In many embedded systems, memory is severely constrained. RAM sizes on the order of a few hundreds of bytes or less is very common. Full multithreading may not be an option because of the memory required for thread stacks. For such systems, the newly released Protothreads Library provides a very lightweight alternative: protothreads are a stackless type of threads with only two bytes of RAM overhead per protothread. The library is implemented in pure C without any machine specific code. Protothreads are currently used in the Contiki OS and will be used in the upcoming version of the uIP embedded TCP/IP stack.