Synchronizing between ISR and main code

The circular buffer is indeed useful, implementing a FIFO in software. I wanted, however, to avoid it and use something simpler, as I don't need a FIFO. I can, however, use a "trivial" FIFO which has depth 1 and "loses" all subsequent packets until the old ones has been read.

Which brings me to the question - are variable assignment and test atomic in Nios ? What I mean is:

int ready;
// is this atomic ?
ready = 1;
// and is this atomic 
if (ready == 1)
{
 ...
}

I ask because I can make my main code "deassert" the 'ready' flag when it finished reading the packet, and the ISR to ignore a new packet if 'ready' is still asserted. This way no stomping occurs (overflow messages are lost, which is acceptable). So I wonder whether it's possible for the ISR to run in the middle of asserting 'ready' ? If "ready = 1" is atomic, it certainly can't.