Smartphone Operating System (779883), страница 6
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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. A Unix operating system builds device software (calleddrivers) into its model of files; the ‘open’ system call executes a devicedriver if the user is opening a device for access. Microsoft Windowscreates an API for each device – usually layers of APIs for each device.Windows has some of the ideas of Unix; it uses the nomenclature offiles to address device ports.
But it also designs the access to devicesthrough unique APIs, not file I/O mechanisms. By contrast, consider theapproach that Symbian OS takes to communication. In Symbian OS,servers are used to allow access to communication resources. While aSymbian OS user would still use an ‘open’ call (again using the Unixfile nomenclature) to gain access to a serial port, she would first have toconnect to the server that provides access to that port before she couldopen the port.In all models, concepts of abstraction and modularity are preserved.Actual implementations are not specified; the abstraction of the conceptual model is the important part at this stage.
Much research and opinionhas been dedicated to the question of which model is best; both modelshave held up well under such scrutiny.COMPUTER STRUCTURES19The evolution of operating systems is usually spurred by changes inhardware and models that address these changes. The roots of SymbianOS give a good example of this evolution. As we discussed in Chapter 1,Symbian OS finds its roots in EPOC, an operating system developedfor handheld computers.1 Mobile phones were a burgeoning technology and EPOC’s designers wanted to address that technology. EPOC,however, had no models to address telephony and therefore did notextend well to phone-based devices.
EPOC evolved into Symbian OS toaddress mobile phones. As evidence of this evolution, one can spot muchcode in Symbian OS that has been derived from EPOC. New modelsaddressing new technology had to be developed, naturally, and one cansee how new hardware and new technology drove operating systemdevelopment.All Operating Systems EvolveAll operating systems evolve. Some go extinct; some survive. There aremany references for operating system evolution.
Check outwww.levenez.com/unix for a ‘genealogical’ look at the Unix operating system. The evolution of Microsoft Windows is documented byMicrosoft at www.microsoft.com/windows/WinHistoryIntro.mspx.The evolution of operating systems that run on Apple computerscan be found at www.kernelthread.com/mac/oshistory. Evolution ofMS-DOS, the Microsoft operating system built for early PCs, continued until Microsoft Windows arrived.
MS-DOS formed a foundationfor Microsoft Windows until Microsoft Windows 2000, but was thenrelegated to extinction.2.2 Computer StructuresAs we consider how operating systems address computer systems, weshould first outline what structures those systems are built from and how1Of course, you can think of the mobile phones on which Symbian OS runs as handheldcomputers. By definition, mobile phones are indeed handheld computers. However, wedistinguish handheld computers from mobile phones by defining handheld computers asa generic term describing computers that do not use telephony.
Mobile phones are, then,handheld computers that use telephony.20THE CHARACTER OF OPERATING SYSTEMSthey are used. The character of an operating system is determined, inpart, by the structures it has to address.System Structure and OperationKernel StructuresThe core programs and data of an operating system together comprisethe kernel. The kernel consists of the code that runs the operating systemon a CPU and the data – typically organized in tables – that are used tokeep track of how things are running on an operating system. The kernelis where the access to hardware is done and the kernel implements theoperating system’s design model.There are several types of kernel. Monolithic kernels are found onsome general-purpose computers; they implement all operating systemfunctions and hardware abstractions within the kernel itself.
This type ofkernel (see Figure 2.1) usually comprises large amounts of code and largeamounts of memory for system tables.Microkernels provide only a small set of system function and hardwaremodels. Much of the remaining functionality that might be found in amonolithic kernel is provided by server applications that run outside amicrokernel (see Figure 2.2).
Servers in Symbian OS provide this type offunctionality.User softwareKernelHardwareFigure 2.1 Monolithic kernel structureCOMPUTER STRUCTURESUsersoftware21ServersKernelHardwareFigure 2.2Structure of a microkernelHybrid kernels are like microkernels, except that some of the externalapplication function is implemented in the kernel for performance reasons(see Figure 2.3).UsersoftwareServersKernelServersHardwareFigure 2.3Hybrid kernel structureLinux is typically considered a monolithic-kernel operating system.Most system functions are implemented in ‘kernel space’ (by the code22THE CHARACTER OF OPERATING SYSTEMSand within the memory of the kernel). Symbian OS is implemented viaa microkernel. The example in Section 2.1 of defining and opening acommunication device serves well here.
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