Concepts with Symbian OS (779878), страница 5
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Distributed systems assume that they are connected bysome sort of communication network.14INTRODUCTION TO MOBILE PHONE SYSTEMSThere are several models of distributed systems that operating systemshave taken advantage of. The client–server model views some computersas servers, that is, providing a service of some sort, and some computersas clients that ask for and receive a service. Web browsing is a distributedactivity that is based on the client–server model. Browsers are clientsthat ask servers for pages.
Peer-to-peer distribution is a model in whichcomputers are both servers and clients, using some and being used byothers. The interdependent model is a peer-to-peer model where peersare tightly interconnected, such that they cannot operate if other peersare not also functioning. In the interdependent model, each peer hasfunctions that are crucial to the entire network’s operations.There are several examples of operating systems for distributed computing systems.
Good examples of the client–server model are the manydistributions of Linux. The appeal of Gentoo Linux is that it is solely basedon the Internet for its distribution. It uses the Internet for upgrading itself,for installing itself and for updating its applications. For these uses, theoperating system is a client, communicating with one of many Gentooservers.For an example of an interconnected distributed operating system wehave to go back to the 1980s. During those years, an operating systemcalled Domain/OS was implemented that ran on computers made by theApollo company. Domain/OS was a version of Unix that was truly distributed between computers on a network.
The execution of a commandor program might occur on the local computer a user was connected to orit might occur on another computer in the network. No matter where thecommand was executed, the results – text or graphics – appeared on thelocal screen. The decision about which specific computer executed anygiven command was based on an algorithm, which made the locationdecision based on factors such as load and network performance.Handheld SystemsAs computers inexorably shrank in size, handheld devices becamefeasible. These computers – usually fully fledged systems with all theperipherals and issues of desktop systems – fit into and can be used withone hand. At first glance, these systems look as if they could simply takeon the operating systems of their bigger siblings, but they pose someunique challenges.First, the internal environment is more restrictive.
Less memory, lessstorage space and slower processors all dictate that the operating systemCOMPUTER SYSTEMS AND THEIR OPERATING SYSTEMS15must be tailored for a handheld environment, not just shrunk. Often,memory becomes ‘disk’ space: memory space is shared between astorage system and memory used by the system to run programs. Theearly Palm handhelds had 2 MB of memory for operating system spaceand file storage. In the face of these restrictions, the conventional modelsof operating systems change to accommodate the different environment.Secondly, resources must be handled with more care. The resourceson a handheld platform are more fragile – in the sense that a restrictedenvironment puts more of a load on a resource. A restricted environmentleaves less room for software to protect a resource.
This means that anoperating system must have a good model in place for dealing withresource access from multiple sources.Thirdly, power restraints are crucial. While desktop systems are alwaysconnected to AC power, handheld systems are almost always run onbatteries. Extensive running of hardware resources drain battery lifedramatically. And power loss must be handled gracefully.These considerations mean that an operating system must be writtenspecifically for a handheld device. It faces many pressures; it must supportthe multiprogramming of a desktop system in a (sometimes severely)restricted environment that must sip battery power while coordinatingaccess to many resources.
This is a considerable task, but operatingsystems have risen to handle it. Linux has been scaled to fit on severalhandheld devices. Microsoft Windows has also been fitted for handheldplatforms. The early versions of Symbian OS were designed for a handheldenvironment.Mobile Phone SystemsAs even handheld devices got smaller, it became possible to fuse a handheld device with a mobile phone. All the considerations of a handheldplatform are multiplied when a handheld device becomes a communications tool.
All the restrictions and issues are present while the systemrequirements take on communication issues as well. The resource modelof the handheld platform is now augmented with communications andthe functionality that comes with those communications.On a mobile phone, the environment restrictions can be even moresevere than on a handheld device. The data requirements of multimediacommunication – text messages, phone calls, photographs, video clipsand MP3s – are tremendous, yet must fit onto a restricted storage space.A mobile phone now has even more resources that must be carefully dealt16INTRODUCTION TO MOBILE PHONE SYSTEMSwith. And power is even tighter than normal, as the power requirementsof a mobile phone are much higher than that of a handheld device.In the face of even tighter constraints, operating systems have risento the challenge.
