To get frequency response using fft from input and output data

Hello...! I'm trying to get the frequency response from simulated input and output data using fft commands but the results are not suitable please check the command as mention below

m=1000
n = pow2(nextpow2(N))
A=fft(x,n);
B=fft(y,n);
C=A./B;
D=abs(C);
E=mag2db(D);
Fs=10e+3;
f = (0:n-1)*(Fs/n);
semilogx(f,E)
grid on

Pease tell me if someone find my mistake. i'm also not sure these commands are correct to draw fft from input and output data. I'll be very thankful if someone provide me correct commands. Thanx