regarding exception frame

Hi,
In the uboot1.1.6 code , there are some constants like STACK_FRAME_OVERHEAD and STACK_UNDERHEAD in
the file "\include\ppc_asm.tmpl " . Could you please tell me what are the use of those constants ?

Here is one statement in the macro CRITICAL_EXCEPTION_PROLOG in the file "ppc_asm.tmpl"

subi r21,r1,INT_FRAME_SIZE+STACK_UNDERHEAD; /* alloc exc. frame */\

what is the need of STACK_UNDERHEAD here ?

In the macro STD_EXCEPTION there is a statement in the same file
addi r3,r1,STACK_FRAME_OVERHEAD

What is the need of STACK_FRAME_OVERHEAD here ?

Please clarify my doubts ....

Thanks
Dileep Kumar