andy said,
What identifies the start of each Word within a packet?
What marks the end of one Word, and the start of the next?

I explained it before but I'll try to make an example. hope it answer your question.

the words are present in a 9 bit fifo, as I explained the 9th bit(when it is 1) identifies the start of a packet, following that word are optional data for example assume a command named "Write following 5 bytes into your flash memory", this command has a code for example 9 which is present in the header byte(command byte, the one with its 9th bit set to 1) the reciever when recieves a command byte refers it to a dispatch routine( the command should be defined for all 8051s) and appropriate handling function. the handling function knows that 5 bytes that follows the mentioned command(code 9) should be written to flash after reading 5 bytes and calculating CRC if it was ok the function will carry on the request and sends an ACK. 8051 reads more words from fifo and look for next command (one in which the 9th bit is 1).

regards
Ebrahim Vakilpour