assembler parse 64-bit immediate

[rain z]

hi, guys:
     I got a problem when I was implementing a pseudo assembler
instruction on ARM-like architecture.
The pseudo instruction is: "dmovl rd, #0xffffffffffffffff"  /* move a
64-bit immediate to rd register */.
 I use the function my_get_expression( ) which is in
gas/config/tc-arm.c to parse the 64-bit immediate, but it can't give
me the right number I want. It seems to me that the assembler can't
parse a 64-bit immediate, maybe I was wrong.
should I implement a special function to parse a 64-bit immediate ?

Best regards

                          ~Rain~