Dual-core MCUs are asymmetric: on NXP's MCXN947, the second Cortex-M33 has no FPU, DSP extension, TrustZone, or MPU. We treat the asymmetry as a design input in the Phase 3 dual-core architecture of SynapticOS, an open-source Zephyr-based runtime: the AI runtime (models, NPU/DSP, scheduler) lives on the capable core, and the application core reaches inference only via a message-based OS service -- a remote system call. The transport is a pair of lock-free single-producer/single-consumer rings in shared SRAM: one writer per index, free-running 32-bit counters, ordering by data-memory barriers alone (the platform has no cross-core atomics). Because ring state is shared, a rebooting application core rejoins unaided. Requests carry priority classes, errors and timeouts propagate to the caller, and tensors stage zero-copy in a shared slot -- a 27 KB frame cannot exist twice in 64 KB of RAM. Measured on the FRDM-MCXN947 (both cores 150 MHz): the application core boots in 1,514 us and completes the handshake in 2,554 us, bit-identical over 11 boots; round trips are 15 us typical / 81 us worst-case (50 us budget); pushes cost 25 cycles; a 1,913-serve two-model soak had zero errors (stub-NPU latencies bracket transport, not silicon). An MPU region on the runtime core guards the application core's RAM (fault-injection verified); protection is one-directional -- the application core has no MPU, and ARMv8-M cannot block privileged reads. Two hardware-revealed defects are reported: releasing the second core into erased flash wedges the whole chip and its debug port (now prevented by a ROM-API blank check), and a Zephyr flash-driver Kconfig silently disarmed the devicetree MPU guard (now programmed at runtime). Firmware is 98.9 KB flash (runtime core) and 32.4 KB (application core, 42.6 of 64 KB RAM); 108 tests in 13 suites pass 100%. Apache 2.0: https://github.com/Dimitrios-Kafetzis/SynapticOS

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