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This paper introduces INTOS, an embedded operating system and language support for multi-threaded intermittent computing on a battery-less energy-harvesting platform. INTOS simplifies programming with a traditional "thread" and a "transaction" with automatic undo-logging of persistent objects in non-volatile memory. While INTOS allows the use of volatile memory for performance and energy efficiency, conventional transactions do not ensure crash consistency of volatile register and memory states. To address this challenge, INTOS proposes a novel replay-and-bypass approach, eliminating the need for users to checkpoint volatile states. Upon power restoration, INTOS recovers non-volatile states by undoing the updates of power-interrupted transactions. To reconstruct volatile states, INTOS restarts each thread bypassing committed transactions and system calls by returning recorded results without re-execution. INTOS seeks to build a persistent, full-fledged embedded OS, supporting priority-based preemptive multithreading while ensuring crash consistency even if power failure occurs during a system call or while some threads are blocked. Experiments on a commodity platform MSP430FR5994 show that when subjected to an extreme power failure frequency of 1 ms, INTOS demonstrated 1.24x lower latency and 1.29x less energy consumption than prior work leveraging idempotent processing. This trend turns out to be more pronounced on Apollo 4 Blue Plus.