Polling UART output held machine interrupts off for an entire string, which
could lose ThreadX ticks. PLIC enable-word changes could overwrite an update
made by an interrupt handler.
The UART now polls with interrupts enabled and protects only the final status
check and byte write. PLIC enable and disable changes are protected across
their read-modify-write. The README explains the new simulator tests.
Both simulator tests failed on the original code and passed with these fixes.
The 100-million-cycle demo run reported all eight threads and five thread 0
wakeups. AI disclosure and port consistency checks passed. No silicon run.
Assisted-by: Codex (GPT-6-Sol) <noreply@openai.com>
Frédéric Desbiens
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