SISTEMAS OPERATIVOS

Conceito de Sistema Operativo

  Um sistema operativo consiste num conjunto de programas que interagem directamente com o computador, descrito segundo uma determinada arquitectura funcional.

Caracteristicas e funções do Sistema Operativo

Um sistema Operativo desempenha as seguintes funções :

   Gestão de recursos

   - Contexto: Ambiente de vários utilizadores em competição pela utilização dos recursos

    - Soluções: Maximizar a utilização e a partilha de tempo (de CPU, memória central e de cache, memória secundária, suporte de entradas/saídas  

   Aumentar a disponibilidade e a eficiencia da  arquitectura do computador do ponto de                                                                               vista dos utilizadores

   Criação de uma máquina virtual

   Nível de abtracção relativamente á arquitactura fisica do computador, criando ao                    utilizador a noção de dispor de uma máquina individual

   Colocar á disposição do utilizador um ambiente mais fácil de utilizar 

   Memória : - divisão e gestão   estruturada de parte da hierarquia de memória            (cache, principal e secundário) em segmentos ou páginas, possibilitando a execução de um programa mesmo que possua uma dimensão superior á da memória principal.

   Conceito de memória virtual

   Sistemas de ficheiros: - Armazenamento de dados e programas na hierarquia de memória secundária, por forma a que possam ser acedidos através de nomes simbólicos.

   Estrutura em árvore: C:\usr\local\README.txt   (MS-DOS)

                                                     /usr/local/README.txt      (UNIX)

   Protecção e tratamento de erros: -Idealmente, a máquina virtual deve prevenir todas as situações de erro por causas  internas ou externas á máquina (tarefa impossivel)

   Fornecer protecção de uns utilizadores  relativamente aos outros

   Defender os recursos priviligiados do Sistema Operativo

   Interacção entre programas: -disponibilizar mecanismos de cooperação entre diferentes programas (Sincronização)

   Troca de mensagens entre processos (comunicação)

   Interacção utilizador-computador: -definição de uma linguagem de comandos:

                                                                 command.com, windows 95/NT     (MS_DOS)

                                                                                 shel, c-shell, x windows    (UNIX)

    Tipos de Sistema Operativo

   Sistemas dedicados

   Monoutilizador

    -máquina virtual para um único utilizador

    -recursos mínimos: -interpretador de comandos, sistema de ficheiros simples, unidades de E/S para monitor e disco

    -ex: -zx-spectrum, cp/ms-dos

   Controlo de processos (tempo real)

    -Sistemas que garantem a estabilidade de processos

    -ex: controlo industrial, piloto automático num avião

   Processamento de transacções

    -sistemas centralizados que se caracterizam pela capacidade de processar milhares de acessos por segundo

    -ex: bancos, seguradoras, reservas de uma companhia aérea

   Sistemas de uso geral

   Caracterizados por um grande número de utilizadores, realizando tarefas distintas em ritmo contínuo, isto é comutando o CPU de tarefa em tarefa

   Cada tarefa é lançada por um utilizador e pode consistir num ou mais programas executáveis

   Suportam uma grande variedade de programas

    -compiladores, assembladores, depuradores, editores, etc.

    - reconhece um grande número de dispositivos de E/S

   Sistemas por lotes

   Tratamento de uma tarefa de cada vez, a qual monopoliza o CPU até terminar a execução

   Sistemas multi-utilizador

   Criação de uma máquina virtual para cada utilizador, oferecendo a ilusão de           disponibilização  de recursos e atenção permanente a todos eles

   Sistemas distribuidos

   Criação de máquinas virtuais sobre uma rede local (LAN), metropolitana (MAN) ou de âmbito alargado (WAN)

