Concepts with Symbian OS (779878), страница 3
Текст из файла (страница 3)
Management of that hardware resource is thejob of the operating system.Software manipulates hardware resources through an application programming interface. APIs can be provided by the operating systemdesigner or by a third party. Software does not usually work with hardware directly, but manipulates resources by communicating with theoperating system through a function call interface. The operating systembuilds a model of the hardware and provides system function calls thataccess that hardware model in specific ways (see Figure 1.2).Consider the previous message manager example. The operating system has many choices to make as it works with messages.
It could, forexample, store the message text in a file and give an application a wayto find the file name and to work with that file directly. The applicationwould have to open the file (again, through the operating system resourcemodel) and process the raw message data. Another way to present themessage would be to store the message in a file, but present an applicationwith an abstract object called a ‘text message’ that the application couldwork with. The application would make function calls that the operatingsystem would intercept, deriving information about the message andreturning that information. The application would not be aware of wherethe object was stored. These are two models of message handling: oneUser ApplicationAPIsSystemCallsOperating SystemFigure 1.2 Structure of access to an operating system6INTRODUCTION TO MOBILE PHONE SYSTEMSmore raw and direct, the other more abstract and object-oriented.
Thechoice that the operating system makes about which one to use buildsthe character of the operating system.A good system model is one that effectively and transparently providessoftware with an intuitive way to access system resources. System modelsare often based on abstraction.
Abstraction involves the hiding of irrelevant data and the presentation of only useful, relevant information. Weoften label abstractions as ‘objects’. For example, system resources arethe abstract objects that the operating system presents to the software.They might represent a resource as a hardware object, with the detailabstracted away, or as a set of functions that can use the hardware. A fileis an abstract object that represents a way to use hardware storage.
A textmessage is an abstract object that represents a way to use both softwareand hardware resources to access that message. These are concepts builtand supported by the operating system and provided to applications.A good operating system has more goals than simply providing a usefulmodel to software applications.• Robustness : a good operating system is reliable and tolerates problems well. The system does not stop working due to isolated hardwareor software errors and fails gracefully if it must deal with severalerrors at the same time. Robust operating systems provide services tosoftware unless the hardware fails.• Scalability : a good operating system incorporates resources as theyare added to the system. This can be transparent to the user – the bestway – or can involve some kind of user interaction.
The plug-and-playconcepts of Microsoft Windows – where devices are discovered andinstalled automatically – is an example of good scalability. On theother hand, old versions of Linux used to require recompilation of theoperating system when new devices were added. This is an exampleof bad scalability.• Extensibility : the operating system should be designed to adapt tonew technologies that extend the operating system beyond the pointat which it was implemented. For example, it should be able toadapt to new forms of file storage without a complete redesign of theoperating system.• Throughput (the work that a processor can complete in a specifictime period) : an operating system must perform well and achieveHISTORY OF OPERATING SYSTEMS7high throughput.
A good operating system minimizes the time spentproviding services while maximizing throughput.• Portability : a good operating system should be portable, that is, ableto be run on many different hardware platforms.• Security : an operating system must be secure. It must prevent unauthorized users and processes from accessing stored data and systemservices.Many Operating Systems Fit the BillEven though the list of criteria for a good operating system looks a bitdaunting, many operating systems have been created over the yearsthat meet these criteria. In addition, many operating systems did someof these very well and steered the industry in one particular area.
A listof operating systems can be found at http://en.wikipedia.org/wiki/List of operating systems.Many operating systems are not very portable. They are specificallydesigned to run on a single platform. In addition, you will note that‘popularity’ is not an item on the criteria list. Most operating systemswere not popular, yet were designed to address a specific systemmodel.1.2 History of Operating SystemsOperating systems are the heart of every general-purpose computer. Since1957, operating systems have been an essential component of computers.This section outlines a brief history of operating systems, highlighting thehistory of Symbian OS.General-Purpose Operating SystemsThe earliest computers did not have operating systems. They were dedicated computing devices that performed a single task, thereby needingonly one ‘program’ to execute.
