack by the operating system. Operating System Concepts 3.14 Silberschatz and Galvin c 1998

26 Exceptions The interrupt mechanism for CPU non maskable interrupts and exceptions conditions utilize Division by zero Invalid memory access (range, alignment ) Invalid opcode Double fault Known as Exceptions, traps, or software interrupts Operating Systems Concepts 27

27 Exceptions (continuation I) Handling Each exception (fault/trap/abort) is assigned an (interrupt) number Table of interrupt vectors When an exception is raised - Call corresponding interrupt handler Handlers Save contents of most registers in the Kernel Mode stack (coded in asm). Handle the exception by means of a high-level C function. Exit from the handler by means of the ret_from_exception( ) function. Operating Systems Concepts 28

28 Exceptions (continuation II) Difference between exceptions and interrupts (hardware and software interrupts) Exceptions are synchronous - The event is linked to a specific point in the code - UNIX signal Interrupts are asynchronous - The event can arrive while any part of the code executes - No UNIX signal Exceptions are processed identically, with a different goal: to correct the action of the faulty program. Operating Systems Concepts 29

29 Interrupts Table of interrupt vectors Vector range Use 0-19 NMI and exceptions Intel-reserved External interrupts 128 Programmed exception for system calls External interrupts 239 Local APIC timer interrupt 240 Local APIC thermal interrupt Reserved by Linux for future use Interprocessor interrupts 254 Local APIC error interrupt 255 Local APIC spurious interrupts Operating Systems Concepts 30

30 System Calls User space (unprivilegierter Modus) Kernel space (privilegierter Modus) Trap Library für OS Dienste Trap System calls, supervisory calls Can be directly achieved in the program (asm), often available within a library (C bindings) Operating Systems Concepts 31

31 Supervisory Calls (SVC) An SVC is a HW instruction used to cause an interrupt to request a service directly from the OS via an SVC routine. Processing of an exception goes hand in hand with a CPU mode change. Exception handler requires extended access rights (similar to interrupts) Mode change from software interrupts is used for OS calls Flow Load register with parameters, including OS services (syscall-nr *) Trigger an SVC (trap), e.g., 128 (0x80) for Linux and Intel Read results from the register. * See /usr/src/kernels/linux /include/asm-i386/unistd.h Operating Systems Concepts 32

32 States of a (Unix) Process new schedule() exit() ready wakeup() (durch einen anderen Prozess) preempt waiting running sleep_on() Blocking SVC: changes state from running waiting Operating Systems Concepts 33

33 Blocking System Calls Assumption: System call for reading data is issued, yet the I/O device is not ready Calling process must suspend itself Actions taken by the processes - Check whether resource (result) is ready, - If not: leave a note that it is interested in the resource (result), - CPU is given to another process or the process itself goes to sleep, - When reactivated: Check resource (result) again. In UNIX: If a resource becomes available (anew), ALL processes that were waiting on it are reactivated. All processes must check resource again: first one to complete the check wins. Operating Systems Concepts 34

34 Pseudo-Code for Blocking in Kernel for(;;) { if (resource_is_ready()) break; } request_the_resource(); sleep_on(resource_condition); /* block */ /* continue and use the resource... */ See Linux files: /usr/src/linux-source-2.x.y/kernel/sched*.c See also later: the Monitor concept Operating Systems Concepts 35

35 Implementation of sleep_on (Linux 2.2.x) sleep_on(struct wait_queue **p, int state) { unsigned long flags; struct wait_queue wait = { current, NULL }; if (!p) return; // process already sleeps if (current == task[0]) panic("task[0] trying to sleep"); current->state = state; add_wait_queue(p, &wait); save_flags(flags); sti(); // set the IF flag of the eflags ctrl reg schedule(); remove_wait_queue(p, &wait); restore_flags(flags); } Operating Systems Concepts 36

36 Implementation of wake_up (Linux 2.2.x) void wake_up(struct wait_queue **q) { struct wait_queue *tmp; struct task_struct *p; if (!q!(tmp = *q)) return; do { if ((p = tmp->task)!= NULL) { if ((p->state == TASK_UNINTERRUPTIBLE) (p->state == TASK_INTERRUPTIBLE)) p->state = TASK_RUNNING; } tmp = tmp->next; } while (tmp!= *q); } Operating Systems Concepts 37

37 Implementation of schedule (Linux 2.2.x) asmlinkage void schedule(void) { int c; struct task_struct *p, *next; sti(); /* Here additional code for UNIX signals handling */ c = -1; for (next = p = &init_task;;) { if ((p = p->next_task) == &init_task) break; if (p->state == TASK_RUNNING && p->counter > c) c = p->counter, next = p; } /*... Continued on next slide... */ Operating Systems Concepts 38

38 Implementation of schedule (continuation) /*... Continued from previous Slide... */ } if (!c) { for_each_task(p) /* this is a Macro! */ p->counter = (p->counter >> 1) + p->priority; } switch_to(next); /* and this hides assembly code */ Explanations of new Linux version can be found, for instance in /usr/src/linux-source /documentation/scheduler/ Operating Systems Concepts 39

39 Observations Regarding Earlier Linux Versions An increasing number of macros to increase portability. Example: sleep_on() in Linux void sleep_on(wait_queue_head_t *q) { SLEEP_ON_VAR current->state = TASK_UNINTERRUPTIBLE; SLEEP_ON_HEAD schedule(); SLEEP_ON_TAIL } Kernel reading becomes more difficult, many indirections. sched.c in 2.4.x has more than 100 LOC, while in more than 11,000 LOC! Latest stable Linux kernel version: Operating Systems Concepts 40

Module 1: Introduction

Module 1: Introduction What is an operating system? Simple Batch Systems Multiprogramming Batched Systems Time-Sharing Systems Personal-Computer Systems Parallel Systems Distributed Systems Real-Time Systems