Exemplos de Sistemas Operativos

Descrição de alguns Sistemas Operativos

   Unix

UNIX is an operating system that originated at Bell Labs in 1969 as an interactive time-sharing system. Ken Thompson and Dennis Ritchie are considered the inventors of UNIX. The name (pronounced YEW-nihks) was a pun based on an earlier system, Multics. In 1974, UNIX became the first operating system written in the C language. UNIX has evolved as a kind of large freeware product, with many extensions and new ideas provided in a variety of versions of UNIX by different companies, universities, and individuals. Partly because it was not a proprietary operating system owned by any one of the leading computer companies and partly because it is written in a standard language and embraced many popular ideas, UNIX became the first open or standard operating system that could be improved or enhanced by anyone. A composite of the C language and shell (user command) interfaces from different versions of UNIX were standardized under the auspices of the IEEE as the Portable Operating System Interface (POSIX). In turn, the POSIX interfaces were specified in the X/Open Programming Guide 4.2 (also known as the "Single UNIX Specification" and "UNIX 95"). Version 2 of the Single UNIX Specification is also known as UNIX 98. The "official" trademarked UNIX is now owned by the The Open Group, an industry standards organization, which certifies and brands UNIX implementations. UNIX operating systems are used in widely-sold workstation products from Sun Microsystems, Silicon Graphics, IBM, and a number of other companies. The UNIX environment and the client/server program model were important elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers. Linux, a UNIX derivative available in both "free software" and commercial versions, is increasing in popularity as an alternative to proprietary operating systems.                                        

   Linux

Linux® (pronounced LIH-nucks) is a UNIX®-like operating system created in the early nineties by Linus Torvalds with the assistance of numerous developers around the world. Linux is publicly available for download on the Internet at no cost and the source code is open to developers. The term “Linux” refers technically only to the kernel or core operating system, but has come to describe the complete operating system and suite of applications. Linux strives for POSIX compliancy (a set of standards defining a UNIX) to stay compatible with other UNIX-like operating systems.
Linux is known for its stability and encompasses all of the features one would expect from an operating system, including virtual memory, true multitasking, incredibly fast TCP/IP networking, shared libraries, and multi-user capabilities enabling hundreds of people to use one computer simultaneously. Originally developed for the home PC, Linux can now be run on a wide array of platforms including Macintosh®, PowerPC, Amiga and many others. Today, the operating system boasts over 10 million users worldwide and is still growing as programmers and general enthusiasts exchange ideas and contribute code to further develop the open source software known as Linux.

   AMIGA

The Amiga's operating system is old, and has had almost no official updates for over four years.

   Windows 95

Windows 95 is the easiest way to a 32-bit desktop with a reduced set of hardware requirements, comprehensive compatibility, and easy installation. Windows 95 makes everything you do on a PC easier, faster, and more fun - whether you use your computer to.

   Windows NT

Windows NT is an excellent choice for any organization that wants to increase productivity and reduce the total cost of ownership for desktop operating systems.

   Aegis/Exo-kernel (Massachusetts Institute of Technology)
The Aegis kernel is built around the idea of an exo-kernel. An evolution from micro-kernels, exo-kernels export a virtual machine that securely multiplexes resourses among mutually distrusting spplications. The exo-kernel philosophy tries to export as few abstractions besides the basic hardware abstractions as possible, and to implement as little policy in the kernel as possible. Like SPIN, Aegis relies on techniques such as downloading code into the kernel to make the system fast.

    AIOS
The AIOS project is aimed toward developing a new computer operating system based on artificial intelligence.

   Amoeba (Vrije Universiteit)
Amoeba is a powerful micro-kernel-based system that turns a collection of workstations or single-board computers into a transparent distributed system. It has been in use in academia, industry, and government for about 5 years.

    Angel (City University of London)
Angel is designed as a generic parallel and distributed operating system, although it is currently targeted towards a high-speed network of PCs. This model of computing has the dual advantage of both a cheap initial cost and also a low incremental cost. By treating a network of nodes as a single shared memory machine, using distributed virtual shared memory (DVSM) techniques, we have addressed both the needs for improved performance and provided a more portable and useful platform for our applications.

    BE OS
Be is a new multithreaded, multiprocessor microkernel OS that has been designed from the ground for multimedia applications. It currently runs on the Intel and some PowerPC platforms. [ Everything I've heard about Be sounds really good. Everyone who uses it seems to be very impressed - Patrick ]

   BSD/OS
BSD/OS is a commercial BSD 4.4-based UNIX operating system for the x86 PC. This is where many of the Berkeley CSRG people went after BSD UNIX research ended at Berkeley. A commercial-quality UNIX implementation with all of the bells and whistles one expects from a modern UNIX system.

