Doing a GAS port

[c.c.chen]

Hi,
I wanted to port the GAS to a new target
I know that basically I will need to create this file:

--GAS target dependent 
  gas/config/tc-CPU.[ch]          
 
--opcode library (CGEN done this part) 
  opcodes/CPU-dis.c            
  opcodes/FOO-opc.c             

--BFD library
  bfd/{elf32|coff}-CPU.c        
  bfd/cpu-CPU.c 
  ...               

(Replace CPU with the name of my target chip)
 
I want to ask a quistion?
why CGEN doesn't do this part which generate tc-CPU.[ch] files?
(because cpu description information is not enough or other reason?) 

if it is worth writing a framewok to automaticly generate tc-CPU.[ch]( input 
CPU.cpu file )? 
(if i just want GAS produce a ELF format object file) 

c.c.chen