t=0;
for(i=n; i>0; i--)
{
  t|=(1<<p);
  p++;
}
t=t&y;
x=x&t;

Sorry above answer ; I did a mistake
t=0;
for(i=n; i>0; i--)
{
  t|=(1<<p);
  p++;
}
x=x&~t
t=t&y;
x=x|t;

t =((~(~0<<n))<<p)
x = x & ~t;
y = y & t;
x=x|y;

It should be p--, not p++. for example y = 217 (11011001) 
and you want to extract the least 4 SB then n = 4 and p = 3 
(remember the number start from 0 to 7 in memory). After 
executing this you will get x = 9 i.e 1001.

t=0;
for(i=n; i>0; i--)
{
      t |= (1<<p);
      p--;
}
x=x&~t;
t=t&y;
x=x|t;