xor encoding / decoding of shellcode

[KF <dotslash () snosoft com>]

I have been working alot with asm lately on the ppc platform... my goal
has been to develop
shellcode for several different situations... I have come up with
shellcode that writes an xinetd.d 
entry and one that binds a shell to a port as well as a few other
typical shellcode examples. The 
problem that I am having is that alot of my instructions cause null. I
have a technique to remove 
some of them but the method I use takes 2 instructions for every one
with null. I have been trying 
to figure out how the stack needs to be layed out in order for me to xor
encode / decode my shellcode. 
I imagine it is similar to <nops><xor decoder><xor encoded
shellcode><return addy>. At the moment 
I am stuck on how to place a xor encoded string on the stack and then
use it as executable code. 
In essence I need to learn how to write a xor decoder does anyone have
any tips on this technique?
I am trying to come up with something similar to ADMmutate that works on
power pc or other RISC processors. 
-KF