struct SharedVars {
    // set by pru before halting
    char const *abort_file;
    int abort_line;
    uint32_t time;
    // read-pointer updated by python
    uint16_t ridx;
    uint16_t cycle_limit;
    uint16_t time_limit;
    uint16_t signal_length;
    uint16_t signal[2000];
};