Operating-System Services

Before we start

Why this matters

Chapter 2 · Lectures 4–6

Outline

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1. Operating system services
2. User and operating-system interface
3. System calls
4. Types of system calls
5. System services
6. Why applications are OS specific
7. Design and implementation
8. Operating system structure
9. Building and booting an operating system
10. Operating system debugging

Operating system services

Operating system services

Operating systems provide an environment for execution of programs and services to programs and users.

One set of operating-system services provides functions that are helpful to the user:

• User interface – almost all operating systems have a user interface (UI)
  • Varies between command-line (CLI), graphics user interface (GUI), touch-screen, batch
• Program execution – the system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error)
• I/O operations – a running program may require I/O, which may involve a file or an I/O device
• File-system manipulation – the file system is of particular interest. Programs need to read and write files and directories, create and delete them, search them, list file information, permission management.

Operating system services

Operating system services (cont.)

One set of operating-system services provides functions that are helpful to the user (cont.):

• Communications – processes may exchange information, on the same computer or between computers over a network
  • Communications may be via shared memory or through message passing (packets moved by the OS)
• Error detection – OS needs to be constantly aware of possible errors
  • May occur in the CPU and memory hardware, in I/O devices, in user program
  • For each type of error, OS should take the appropriate action to ensure correct and consistent computing
  • Debugging facilities can greatly enhance the user’s and programmer’s abilities to efficiently use the system

Operating system services

Operating system services (cont.)

Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing:

• Resource allocation – when multiple users or multiple jobs are running concurrently, resources must be allocated to each of them
  • Many types of resources – CPU cycles, main memory, file storage, I/O devices
• Logging – to keep track of which users use how much and what kinds of computer resources
• Protection and security – the owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other
  • Protection involves ensuring that all access to system resources is controlled
  • Security of the system from outsiders requires user authentication, extends to defending external I/O devices from invalid access attempts

Operating system services

A view of operating system services

User and operating-system interface

Command line interpreter

$ ls
notes.txt  fork.c  a.out
$ cp notes.txt backup.txt
$ echo $SHELL
/bin/bash

User and operating-system interface

User operating system interface – GUI

• User-friendly desktop metaphor interface
  • Usually mouse, keyboard, and monitor
  • Icons represent files, programs, actions, etc.
  • Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder))
  • Invented at Xerox PARC
• Many systems now include both CLI and GUI interfaces
  • Microsoft Windows is GUI with CLI “command” shell
  • Apple Mac OS X is “Aqua” GUI interface with UNIX kernel underneath and shells available
  • Unix and Linux have CLI with optional GUI interfaces (CDE, KDE, GNOME)

User and operating-system interface

Touchscreen interfaces

• Touchscreen devices require new interfaces
  • Mouse not possible or not desired
  • Actions and selection based on gestures
  • Virtual keyboard for text entry
• Voice commands

System calls

System calls

• Programming interface to the services provided by the OS
• Typically written in a high-level language (C or C++)
• Mostly accessed by programs via a high-level Application Programming Interface (API) rather than direct system call use
• Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)
• Note that the system-call names used throughout this text are generic

Interactive

Example of system calls

System call sequence to copy the contents of one file to another file:

Interactive

Example of system calls (cont.)

Code and output

The copy program in C

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(void)
{
    char buf[64];
    ssize_t n, total = 0;

    int in  = open("notes.txt", O_RDONLY);                       /* system call */
    int out = open("copy.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644); /* system call */
    if (in < 0 || out < 0) { perror("open"); return 1; }

    while ((n = read(in, buf, sizeof buf)) > 0) {  /* system call */

Code and output

The copy program in C (cont.)

        write(out, buf, n);                        /* system call */
        total += n;
    }
    close(in);                                     /* system call */
    close(out);                                    /* system call */
    printf("copied %zd bytes\n", total);           /* write() inside */
    return 0;
}

$ ./copy        # copies notes.txt to copy.txt
copied 48 bytes

Run strace ./copy on Linux to see each call listed with its arguments and return value.

System calls

API – system call – OS relationship

System calls

API – system call – OS relationship (cont.)

• The program calls open() in the API (a library function).
• The library puts the system-call number in a register and traps into the kernel.
• The system-call interface looks up entry i in its table and runs the kernel implementation of open().
• The return status and any values go back to the program.

