UART interrupt handler

This is the better disussion on interrupt over UART, so I won't open a new one.

I understand that there is device driver provided in system librairy, and you can use it with open(), read(), write(), ... or fopen(), fread(), fwrite(), fprintf(), fscanf(),...

I understand that you can re write device driver and interrupt handler.

I think interrupt are driven RTS, CTS signals if hardware flow control is done.

I think datas are in Rx and Tx buffers of UART driver at the interrupt time.

When you read Rx buffer through read() function, I think it's not an interrupt.

Interrupt just fill the Rx buffer, read() function take data stored into the Rx buffer, no ?

What happens if you don't get datas into Rx buffer before the buffer is full ? I think the circular buffer continue, and datas are lost ? Is there a signal or something telling the buffer is almost full ?

How getting datas at interrupts time ? (I'm working at 921Kbds..)

Is it necessary to rewrite the driver and the interrupt handler ?