OS Services and Structures

Operrating 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.

  • Communications - Processes may exchange information, on the same computer or between computers over a network (IPC: internal procedure call / RPC: remote procedure call)

    • 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
  • 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 running concurrently, resources must be allocated to each of them
    • Many types of resources - CPU cycles, main memory, file storage, l/O devices.
  • Logging - To keep track of which users use how much and what kinds of computer resources
    • Eg. top can monitor the usage information of 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

A View of Operating System Services

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
  • Eg. libc on Linux
• 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)

Types of System Calls

• Process control
  • create process, terminate process
    • For UNIX: Create = fork, terminate: exit
  • end, abort
  • load, execute
  • get process attributes, set process attributes
  • wait for time
    • For UNIX: wait
  • 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: UNIX: open, read, write, close
  • create file, delete file
  • open, close file
  • read, write, reposition
  • get and set file attributes
• Device management: UNIX: ioctl, read, write
  • request device, release device
  • read, write, reposition
  • get device attributes, set device attributes
  • logically attach or detach devices
• Information maintenance Eg. getpid...
  • get time or date, set time or date
  • get system data, set system data
  • get and set process, file, or device attributes
• Communications Eg. pipe, nmap
  • 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 Eg. umask, chown
  • Control access to resources
  • Get and set permissions
  • Allow and deny user access

Architecture Examples

MS-DOS Execution (Single Tasking OS)

• Command interpreter = special program = shell (cshell, korn shell)
  • Created at First Login / Program launch
  • Execute User-specified command

MS-DOS Layer Structure

UNIX system structure

• UNIX -
  • Limited hardware functionality -> the original UNIX operating system had limited structuring
  • UNIX OS consists of two separable parts
• Systems programs
• The kernel
  • Everything below the system-call interface and above the physical hardware
  • Provides
    • File system
    • CPU scheduling
    • Memory management
    • Etc.
  • A large number of functions for one level 同一层 / Same address base

UNIX Structure (Simplest Moderm System Structure) / Monolithic Kernel 单体核心操作系统设计原则

UNIX Running Multiple Programs

Review: How BSD Operate 1:17:00

Communication Models

• Message Passing
  • Through Kernel (System Call)
• Shared Memory
  • 开销更低

Layered Approach

VS Monolithic Design?

• 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

• High-level layers can invoke low-level layers
• Friendlier to modify / maintain
• BUT high performance overhead