conditional multivariate normal distribution

Hi all

First of all, I am new here but I am not new to statistics. But I need your help on this one:

I do have a multivariate normal distribution: x~p(mu,sig)

the vector x has to groups of variables, those that I know are below zero (x_bz), and those that I know are above zero (x_az).
I am interested in the conditional distribution of the x above zero: p(x_az|x_bz<0). Can someone help me derive this distribution or is this a known distribution I was to stupid to find?

thanks for all input, J
.