int Timer::read_us() {
    core_util_critical_section_enter();
    int time = _time + slicetime();
    core_util_critical_section_exit();
    return time;
}

int Timer::slicetime() {
    core_util_critical_section_enter();
    int ret = 0;
    if (_running) {
        ret = ticker_read(_ticker_data) - _start;
    }
    core_util_critical_section_exit();
    return ret;
}

void core_util_critical_section_enter(void)
{
    bool interrupts_disabled = !core_util_are_interrupts_enabled();
    __disable_irq();

    /* Save the interrupt disabled state as it was prior to any nested critical section lock use */
    if (!interrupt_enable_counter) {
        critical_interrupts_disabled = interrupts_disabled;
    }

    /* If the interrupt_enable_counter overflows or we are in a nested critical section and interrupts
       are enabled, then something has gone badly wrong thus assert an error.
    */
    MBED_ASSERT(interrupt_enable_counter < UINT32_MAX); 
// FIXME
#ifndef   FEATURE_UVISOR
    if (interrupt_enable_counter > 0) {
        MBED_ASSERT(interrupts_disabled);
    }
#else
#warning "core_util_critical_section_enter needs fixing to work from unprivileged code"
#endif /* FEATURE_UVISOR */
    interrupt_enable_counter++;
}

void core_util_critical_section_exit(void)
{
    /* If critical_section_enter has not previously been called, do nothing */
    if (interrupt_enable_counter) {

// FIXME
#ifndef   FEATURE_UVISOR
        bool interrupts_disabled = !core_util_are_interrupts_enabled(); /* get the current interrupt disabled state */

        MBED_ASSERT(interrupts_disabled); /* Interrupts must be disabled on invoking an exit from a critical section */
#else
#warning "core_util_critical_section_exit needs fixing to work from unprivileged code"
#endif /* FEATURE_UVISOR */

        interrupt_enable_counter--;

        /* Only re-enable interrupts if we are exiting the last of the nested critical sections and
           interrupts were enabled on entry to the first critical section.
        */
        if (!interrupt_enable_counter && !critical_interrupts_disabled) {
            __enable_irq();
        }
    }
}