Networked Embedded Systems

Ten years after the Internet revolution are we standing on the brink of another revolution: networked embedded systems that connect the physical world with the computers, enabling new applications ranging from environmental monitoring and wildlife tracking to improvements in health care and medicine. Only 2% of all microprocessors that are sold today are used in PCs; the remaining 98% of all microprocessors are used in embedded systems. The microprocessors used in embedded systems have much smaller amounts of memory than PC computers. An embedded system may have as little has a few hundred bytes of memory, which is thousands of millions times less than the memory in a modern PC. The memory constraints make programming embedded systems a challenge. This thesis focus on three topics pertaining to programming memoryconstrained networked embedded systems: the use of the TCP/IP protocol suite even in memory-constrained networked embedded systems; simplifying eventdriven programming of memory-constrained systems; and dynamic loading of program modules in an operating system for memory-constrained devices. I show that the TCP/IP protocol stack can, contrary to previous belief, be used in memory-constrained embedded systems but that a small implementation has a lower network throughput. I present a novel programming mechanism called protothreads that is intended to replace state machine-based event-driven programs. Protothreads provide a conditional blocked wait mechanism on top of event-driven systems with a much smaller memory overhead than full multithreading; each protothread requires only two bytes of memory. I show that protothreads significantly reduce the complexity of event-driven programming for memory-constrained systems. Of seven state machine-based programs rewritten with protothreads, almost all explicit states and state transitions could be removed. Protothreads also reduced the number of lines of code with 31% on the average. The execution time overhead of protothreads is on the order