Can I use the system call provided by uboot in an assembly language program as in ppcbug?

Hello,
I want to call some system calls to do input/ouput in assembly language
program, and does uboot provide such syscall? As in PPCBUG, I can
program the following code:
  li r10,0x0022
  sc
to do an output to console.
 
I know I can program a standalone application using printf() to achieve
this function, as illustrated by the example uboot provides. Instead, I
want to use assembly language. 
 
Thanks and Regards.
 
Zhang Xin
MOTO Networks CDC
Phone: 86 10 58830720