   C Executive and PSX
C EXECUTIVE is a operating system kernel for embedded applications - providing a small, efficient, real-time software environment for programs written in C. C EXECUTIVE, as small as 5 KB in ROM space, is available on 8-, 16- and 32-bit CISC and RISC processors, providing the foundation for a common, corporate-wide, portable software strategy. PSX provides a single-user, single-group, subset of POSIX.1, with up to 32,000 preconfigured processes. PSX adds a substantial subset of the POSIX.1 system calls to the basic C EXECUTIVE kernel. Using these calls allows applications to migrate from POSIX-conformant UNIX platforms to board-level systems, or vice versa.

    Cache kernel (Stanford University Distributed Systems Group)

The supervisor-mode component of the V++ operating system. The Cache Kernel caches operating system objects such as threads and address spaces just as conventional hardware caches memory data. User-mode application kernels handle the loading and write-back of these objects, implementing application-specific management policies and mechanisms.

 Chimera  (Carnegie Mellon University)
The Advanced Manipulators Laboratory, at Carnegie Mellon University, has developed the Chimera Real-Time Operating System, a next generation multiprocessor real-time operating system (RTOS) designed especially to support the development of dynamically reconfigurable software for robotic and automation systems. Version 3.0 and later of the software is already being used by several institutions outside of Carnegie Mellon, including university, government, and industrial research labs.

   Choices (University of Illinois, Urbana-Champaign)
Choices is written as an object-oriented operating system in C++. As an object-oriented operating system, its architecture is organized into frameworks of objects that are hierarchically classified by function and performance. The operating system is customized by replacing subframeworks and objects. The application interface is a collection of kernel objects exported through the application/kernel protection layer. Kernel and application objects are examined through application browsers

   Chorus (Sun Microsystems)
CHORUS is a family of open micro-kernel-based operating system components to meet advanced distributed computing needs in areas such as telecommunications, internetworking, embedded systems, realtime, "mainframe UNIX", supercomputing and high availability. The CHORUS/MiX multiserver implementations of UNIX allow to dynamically integrate part or all of standard UNIX functionalities and services in the\ above application areas.

   COSY (University of Karlsruhe, University of Paderborn)
Cosy is an operating system for highly parallel computers, with hundreds or thousands of processors. All parts of the system are designed to scale up with the number of processors, without any one becoming a bottleneck.

    EROS (University of Pennsylvania)
EROS (Extremely Reliable Operating System) is a new operating system being implemented at the University of Pennsylvania. The system merges some very old ideas in operating systems with some newer ideas about scheduling and performance. The result is a small, secure, high-performance operating system that provides transparent orthogonal persistence coupled with microkernel-style critical paths, including a high performance IPC subsystem.

    Flux (University of Utah)
The Flux Project's objective is to develop a nanokernel-based decomposed operating system that achieves high performance while retaining inter-component protection and rich functionality. Such a system will overcome the performance/protection and performance/functionality tradeoffs that thwart traditional micro-kernel-based operating systems. This objective includes integration of selected research results of others, and free distribution of an unencumbered and usable version of the entire system.

    FreeBSD
FreeBSD is one of several free, monolithic BSD 4.4-lite derivative operating systems. It provides full UNIX support, including networking, X Windows, and almost all other normal UNIX services.

    GLUnix (University of California, Berkeley)
Currently, modern workstation operating systems do not provide support for efficient distributed program execution in an environment shared with sequential applications. The goal of our research is to pool resources in a NOW to provide better performance for both parallel and sequential applications. To realize this goal, the operating system must support gang-scheduling of parallel programs, identify idle resources in the network, allow for process migration to support dynamic load balancing, and provide support for fast inter-process communication.

    Grasshopper (University of Sydney)
Despite the fact that the basic idea behind orthogonal persistence is very simple, research groups are finding it extremely hard to develop scalable and efficient persistent stores. One of the major difficulties derives from the fact that persistence provides a fundamentally different model of computing from that supported by conventional operating systems. In this project we are investigating the requirements of an operating system to support persistence and propose to design and construct a new operating system, known as Grasshopper, which has explicit support for persistent systems.

    GUIDE
Guide (Grenoble Universities Integrated Distributed Environment) is an object-oriented distributed operating system for the development and operation of distributed applications on a local area networks connecting workstations and servers. Guide is a joint project of Bull and the IMAG Research Institute (Universities of Grenoble), which have created the Bull-IMAG joint Research Laboratory. It also has strong links with the COMANDOS Esprit Project (Construction and Management of Distributed Open Systems) and the BROADCAST Esprit Basic Research project.

