How to write this insn define?

["Ruan Beihong" <ruanbeihong@xxxxxxxxx>]

There is a instruction:
mult rega,regb,regc
Lower 32-bit of sign-extended regb is multiplied by lower 32-bit of
sign-extended regc, and the lower 32-bit of product then sign-extends
to 64-bit and stores in rega.
How to write a (define_insn ) to describe this instruction?
Thanks.

James Ruan