what does this iteration do?

[sanctus]

I'm working on an existing code and don't understand what the following iteration iterates over:

rows = dim[0];
pagesize = dim[0]*dim[1];
    
	for(ix=1;ix<dim[0]-1;ix++) 
		for(iy=1;iy<dim[1]-1;iy++) 
			for(iz=1;iz<dim[2]-1;iz++) 
			{
				index   = ix + iy*rows + iz*pagesize;
                               if(MASK[index]==0)
                                     Gradient[index]=0;
                               else
                                  (...)
                        }

I mean what is index exactly? MASK is an array of dimensions
dim[0] x dim[1] x dim[2] but index doesnt seem to me to iterate ovaer all elements of MASK (this can for example seen by the fact the first iteration gives index=1+rows+pa gesize=1+rows*( 1+dim[1]))


EDIT: to be precise MASK is imported from Matlab in the following way:
in the mexfunction there is

MASK = mxGetPr(prhs[1]);

hence, Mask is a pointer to the second input element from the calling matlab function. And this calling Matlab function has as second argument an array of ones and zeros of the dimensions dim[0] x dim[1] x dim[2].

I hope it is clear.

[sanctus]

Mo need to reply anymore, it took me an afternoon but I figured it out, thanks in case someone tried to understand