I'm not an expert on ppc assembly, but I can answer a few of your  
questions:

>         .text  ; what does the dot here mean?
>        .globl  _start ;what does .globl  mean ?

The assembler syntax used by gas indicates (most?) assembler  
directives by starting them with a dot. ".text" means that the  
following stuff goes into the text segment. ".globl" indicates that  
_start is a global symbol, that is, it is visible outside of this  
file --- otherwise it'd be local to the file (like a static symbol in  
C).

>        lis     1,stack@ha 	 ; r1 = stack<<16 -- what does @ha mean?
>        addi    1,1,stack@l 	; r1 = r1 | 0 +stack - what does @l mean ?
>        addi    1,1,4096  	 ; r1  = r1 | 0 +4096 -- ditto
>			what's the effect of this  ? why is it done?

Typically, all instructions on a RISC processor are the same length.  
Most of the time they're also the same width as the general-purpose  
registers. This means that immediate operands have to be crammed into  
only part of the instruction word, and can't actually be as wide as  
a GPR. So to load an arbitrary value takes two instructions ---  
usually, the first one loads the upper half of the register and  
zeroes the lower half, and the second instruction adds or ors in the  
lower half. So after these instructions, r1 contains "stack + 4096".

Oftentimes there'll be a macro in a header somewhere to produce the  
two-instruction immediate load automatically...

r0 is hardwired to always contain 0 on some processors; I don't know  
if the ppc is one of them.