UART RS232 receive missing bytes

I'd guess that the kernel has interrupts disabled (or a long interrupt routine) for long enough for you to lose bytes.

The only real solution is to add a hardware rx fifo to the uart (or rx dma - but that is just a fifo in host memory).

With a small OS (aka no real OS at all) you can probably ensure that the interrupt latency is always short enough. But I doubt you'll manage with anything large!