From the ancient Incas in Central Americato the Difference Engine constructed by Charles Babbage in 1847 tothe early days of modern computing (the ENIAC in 1946, the Mark I in1948), early computers focused on single tasks that had direct access tohardware and no operating system.8INTRODUCTION TO MOBILE PHONE SYSTEMSOperating systems were invented when it became clear that accessto ‘the system’ needed to be standardized.
Until the mid-1950s, programmers wrote their own routines for accessing resources, particularlysystem input and output. Patterns of programming were beginning toemerge, such as repeated use of certain mathematical functions. Theneed for basic, standardized operating system functionality, includingdevice drivers and execution libraries, was becoming apparent. Criticalmass was reached as computer systems were designed to allow queuingof jobs, or programs, to run one after the other.The first operating system was released in 1957. Called BESYS, thisoperating system was implemented by Bell Labs to handle the executionof many short programs, queued up so that the operators did not haveto load each program just prior to its execution.
BESYS shared CPUtime between several jobs at once, thus making it the first multitaskingoperating system.Operating system research and implementation moved very fast in the1960s. Two influential examples were OS/360, released by IBM in 1964,and MULTICS, released by Bell Labs, MIT and General Electric in 1965.OS/360 was influential because it combined a powerful commandlanguage with the ability to run many jobs at once.
The commandlanguage controlled job execution and specified how each job wasto access resources. In addition, OS/360 worked on various computermodels; it became the standard among batch processors.MULTICS was influential because it took a very different approach fromOS/360: it allowed users to use the operating system directly. It had aunique structure – using a central core of software called a ‘kernel’ – andallowed users to extend the operating system through software basedon the kernel.
Based on the foundational ideas introduced in MULTICS,Unix was invented at Bell Labs by a man named Ken Thompson in 1972.Thompson teamed with Dennis Ritchie, the author of a programminglanguage called ‘C’, to produce the source code of the Unix operatingsystem in that language. Unix was distributed almost free of charge and,in the 1970s, it spread to many platforms.Since the spread of Unix, there have been many developments in operating systems. One of the biggest was brought about by a development incomputers: the personal computer. The ideas invented by MULTICS andhoned by Unix were streamlined to fit into a personal computer with theintroduction of MS-DOS in 1981. MS-DOS ran on an IBM PC using theIntel 8088 chipset. Its first version was indeed primitive, but as hardwareresources were improved upon and faster processors with more memoryHISTORY OF OPERATING SYSTEMS9were packaged as desktop computers, MS-DOS evolved into MicrosoftWindows and has taken on many of the foundational concepts embeddedin Unix.As we look at the evolution of operating systems, it is interesting to seethe progression of computer resources that also evolved:• computers started by running one task at a time and have progressedto running many tasks at the same time• storage hardware has evolved from needing a large physical size foronly 100 KB of data to packing 100 GB into a matchbox-sized disk• electronic storage has made access much faster• memory has progressed from only a few kilobytes to many gigabytes;even handheld and mobile phone platforms sport 128 MB (and larger)memories• communication has gone from none to a large collection of possibilities: wired and wireless, serial and parallel, radio and infrared.Operating systems have developed to take advantage of all of theseaspects of computer hardware.Symbian OSHandheld devices were developed in the late 1980s as a way to capturethe usefulness of a desktop device in a smaller, more mobile package.Although the first attempts at a handheld computer (for example, the AppleNewton) were not met with much excitement, the handheld computersdeveloped in the mid-1990s were better tailored to the user and the waythat they used computers ‘on the go’.
By the turn of the 21st century,handheld computers had evolved into smartphones – a combination ofcomputer technology and mobile phone technology. Symbian OS wasdeveloped specifically to run on the smartphone platform.The heritage of Symbian OS begins with some of the first handhelddevices. The operating system began its existence in 1988 as SIBO (anacronym for ‘16-bit organizer’).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
