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Chapter: Embedded Systems Design : Real-time operating systems

Choosing an operating system

Comparing an operating system from 10 years ago with one offered today shows how operating system technology has devel-oped over the past years.

Choosing an operating system


Comparing an operating system from 10 years ago with one offered today shows how operating system technology has devel-oped over the past years. Although the basic functions provided by the old and the newer operating systems — they all provide multitasking, real-time responses and so on — are still present, there have been some fundamental changes in the improvement in the ease of use, performance and debugging facilities. Compar-ing a present-day car with one from the 1920s is a good analogy. The basic mechanics and principles have largely remained un-changed — that is, the engine, gearbox, brakes, transmission — but there has been a great improvement in the ease of driving, comfort and facilities. This is similar to what has happened with operating systems. The basic mechanisms of context switches, task control blocks, linked lists and so on are the basic fundamen-tals of any operating system or kernel.


As a result, it can be quite difficult to select an operating system. To make such a choice, it is necessary to understand the different ways that operating systems have developed over the years and the advantages that this has brought. The rest of this chapter discusses these changes and can help formulate the crite-ria that can be used to make a decision.

Assembler versus high level language


In the early 1980s, operating systems were developed in response to minicomputer operating systems where the emphasis was on providing the facilities and performance offered by mini-computers. To achieve performance, they were often written in assembler rather than in a high level language such as C or PASCAL. The reason for this was simply one of performance: compiler technology was not advanced enough to provide the compact and fast code needed to run an operating system. For example, many compilers from the early 1980s did not use all the M68000 address and data registers and limited themselves to only one or two. The result was code that was extremely inefficient when compared with hand coded assembler which did use all the processor’s registers.


The disadvantage is that assembler code is harder to write and maintain compared to a high level language and is extremely difficult to migrate to other architectures. In addition, the interface between the operating system and a high level language was not well developed and in some cases non-existent! Writing interface libraries was considered part of the software task.


As both processor performance and compiler technology improved, it became feasible to provide an operating system written in a high level language such as C which provided a seamless integration of the operating system interface and appli-cation development language.


The choice of using assembler or a high level language with some assembler compared to using an integrated operating sys-tem and high level language is fairly obvious. What was accept-able a few years ago is no longer the case and today’s successful operating systems are highly integrated with their compiler tech-nology.


ROMable code


With early operating systems, restrictions in the code devel-opment often prevented operating systems and compilers from generating code that could be blown into read only memory for an embedded application. The reasons were often historic rather than technical, although the argument that most applications were too big to fit into the relatively small size of EPROM that was available was certainly true for many years. Today, most users declare this requirement as mandatory, and it is a standard offer-ing from compilers and operating system vendors alike.


Scheduling algorithms


One area of constant debate is that of the scheduling algo-rithms that are used to select which task is to execute next. There are several different approaches which can be used. The first is to switch tasks only at the end of a time slice. This allows a fairer distribution of the processing power across a large number of tasks but at the expense of response time. Another is to take the first approach but allow certain events to force switch a task even if the current one has not used up all its allotted time slice. This allows external interrupts to get a faster response. Another event that can be used to interrupt the task is an operating system call.


Others have implemented priority systems where a task’s priority and status within the ready list can be changed by itself, the operating system or even by other tasks. Others have a fixed priority system where the level is fixed when the task is created. Some operating systems even allow different scheduling algo-rithms to be implemented so that a designer can change them to give a specific response.


Changing algorithms and so on are usually indicative of trying to squeeze the last bit of performance from the system and in such cases it may be better to use a faster processor, or even in extreme cases actually accept that the operating system cannot meet the required performance and use another.


Pre-emptive scheduling


One consistent requirement that has appeared is that of pre-emptive scheduling. This refers to a particular scheduling algo-rithm where the highest priority task will interrupt or pre-empt a currently executing task irrespective of whether it has used its allotted time slice, and will continue running until a higher level task is ready to. This gives the best response to interrupts and events but can be a little dangerous. If a task is given the highest priority and does not lower its priority or pre-empt itself, then other tasks will not get an opportunity to execute. Therefore the ability to pre-empt is often restricted to special tasks with time critical routines.


Modular approach


The idea of reusing code whenever possible is not a new one but it can be difficult to implement. Obvious candidates with an operating system are device drivers for I/O , and kernels for different processors. The key is in defining a standard interface which allows such modules to be reused without having to alter or change the code. This means that memory maps must not be hardwired, or assumptions made by the driver or operating system. One of the problems with early versions of many operat-ing systems was the fact that it was not until fairly late in their development that a modular approach for device drivers was available. As a result, the standard release included several driv-ers for the same peripheral chip, depending on which VMEbus board it was located.


Today, this approach is no longer acceptable and operating systems are more modular in their approach and design. The advantages for users are far more compact code, shorter develop-ment times and the ability to reuse code. A special driver can be re-used without modification. This coupled with the need to keep up with the number of boards that need standard ports has led to the development of automated build systems that can take modular drivers and create a new version extremely quickly.


Re-entrant code


This follows on from the previous topic but has one funda-mental difference in that a re-entrant software module can be shared be many tasks and also interrupted at any point and reused without any problems. For example, consider module A which is shared by two tasks B and C. If task B uses module A and exits from it, the module will be left in a known state, ready for another task to use it. If task C starts to use it and in the middle of its execution is switched out in favour of task B, then the problem may appear. If task B starts to use module A, it may experience problems because A will be in an indeterminate state. Even if it does not, other problems may still be lurking. If module A uses global variables, then task B will cause them to be reset. When task C returns to continue execution, its global data will have been destroyed.


A re-entrant module can tolerate such interruptions with-out experiencing these types of problems. The golden rule is for the module to only access data associated with the task that is using the module code. Variables are stored on stacks, in registers or in memory areas specific to the task and not to the module. If shared modules are not re-entrant, care must be taken to ensure that a context switch does not occur during its execution. This may mean disabling or locking out the scheduler or dispatcher.


Cross-development platforms


Today, most software development is done on cross-development platforms such as Sun workstations, UNIX systems and IBM PCs. This is in direct contrast to early systems which required a dedicated software development system. The degree of platform support and the availability of good development tools which go beyond the standard of symbolic level debug have become a major product selling point.


Integrated networking


This is another area which is becoming extremely impor-tant. The ability to use a network such as TCP/IP on Ethernet to control target boards, download code and obtain debugging information is fast becoming a mandatory requirement. It is rapidly replacing the more traditional method of using serial RS232 links to achieve the same end.


Multiprocessor support


This is another area which has changed dramatically. Ten years ago it was possible to use multiple processors provided the developer designed and coded all the inter-processor communi-cation. Now, many of today’s operating systems can provide optional modules that will do this automatically. However, mul-tiprocessing techniques are often misunderstood and as this is such a big topic for both hardware and software developers it is treated in more depth later in this text.

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