AVR-Chat

I have a question of some concern to me, perhaps you could help.

In overview, it involves mixing assembly language and C code.

In detail:

The CPU that I am using is ATmega328 using AVR Studio 5 as a development
environment.

I an writing a motor control application that involves a quadrature encoder
attached to the motor shaft and a corresponding decoder with the following
characteristics:

[1] The decoder itself is implemented entirely in assembly language.
[2] All the rest of the application will be written in C.
[3] It maintains a running 24-bit count of the encoder's position.
[4] It will be called by a periodic interrupt at 25 kHz sample rate.
[5] THE DECODER RESERVES FIVE OF THE MACHINE REGISTERS FOR ITS EXCLUSIVE 
USE.
     This is the big deal I want to question you about.

What I want to do is this:  Can I somehow tell the C compiler that comes 
with AVR
Studio that these certain five registers are reserved for the exclusive 
use of the
decoder and therefore avoid using them in any way?

erikc