Shadow Register Sets in Nios II

Hi,

i'am using the Nios II with an external interrupt controller (VIC)

Now, i can choose the number of shadow register sets (0..63) in

the "advanced feature tab" of the Nios II.

I need 4 shadow register sets.

SOPC Builder generate the following warning:

Warning: Number of shadow register sets is set to a non-optimal

value for the selected device family. 7 is the recommended value.

My device family is Cyclone (Cyclone III EP3C16F484C7)

This warning appears on all choices of register sets except 7.

My question:

Why does the number of shadow register sets depend on the selected device family ?

I think it is part of the Nios II and not device specific ?

Can anyone explain the warning ?

Regards,

Nicolas