Subtraction with numeric_Std doesn't work

Hi all,

I have a subtraction part in my code:

x<=unsigned(DataSent);

y<=N/2;

y1<=(unsigned(x)-y);

my variables datasent ,x,y,y1,all are integers.

when i comment the statement

y1<=(unsigned(x)-y);

the variable x rightly has the datasent value but when the above statement is not commented the code breaks at y1<=unsigned(x)-y

How can x loose its value?

It shows the -214748368

how do i make x store the right value ?

say if datasent is 231 and y-N/2 is 6 for N=12

what correction has to be done to get y1= 231-6=225

Can someone verify why the processing stops at the line y1<=x-y?

Thanks in Advance