mortgage calculation, please

Can anyone show me, even simply algebric, how to calculate morgage. Relation between:

var r = rate (/per month)
var m = number of month
var val = total initial value

var mort (/per month) = ?

I found ... sorry for bother you, but I have an account who do this stuff in my bussines :o

var r = rate (/per month)
var m = number of month
var val = total initial value
var mort = mortgage(/per month)


var mort = val * (r / (1 - (1 / Math.pow(1+r,m))))