Animated

The journey of a system call

Types of system calls

Types of system calls

Types of system calls

Types of system calls (cont.)

Types of system calls

Examples of Windows and Unix system calls

Interactive

Classify the calls

Types of system calls

Standard C library example

C program invoking the printf() library call, which calls the write() system call.

#include <stdio.h>
int main()
{
   printf("Greetings");
   return 0;
}

Types of system calls

Standard C library example (cont.)

Code and output

printf( ) and write( ): run it

#include <stdio.h>
#include <unistd.h>

int main(void)
{
    printf("Greetings from printf\n");     /* C library: formats, then calls write() */
    write(1, "Greetings from write\n", 21); /* system call: fd 1 is the screen */
    return 0;
}

$ gcc greet.c -o greet -pthread && ./greet
Greetings from printf
Greetings from write

System services

System services

System services

System services (cont.)

• 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

System services

System services (cont.)

• File modification
  • Text editors to create and modify files
  • Special commands to search contents of files or perform transformations of the text
• Programming-language support – compilers, assemblers, debuggers and interpreters sometimes provided
• Program loading and execution – absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language
• Communications – provide the mechanism for creating virtual connections among processes, users, and computer systems
  • Allow users to send messages to one another’s screens, browse web pages, send electronic-mail messages, log in remotely, transfer files from one machine to another

System services

System services (cont.)

Why applications are OS specific

Why applications are operating system specific

• Apps compiled on one operating system usually not executable on other operating systems
• Each operating system provides its own unique system calls
  • Own file formats, etc.
• Apps can be multi-operating system
  • Written in interpreted language like Python, Ruby, and interpreter available on multiple operating systems
  • App written in language that includes a VM containing the running app (like Java)
  • Use standard language (like C), compile separately on each operating system to run on each
• Application Binary Interface (ABI) is architecture equivalent of API, defines how different components of binary code can interface for a given operating system on a given architecture, CPU, etc.

Design and implementation

Design and implementation

• Design and implementation of OS is not “solvable”, but some approaches have proven successful
• Internal structure of different operating systems can vary widely
• Start the design 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
• Specifying and designing an OS is highly creative task of software engineering

Design and implementation

Implementation

Operating system structure

Operating system structure

Operating system structure

Monolithic structure – original UNIX

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring.
• The UNIX OS consists of two separable parts
  • Systems programs
  • 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

Operating system structure

Monolithic structure – original UNIX (cont.)

Operating system structure

Linux system structure

Operating system structure

Linux system structure (cont.)

• The whole kernel runs in one address space (monolithic): fast, because services call each other directly.
• Loadable kernel modules (for example device drivers) can be added and removed while the system runs.

Operating system structure

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

Operating system structure

Layered approach (cont.)

Operating system structure

Microkernels

• Moves as much from the kernel into user space
• Mach is an example of microkernel
  • Mac OS X kernel (Darwin) partly based on Mach
• Communication takes place between user modules using message passing
• 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

Operating system structure

Microkernels (cont.)

Operating system structure

Modules

• Many modern operating systems implement loadable kernel modules (LKMs)
  • 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
  • Linux, Solaris, etc.

Operating system structure

Hybrid systems

• Most modern operating systems are not one pure model
  • Hybrid combines multiple approaches to address performance, security, usability needs
  • Linux and Solaris kernels in kernel address space, so monolithic, plus modular for dynamic loading of functionality
  • Windows mostly monolithic, plus microkernel for different subsystem personalities
• Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa programming environment
  • Below is kernel consisting of Mach microkernel and BSD Unix parts, plus I/O kit and dynamically loadable modules (called kernel extensions)

Operating system structure

Hybrid systems (cont.)

Operating system structure

iOS

• Apple mobile OS for iPhone, iPad
  • Structured on Mac OS X, added functionality
  • Does not run OS X applications natively
    • Also runs on different CPU architecture (ARM vs. Intel)
  • Cocoa Touch Objective-C API for developing apps
  • Media services layer for graphics, audio, video
  • Core services provides cloud computing, databases
  • Core operating system, based on Mac OS X kernel

Operating system structure

iOS (cont.)

