print result of nios ii custum instruction on model sim?

Hello

I add sqrt as custom instructio to nios ii and generate built system in sopc and all things ok.

I write simple code in nios2 ide and simulate it with modelsim but not any result show .

print NaN or 0.0000!!!

How can i solve this problem?

quartus 9.0 and modelsim 6.4a and niosii9 ide used.

Thank you