April 1998, Issue 93

Software Development for RTOSs

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REAL-TIME PROGRAMMING LANGUAGES

The most common is development language for real-time PC development is C. As you likely already know, C was developed originally to implement Unix in a portable way and has now been around for quite some time.

C has been a favorite with embedded-systems developers, too. While not as efficient as coding in assembler language, C is portable and enables the programmer to access memory and I/O port resources.

C compilers exist for almost any microprocessor architecture from supercomputer architectures like Cray, and 64- and 32-bit processors, like Pentium and PowerPC, all the way down to PIC processors. Well, OK, I haven’t seen a C compiler for a DEC PDP-8.

While it’s possible to use assembly language in all PC-based RTOSs, it is probably wise to avoid coding in assembly unless absolutely necessary. Most modern C compilers are good at optimizing code for particular architectures (i.e., i486 vs. Pentium).

It is almost never necessary to code in assembly language for speed. Besides, unless your application is very high volume, the increased time and costs associated with developing in using assembly are almost always higher than just going with a faster processor.

For example, it may take 80% of the development effort to squeeze that last 20% of speed from an application by coding up critical routines in assembly language and hand-optimizing them. Speed increases achieved by using a faster CPU and bigger faster cache architecture are usually cheaper.

About the only time you want to code in assembly, is when you’d like to implement low-latency interrupt service routines, or stubs for ISRs, which allow setting up the environment necessary to call C routines. Luckily, many RTOS vendors have done a good job providing these stubs as part of their API.

C++ has been popular in desktop systems for quite some time now and is starting to become more popular in embedded systems. While there is nothing magic about using C++, tool support for C++ in development systems for RTOS has been slow in coming. However, currently many RTOS vendors have tool and library support for C++.

However, just because you code your applications in C++ doesn’t mean you’re necessarily using object-oriented programming (OOP) methodology. But, OOP isn’t simply a methodology. You can also do OOP in C or assembly language. C++ is just a tool.

Another OOP language you may have heard getting a lot of press lately is Java. Java was developed by Sun Microsystems as a new network programming technology.

The idea here is to use the Java programming language to develop code, which gets compiled into byte code to be executed in an interpreter on a target systems. It promises architecture-independent application development.

Several RTOS vendors offer Java support, but there are still some issues to be worked out. In particular, the Java run time relies on garbage collection, which in its current reference implementation from Sun is nondeterministic and thus not suitable for hard real-time applications. Work is being done on this, but more about Java in a later column.

Fortran is still used in embedded systems. In particular, there are numerical libraries which are coded in Fortran. These libraries represent much time in development, testing, and tuning.

Since it’s harder to find a Fortran compiler that can be hosted on current development platforms, some libraries are being converted to C. One solution is to use Fortran-to-C converters, which you can find on the Internet.

Unless you code for the government or systems that need to be flight qualified, you probably won’t see ADA. ADA is a good for implementing large systems where many developers are working in different organizations, but it’s sometimes difficult to use for expressing real-time issues.

That’s not to say that there aren’t some RTOSs for embedded systems which are written in Ada. For example, RTEMS, which is developed by the US Army and distributed for free, is available in Ada or C source.

Also, there’s trend towards using high-level behavioral modeling language for systems. In these systems, the modeling language describes what the system components do. Typically, hardware components modeled in these systems are then synthesized into hardware-description languages like VHDL or Verilog, and software components with languages like in Ada or C, which are then compiled using standard compilers.