Operating system structure

Android

• Developed by Open Handset Alliance (mostly Google)
  • Open source
• Similar stack to iOS
• Based on Linux kernel but modified
  • Provides process, memory, device-driver management
  • Adds power management
• Runtime environment includes core set of libraries and Dalvik virtual machine
  • Apps developed in Java plus Android API
  • Java class files compiled to Java bytecode then translated to executable that runs in Dalvik VM
• Libraries include frameworks for web browser (webkit), database (SQLite), multimedia, smaller libc

Operating system structure

Android (cont.)

Building and booting an operating system

Building and booting an operating system

• Operating systems generally designed to run on a class of systems with variety of peripherals
• Commonly, operating system already installed on purchased computer
  • But can build and install some other operating systems
  • If generating an operating system from scratch
    • Write the operating system source code
    • Configure the operating system for the system on which it will run
    • Compile the operating system
    • Install the operating system
    • Boot the computer and its new operating system

Building and booting an operating system

Building and booting Linux

$ make menuconfig
$ make
$ make modules
$ sudo make modules_install
$ sudo make install

Building and booting an operating system

System boot

• When power initialized on system, execution starts at a fixed memory location
• Operating system must be made available to hardware so hardware can start it
  • Small piece of code – bootstrap loader, BIOS, stored in ROM or EEPROM locates the kernel, loads it into memory, and starts it
  • Sometimes two-step process where boot block at fixed location loaded by ROM code, which loads bootstrap loader from disk
  • Modern systems replace BIOS with Unified Extensible Firmware Interface (UEFI)
• Common bootstrap loader, GRUB, allows selection of kernel from multiple disks, versions, kernel options
• Kernel loads and system is then running
• Boot loaders frequently allow various boot states, such as single user mode

Animated

System boot, animated

Operating system debugging

Operating-system debugging

• Debugging is finding and fixing errors, or bugs
• Also performance tuning
• OS generate log files containing error information
• Failure of an application can generate core dump file capturing memory of the process
• Operating system failure can generate crash dump file containing kernel memory
• Beyond crashes, performance tuning can optimize system performance
  • Sometimes using trace listings of activities, recorded for analysis
  • Profiling is periodic sampling of instruction pointer to look for statistical trends

Operating system debugging

Performance tuning

Tasks: 212 total,   1 running, 211 sleeping
%Cpu(s): 12.5 us,  3.1 sy,  0.0 ni, 84.0 id
MiB Mem :  7841.2 total,  2210.5 free

  PID USER   %CPU %MEM COMMAND
 1432 ayan   35.2  4.1 firefox
  877 root    6.0  0.8 Xorg
 2210 ayan    1.3  0.5 bash

Operating system debugging

Tracing

• Collects data for a specific event, such as steps involved in a system call invocation
• Tools include
  • strace – trace system calls invoked by a process
  • gdb – source-level debugger
  • perf – collection of Linux performance tools
  • tcpdump – collects network packets

openat(AT_FDCWD, "notes.txt", O_RDONLY) = 3
read(3, "OS notes\n", 131072)         = 9
write(1, "OS notes\n", 9)              = 9
read(3, "", 131072)                    = 0
close(3)                               = 0

Worked example 1 of 2

Worked example 1

Worked example 2 of 2

Worked example 2

Practice

Practice (1 of 2)

Try each one on paper first, then check your answer.

Practice

Practice (2 of 2)

Try each one on paper first, then check your answer.

Check yourself

Quiz

Resources

Further learning

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Summary · End of Chapter 2

Summary

• The OS offers services for users (UI, program execution, I/O, files, communication, error detection) and for the system (resource allocation, logging, protection).
• Programs reach the services through system calls, usually via an API such as POSIX or Win32; the system-call interface dispatches by number.
• System calls fall into six types: process control, file management, device management, information maintenance, communications, protection.
• System programs define most users’ view of the OS; apps are OS specific because system calls and ABIs differ.
• Structures: monolithic (UNIX, Linux), layered, microkernel (Mach), modules, and hybrids (Windows, macOS, iOS, Android).
• A bootstrap loader (BIOS/UEFI, GRUB) loads the kernel; logs, core dumps, top, strace, gdb and perf help debugging and tuning.