How to exclude some fields from register testing??

Hi,

I have a doubt.

For e.g. in a 32 bit register only 5 bits are used  while rest are reserved or unused.

What setting I need to do in register model so that reserved bits are never touched or tested??

For the complete register we can set attribute, but what should be done for some bits of a register??

Thanks and Regards,

Gaurav Gupta