Standard Asynch "clock extraction" really is a framing technique typically based upon have a 16X sampling rate for the incomming data (i.e. the 1604 type UART). The 8051 does not do this and that is why you read (or used to anyway) about 8051 framing errors.

The old RS-232 synch exchange specified a terminal/modem interface at a time when data comm occured through modems.
The synch modem provide (or accepted) both a tx clock and a rx clock (and separate Rx/Tx data signals). Hence the RX/TX clock signals on the older DB-25 PC asynch connector.

It was even common to exchange 300 bps. asynch on a 1200 bps synch modem (4/1 under-clocking i think we called it) when an asynch was not available.

Mode 0 is a very cool way of sending data with few signals and a simplex protocol.

regards,
p