K15 — Explicit FromISR, UART and GPIO wrappers

K15 routes a real Hazard3 machine-external interrupt through the official FreeRTOS RISC-V trap path. The C++ layer keeps task and ISR views explicit: UartTaskView/GpioTaskView contain configuration and output operations, while UartIsrView/GpioIsrView expose only bounded register reads and clears.

One IsrContext accumulates wake requests from two primitives:

UART RX bytes: 0x41, 0x42
static queue capacity: 1
accepted: 0x41
overflow policy: drop-newest -> 0x42 dropped
GPIO edge: task notification
higherPriorityTaskWoken: true
yield_if_needed calls: exactly 1

The high-priority receiver consumes byte 0x41 and notification count 1, then drives GPIO output high. It runs before the interrupted stimulus continues, which proves the ISR-requested context switch.

Build

make check

The gate includes a host contract test, freestanding ABI/storage checks and a real external-IRQ simulation on Hazard3.

Debug

riscv64-unknown-elf-gdb build/task01_isr_drivers/prog.elf
b isr_drivers_debug_checkpoint
p/x g_mcause
p g_rx_queued
p g_rx_dropped
p/x g_receiver_byte
p/x g_dropped_byte
p g_higher_priority_task_woken
p g_isr_yield_calls
p g_stimulus_after_seen_by_receiver
p g_receiver_done_at_stimulus_resume
p g_isr_drivers_pass

Scope boundary

ISR code cannot call a blocking task API. Parsing, debounce and GPIO policy belong to a task. Backpressure is explicit: this exercise uses drop-newest and counts the dropped byte. The handler creates one IsrContext and calls yield_if_needed once, after all FromISR publications and peripheral clears.