Embedded Systems September 2000 Vol13_10

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M ICHAEL MELKONIAN ,ffb, ,,,, .• Get by Without an RTOS Too many simple systems use a commercial RTOS. All that's sometimes needed is a way to implement even-dri- ven and periodic functions. Here's an easy way to do just that. t's a fac t of life that many embedded systems sur- vive perfectly well without a multitasking real-time operating system (RTOS). I have always wondered if I could find a single cr -iterion that would tell me if it would be an advantage or a liability to include an RTOS fo r a particular proj ect. How hard is it to guess if an RTOS is hidden inside an embedded box simply by looking at it, using it, and reading the owner's manuals? Do you think your VCR has an RTOS hidden inside? What about your car? And your mobile phone? (Maybe, just as some car models proudly state their engine size and the number of valves on the back, an embedded system should clearly state "DRIVEN BY XOS. umbe r of tasks: 24. RAM size: 2MB.") Chances a re your guess will be wrong. A really com- plicated, large sys tem may use an infinite loop plus a couple of ha rd wo rking interrupts. On th e oth er hand, a small e r system with less than 20,000 lin es of C code may 146 SEPTEMBER 2000 Embedded Systems Programming have a full-bl own commercial RTOS. Le t 's consider two extreme cases. First, conside r a primitive software-driven refrigera tor. Our refri ge ra to r software simply monitors and control th e tempe ra ture and handles the door- open switch and the lamps. Even a ha rd-core advocate of ope rating sys tems would hardly insist on procuring a suita ble JUOS. At the other end of the scale, consider something much more complex (and not normally viewed as an embedded system), something probably sitting in a close proximity to you right now. A personal compute r. I guess only a few real- ly brave individuals would proclaim that a proper multitask- ing operating system on the de ktop i · an unnecessary luxu- ry. But wait! You only need to look back a decade or so. Back then, the world 's PCs weren't driven by multitasking operat- ing systems. In fact, DOS simply provided some software interfaces to the ha rdware, some memory management functions, and a few other bits and pieces.

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