Status register as C bitfields

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

typedef struct {
    uint32_t rx_done : 1;   /* W1C */
    uint32_t tx_done : 1;   /* W1C */
    uint32_t error   : 1;   /* W1C */
    uint32_t         : 29;
} status_bits_t;

#define PERIPH_STATUS (*(volatile status_bits_t *)0x40010010u)

void periph_isr(void)
{
    if (PERIPH_STATUS.rx_done) {
        handle_rx();
        PERIPH_STATUS.rx_done = 1;
    }
    if (PERIPH_STATUS.tx_done) {
        handle_tx();
        PERIPH_STATUS.tx_done = 1;
    }
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

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