About assembler parse

[libra]

Hello,all:

In binutils, when i try to add new assembler language grammar for my new 
machine (ISA), i can hack the binutils source code by adding my_start_line_hook
() in tc-my.c. 

My question is that :

Where the assember to parse SPACE and LABEL that is in assembly language?

Because my_start_line_hook() is just to parse my new grammar, it cann't parse 
the SPACE and LABEL.

In the origin assember(as.c), how is it work? 

                                                         thanks a lot!!