    Harmony (National Research Council of Canada)
Harmony is a multitasking, multiprocessing operating system for realtime control, developed at the National Research Council to serve a need for a flexible system for realtime control of robotics experiments and for other applications of embedded systems where predictable temporal performance is a requirement. Harmony is extensible, configurable and portable, both across different target computers (typically assembled from single-board computers), and across different development hosts.

    Helios (Perihelion Distributed Software)
Helios is a micro kernel operating system for embedded and multiprocessor systems. The operating system is modular in design and can scale from an embedded runtime executive up to a fully distributed operating system.

    Hive (Stanford University Flash Project)
The Hive OS Team is designing an operating system that is able to operate effectively in a traditional supercomputer environment as well as in a general-purpose, multiprogrammed environment. The latter environment poses significant challenges since general-purpose environments typically contain large numbers of processes making many system calls and many small I/O requests.

    HURD (Free Software Foundation GNU Project)
The HURD is the operating system being developed by the Free Software Foundation as the basis for the GNU Project, which has already produced such well known tools as Emacs and GCC. The Hurd is a personality for the Mach micro-kernel which exports a bevy of services to the application. The Hurd will provide the standard UNIX interface, but should also be much more flexible than standard UNIX.

    Hurricane
The Hurricane operating system is a hierarchically clustered operating system implemented on the Hector multiprocessor. Hierarchical clustering manages the system resources in clusters, using tight coupling within a cluster, and loose coupling across clusters. Distributed systems principles are applied by distributing and replicating system services and data objects to increase locality, increase concurrency, and to avoid centralized bottlenecks, thus making the system scalable.

    Inferno
Inferno(tm) is a new network operating system and programming environment to deliver content in a rich environment of heterogenous networks, clients and servers. The Inferno system includes the Inferno kernel, the Limbo(tm) programming language, reference APIs that include interfaces for networking and graphics, network protocols, security and authentication, and various toolkits. Inferno was developed by members of the Computing Sciences Research Center of Bell Laboratories, the research arm of Lucent Technologies.

    ITRON
ITRON (industrial TRON) is a real-time operating system specification for embedded systems. Many products have been developed based on the ITRON specifications. ITRON is a de-fact standard operating system specification in Japan for consumer applications.

    Kea (University of British Columbia)
Kea is an operating system architecture designed to provide the applications running on top of it with the ability to dynamically replace parts of the system. Don't like your file-systems buffering policy? Write your own, the system will replace it for your application! Of course, there are lots of research and design issues over how we do this on a fine-grained scale, whilst maintaining system integrity and efficiency.

    KeyKOS
KeyKOS is an operating environment for S/370 computers which provides a high level of security, reliability, performance, and productivity. It allows emulation of other environnzens such as VM, MVS, and POSIX. Development of KeyKOS ceased in 1990. Many of the ideas of this system have been carried forward into EROS

    Lites
Lites is a 4.4 BSD Lite based server and emulation library that provides free unix functionality to a Mach based system. Lites provides binary compatibility with 4.4 BSD. NetBSD (0.8, 0.9, and 1.0), FreeBSD (1.1.5 and 2.0), 386BSD, UX (4.3BSD) and Linux on the i386 platform. It has also been ported to the pc532, and PA-RISC. Preliminary ports to the R3000 and Alpha processors have also been made.

      Lynx (Lynx Real-time Systems)
LynxOS is a proprietary UNIX-like real-time operating system. LynxOS looks and feels like UNIX from the user/programmer point of view. It was developed from the ground-up with high performance, deterministic hard real-time response in mind. Although LynxOS is conformant with POSIX 1003.1 it is not derived from any AT&T/USL/Novell source code. The OS is in effect a complete re-implementation of UNIX from a real-time perspective.

    Mach (Carnegie Mellon University)
Mach is one of the giants in the operating systems research community. Originally started at CMU, Mach has become the basis for many research systems. Although work on Mach at CMU has largely stopped except real-time work and multi-server work, many other groups are still using Mach as the basis for research.

