Perhaps we should say this:
An interrupt happens (when enabled), all it says is "Somethings happened, I will jump somewhere special,(The ISR) and you Mr programmer better do something about it."

The serial HARDWARE is effectively separate from the CPU, the CPU says "Here, transmit this character, and tell me when its gone" and the serial hardware likewise says, " I have received a character" A received character is held in a DIFFERENT place to the transmitted character, but it is considered to be logically the same place, SBUF.A WRITE to SBUF transmits the character, a READ from SBUF picks up a received character.

The buffers/FIFO are mechanisms which go in the ISR and keep characters fed to the serial hardware.



Steve