Chris Moos has published a new Arduino library - avr-os, that enables an Arduino to multitask. in Chris' words
The library uses pre-emptive multitasking to switch tasks and each task has its own stack that is restored when a task is resumed. An AVR timer is used to provide ticks and this interrupt is used to switch tasks.
This looks promising and adds a new dimension to the possible uses of an Arduino board. Chris provides demonstration sketches and the complete code download so you can try it out for yourself.
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