    Mach at OSF (OSF Research Institute)
The OSF Research Institute is using the Mach technology started at CMU and is using it as the basis for several areas of research, including operating systems for parallel machines, trusted object-oriented kernels, and other OS research areas.

    Mach-US (Carnegie Mellon University)
The Mach-US system is an OS developed as part of the CMU MACH project. It is comprised of a set of servers, each of which supports orthogonal system services. For example, instead of one server supplying all of the system services as under the Mach BSD4.3 single server (UX), the Mach Multiserver (Mach-US) has several servers: a task server, a file server, a tty server, an authentication server, a network server, etc. It also has and emulation library that is mapped dynamically into each user process, and uses the system servers to support the application programmers interface (API) of the UNIX operating system.

    Maruti (University of Maryland)
Maruti is a time-based operating system research project at the University of Maryland. With Maruti 3.0, we are entering a new phase of our project. We have an operating system suitable for field use by a wider range of users, and we are embarking on the integration of our time-based, hard real-time technology with industry standards and more traditional event-based soft- and non-real-time systems.

    Masix (Blaise Pascal Institute MASI Laboratory)
Masix is a distributed operating system, based on the Mach micro-kernel, currently under development at the MASI Laboratory. Its primary goal is the simultaneous execution of multiple personalities, in order to run concurrently on a same workstation applications from the Unix, DOS, OS/2 and Win32 worlds. Furthermore, Masix pools the resources of a workstation local area network, independently from the personalities that run on each node. Masix also provides distributed services to the personalities.

    Merlin (University of Sao Paulo)
An object-oriented, reflective operating system based on the Self programming language.

    MetaOS (University of Victoria)
MetaOS is an object-oriented system model, based on meta-levels, meta-spaces, meta-objects, and meta-interfaces, that allows applications to securely customize their run-time environment on the fly. Furthermore, it allows applications to share customizations with other applications, allows different types of security schemes to be implemented, and permits secure, remote troubleshooting of software.

    MOSIX (Hebrew University, Jerusalem, Israel)
A solution to the NOW problem is now available in the form of a multicomputer operating system enhancements, called MOSIX. MOSIX is an enhancement of UNIX which allows users to use the resources of a NOW configuration, without any change to the application level. By using transparent, dynamic process migration algorithms, MOSIX enhances the network services, i.e. NFS, TCP/IP, of UNIX, to the process level, by supporting load balancing and a dynamic work distribution (leveling) in clusters of homogeneous computers.

    Mungi (University of New South Wales)
A new operating system based on a single, flat virtual address space, orthogonal persistence, and a strong but unintrusive protection model.

    Nemesis (University of Cambridge, UK)
Pegasus has produced an entirely new operating system, whose design is geared to support of high-performance applications which require a consistent quality of service (QoS), such as those which use multimedia. This operating system, called Nemesis, currently runs on a number of platforms including Intel, Alpha (21064 and 21164) and StrongARM. Nemesis is a single-address-space system, with an extremely light-weight kernel ('Nemesis Trusted Supervisor Code', or NTSC), and strong emphasis on performing operating system functions in the user's domain (thus avoiding the need for expensive protection regime changes).

    NetBSD
NetBSD is one of two free, monolithic BSD 4.4-lite derivative operating systems. It provides full UNIX support, including networking, X Windows, and almost all other normal UNIX services. Unlike many other free UNIX implementations, NetBSD has also been ported to a large variety of hardware platforms.

    Oberon
Oberon is a freely distributable OS written in the Oberon language in the Pascal-Modula tradition, developed by Niklaus Wirth and Juerg Gutknecht at the Institute for Computer Systems at the Swiss Federal Institute of Technology in Zurich (ETHZ). The most recent development of it is called Oberon System 3. It is available as Native Oberon for Intel-based machines which is self-contained and makes no use of any alien software layer. In addition, ports exist for all flavors of Windows: 3.1, 3.11, 95 and NT, as well as for Linux and for Macintosh.

    Opal (University of Washington)
Opal is a single-address-space operating system for 64-bit architectures. All Opal threads execute with a single global address space. The existence of a single address space simplifies sharing of complex (pointer-rich) data among cooperating applications, as well as persistent storage of that data, because pointers have the same meaning to all threads for potentially all time. Opal provides protection in the single address space; each thread executes within a protection domain that defines which pages it has the right to access.