PCC ABI question

Hi Folks

It seems to me that linux doesn't follow the ABI for PowerPC.
According to http://refspecs.freestandards.org/elf/elfspec_ppc.pdf,
Figure 3-31(Initial Process Stack) should the stack ptr point to a NULL word, instead
linux's stack ptr points to argc and there is no NULL word, correct?

The initial register map, on p. 3-28, register r7 should point to a
"termination function" or hold zero. Does linux do this?
If so, where does it get the "termination function" from?
If not, can one assume that r7 is NULL at all times?

Are there other deviations from the above spec in linux?

 Jocke