How "can't compute CFA for this frame" and "No enough registers or memory available to unwind further" happens?

[zhihua che]

Hi, everyone
       I'm not sure this is right place for the help. I'm writing a
toy os and coding with mixed assembly and C language, debugging with
GDB. But I'm trapped with an annoying problem. This is my situation:
During the os booting time, after the os control transfers from real
mode assembly codes to real mode C codes, I wish I can exam the stack
frames and local variable as I do in regular application program, but
I always get "can't compute CFA for this frame" or "No enough
registers or memory available to further unwind" if I issue "print
xxx" or "backtrace" command respectivelly. The codes seem work, but I
can only exam registers or memory using "info reg" or "x" command,
it's way unfriendly and time-consuming. I have searched a lot but
don't figure out how these happen. I need your help. Thanks.
Harvey