Several operating systems, such as Symbian OS, havebeen tailored for mobile phones.Real-time SystemsA real-time system is a special-purpose computer system where rigid timerequirements have been placed on either the processor or input/outputoperations. These time constraints are well-defined and system failureoccurs when they are not met.Real-time systems come in two varieties. Hard real-time systems guarantee that time constraints are met. Soft real-time systems place a priorityon time-critical processes. In both cases, real-time systems have a specific structure.
Any time-consuming task or device is eliminated andreal-time service often comes from a dedicated computer. Disk drives orslow memory cannot be tolerated. All system services – hardware or software – must be bounded ; that is, they must have specific response-timeboundaries or they cannot be used.In a sense, some mobile phone functions are real-time functions. Theservice of a phone call, for example, is a real-time service. But mostfunctions of a mobile phone can be carried out by a non-dedicated,general-purpose operating system designed for the mobile phone platform.Symbian OS was not initially a real-time operating system but the latestversions (Symbian OS v9 onwards) are powered by a real-time kernel.1.4 SummaryThis chapter has introduced the idea of an operating system and its relationships to both hardware and software.
We defined what an operatingsystem is and discussed the modeling that an operating system does forboth hardware and software. We examined the operating systems from ahistorical perspective and an operational perspective.The next chapter considers the character of operating systems. Itdiscusses some of the common features of operating systems as they existtoday and makes some working definitions that we use throughout thebook.
We also take a much closer look at the central operating system ofthis book: Symbian OS.2The Character of Operating SystemsLike humans, operating systems have a character. The character of anoperating system is the collection of design ideas, software componentsand usage policies that you find in its implementation. This collection gives an operating system identifying marks and is the reason thatpeople can celebrate certain facets or commiserate about features theystruggle with.The character of an operating system can be found in how it isimplemented on various types of hardware.
That character evolves overtime, especially as the operating system takes various shapes throughversions of its implementation.This chapter looks at the various aspects of an operating system’scharacter by introducing operating system concepts. We begin by lookingat how operating systems evolve over time and see how operating systemsview certain concepts – from ideas of disk storage to software protection.We then take a look at several examples: specific operating systems andtheir character.2.1 The Evolution of Operating SystemsAs we saw in Chapter 1, the first operating system was released in 1957.This operating system, BESYS, was closely matched to the hardware it ranon. Since this first introduction, operating systems have evolved as thehardware they run on has evolved.
New concepts have been designedand implemented; some have caught on and some have died out.18THE CHARACTER OF OPERATING SYSTEMSOperating system design begins with a conceptual model of computerstructures. Each operating system embodies a model of the hardware onwhich it is running. Good operating systems weave this model throughouttheir design. They set up and implement abstract concepts and let variousimplementations put those concepts to use on the hardware platform onwhich the implementations run.A good example here is the concept of a server. The idea of a serverwas initially developed as a provider of service to other computers.Operating system designers needed a way to protect a system resourcewhile providing easy access to it.
In addition, this access needed tobe provided abstractly, that is, in a modular way that hid the server’simplementation. So the idea of a server was an application that wouldprotect resources while providing access to those resources by answeringmessage-based requests. This is an interesting implementation. Certainly,this could have been done using different concepts and in different ways,but the server has proven to be an effective conceptual model of resourcemanagement. Servers are used in many operating systems to work withall kinds of resources.As another example, consider two different approaches to communications. The Unix operating system uses a file-oriented model to provideaccess to communication resources. In a Unix system, if a user needsaccess to a serial port, for example, she might ‘open’ a ‘file’ called/dev/ttya.
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