CSC520 - Chapter 2: Operating-System Structures

Chapter 2: Operating-System Structures

2.1 Operating System Services

  • Operating systems provide an environment for the 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 (UI):
      • Almost all operating systems have a user interface.
      • Varies between Command-Line (CLI), Graphics User Interface (GUI), and Batch (e.g., backup files).
    • Program execution: The system must be able to load a program into memory and run it, ending 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: Programs need to read and write files and directories, create and delete them, search them, list file information, and manage permissions.
    • 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: The OS needs to be constantly aware of possible errors.
      • May occur in the CPU and memory hardware, in I/O devices, or in user programs.
      • For each type of error, the OS should take the appropriate action to ensure correct and consistent computing.
  • 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.
    • Accounting: 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 the 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 and extends to defending external I/O devices from invalid access attempts.

A View of Operating System Services

  • The operating system provides various services to users and other system programs, including user interfaces (GUI, batch, command line), system calls, program execution, I/O operations, file systems, communication systems, resource allocation, accounting, error detection services, protection, and security. These services sit between the hardware and the user interface.

2.2 User and Operating System Interface

  • CLI (Command-Line Interface)
    • Allows direct command entry.
    • Sometimes implemented in the kernel, sometimes by systems program.
    • Sometimes multiple types implemented – shells.
    • Primarily fetches a command from the user and executes it.
    • Sometimes commands are built-in, sometimes just names of programs.
      • If the latter, adding new features doesn’t require shell modification.
  • Shells are like command interpreters that allow you to communicate with the computer using text-based commands. They take your text-based instructions (commands) and translate them into actions the computer can understand, such as running programs, managing files, and configuring settings. In essence, shells provide a way for you to interact with the computer without needing to use a complex graphical user interface (GUI).
  • GUI (Graphical User Interface)
    • Usually involves a 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).
    • User-friendly desktop metaphor interface.
    • Examples:
      • Microsoft Windows: GUI with CLI “command” shell
      • Apple Mac OS X: “Aqua” GUI interface with UNIX kernel underneath and shells available
      • Unix and Linux: CLI with optional GUI interfaces (CDE, KDE, GNOME)
    • Many systems now include both CLI and GUI interfaces.
  • Touchscreen Interfaces
    • Touchscreen devices require new interfaces.
      • Mouse is not possible or not desired.
      • Actions and selection based on gestures.
      • Virtual keyboard for text entry.
      • Voice commands.

2.3 System Calls and System Services

  • System-call names used throughout this text are generic POSIX - Portable Operating System Interface.
  • 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)
    • Java API for the Java virtual machine (JVM)
  • Example: System call sequence to copy the contents of one file to another file.
System Call Implementation
  • Typically, a number is associated with each system call.
  • The system-call interface maintains a table indexed according to these numbers.
  • The system call interface invokes the intended system call in OS kernel and returns the status of the system call and any return values.
  • The caller need know nothing about how the system call is implemented; just needs to obey the API and understand what the OS will do as a result of the call.
  • Most details of OS interface are hidden from the programmer by the API, and managed by a run-time support library (set of functions built into libraries included with compiler).
System Call Parameter Passing

  • System Call Parameter Passing is directly related to the operating system (OS) because it is a mechanism used by programs to interact with the OS. When a program requires services that it cannot perform on its own (like reading files, writing data to disk, or accessing hardware), it makes a system call.

    • Simplest (Register Passing): Programs that perform basic calculations (like addition, subtraction, multiplication) often use simple, single-value parameters (e.g., two numbers to add). Register passing works well here since the parameters are limited in number and can easily fit.
    • Block/Memory Table Passing: When a program needs to work with complex data—such as file details (name, size, location) in a file manager or image settings in graphics software—it often bundles this information into a block or table in memory.
    • Stack Passing: Programs that handle multiple, sequential tasks, such as web browsers processing user commands or servers handling multiple client requests, often use stack- based parameter passing.

  • Often, more information is required than simply the identity of the desired system call; the exact type and amount of information vary according to OS and call.

  • Three general methods used to pass parameters to the OS:

    • Simplest: pass the parameters in registers
      • In some cases, may be more parameters than registers
    • Parameters stored in a block, or table, in memory, and the address of the block is passed as a parameter in a register.
      • This approach is taken by Linux and Solaris.
    • Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system.
    • Block and stack methods do not limit the number or length of parameters being passed.
Types of System Calls
  • Process control:
    • create process, terminate process
    • end, abort
    • load, execute
    • get process attributes, set process attributes
    • wait for time
    • wait event, signal event
    • allocate and free memory
    • Dump memory if error
    • Debugger for determining bugs, single step execution
    • Locks for managing access to shared data between processes
  • File management:
    • create file, delete file
    • open, close file
    • read, write, reposition
    • get and set file attributes
  • Device management:
    • request device, release device
    • read, write, reposition
    • get device attributes, set device attributes
    • logically attach or detach devices
  • Information maintenance:
    • get time or date, set time or date
    • get system data, set system data
    • get and set process, file, or device attributes
  • Communications
    • create, delete communication connection
    • send, receive messages if message passing model to host name or process name
      • From client to server
    • Shared-memory model create and gain access to memory regions
    • transfer status information
    • attach and detach remote devices
  • Protection
    • Control access to resources
    • Get and set permissions
    • Allow and deny user access
System Programs
  • System programs provide a convenient environment for program development and execution. They can be divided into:
    • File manipulation
    • Status information (sometimes stored in a file)
    • File modification
    • Programming language support
    • Program loading and execution
    • Communications
    • Background services
    • Application programs
  • Most users’ view of the operation system is defined by system programs, not the actual system calls.

2.4 Operating System Design and Implementation

  • Design and Implementation of OS 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
    • Important principle to separate:
      • Policy: What will be done?
      • Mechanism: How to do it?
      • Mechanisms determine how to do something, policies decide what will be done
      • The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later (example – timer)
  • Specifying and designing an OS is highly creative task of software engineering
Implementation
  • Once an operating system is designed, it must be implemented
  • Much variation
    • Early OSes in assembly language
    • Then system programming languages like Algol, PL/1
    • Now C, C++
    • Actually usually a mix of languages
      • Lowest levels in assembly
      • Main body in C
      • Systems programs in C, C++, scripting languages like PERL, Python, shell scripts
    • More high-level language is easier to port to other hardware, but slower
    • Emulation can allow an OS to run on non-native hardware.

2.5 Operating System Structure

  • General-purpose OS is very large program
  • Various ways to structure ones
    • Simple structure – MS-DOS
    • More complex -- UNIX
    • Layered – an abstraction
    • Microkernel -Mach Open Systems Interconnection
Simple Structure -- MS-DOS
  • MS-DOS – written to provide the most functionality in the least space
    • Not divided into modules
    • Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated
Non Simple Structure -- 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
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
Microkernel System Structure
  • Moves as much from the kernel into user space
  • Mach 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 (a cause of harm or damage):
    • Performance overhead of user space to kernel space communication
Operating-System Debugging
  • Debugging is finding and fixing errors, or bugs
  • OS generate log files containing error information
  • Failure of an application can generate a core dump file capturing the memory of the process.
  • Operating system failure can generate a 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 the instruction pointer to look for statistical trends
  • Kernighan’s Law: “Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. ”