[U-Boot-Users] how does stack work?

[Seingier François-Xavier]

hello,
I'm trying to understand how the initial stack works on a ppc4xx. According to start.S this is how the layout looks to me:
      ___________
     |           |
     |           |
     |           |
     |    gd_t   |
     |___________|  
     |     0     |
     |___________|  
     |     0     |  
     |___________|<-+
     | reset_vec |  |
SP ->|___________|  |
     |           |--+
     |___________|<-+
     |           |  |
     |___________|  |
     |           |--+
     |___________|
     |   ...     |

I can't figure out precisely how the last stack frame works:
- /* clear final stack frame so that	*/
  /* stack backtraces terminate cleanly	*/
I'm not sure to understand the "terminate cleanly".

- why will the reset_vec be used in case of a stack underflow?
I tried to find an answer in the "PowerPC compiler writer's guide" about the ABI, but I am not even sure gcc uses an AIX or System V implementation...

If somebody could explain how this works in details, I would be very grateful (or point me to the right doc)

Thanks in advance

François-Xavier SEINGIER