how to scramble it?

here's codes

I=imread('filename'); %256x256 pixels
%split it into block 64x64
ul=I(1:256,1:256);
ur=I(1:256,257:512);
ll=I(257:512,1:256);
lr=I(257:512,257:512);
s=[ul,lr;ur,lr];
figure,imshow(s)

but i want to split it 8x8 pixels,

a=I(1:8,1:8);
b=I(1:8,9:16);

and so on.. until

x=I(249:256,249:356);

then scramble it s=[a,b,..;..,..];

if i use those codes, it'll very long codes. what should i do?