Flickering graphic

I need to move some graphic objects on the screen without they flicker.
So (i've Turbo C 1.0) i tried to use the 'setvisualpage' and the
'setactivepage', modifying graphic in the invisible page and switching
beetween pages. I thought in this way i could have stable images.
But it doesn't work :(

To be more precise: the following code is a good example of my problem.
Let there be 2 pages (0 and 1)

Does anyone know why the line on the screen flickers?

setcolor(YELLOW);    
setvisualpage(0);
setactivepage(0);

line(40,40,300,140);    // draws a yellow line on page 0
setactivepage(1);
line(40,40,300,140);    // draws an identical line on page 1

int p=0;              // starting 'visual page'

for (long i=0; i<1000; i++)    // just a few time of suffering :)
{
 p=1-p;                   // switch beetween pages
 setactivepage(p);    // sets active page as the invisible one
 setcolor(BLACK);      
 line(40,40,300,140);  // deletes the line
 setcolor(YELLOW);
 line(40,40,300,140);  // redraws the line in the same place
 setvisualpage(p);     // shows the hidden page
}