const int bufsize = 256;
unsigned char ringbuffer[bufsize];
unsigned char *ringbuffer_a = ringbuffer;

void add_byte(unsigned char byte) {
    ringbuffer_a++;
    
    // The original code checks just the lower byte.
    if (ringbuffer_a == ringbuffer + bufsize + 1)
        ringbuffer_a = ringbuffer;
    
    *ringbuffer_a = byte;
}