Watcom Page Flip 12h

#define SCREEN_WIDTH_IN_BYTES 80
#define PAGE_0_START 0
#define PAGE_1_START (480 * SCREEN_WIDTH_IN_BYTES) //I modified this to 480 instead of 400, but doesn't work

/* Flip to page 1 */
outpw(0x3D4, 0x0C | ((PAGE_1_START >> 8) << 8));
/* Put my graphics on the other page with built-in function */

_putimage(......);

/* Flip to page 0 */
outpw(0x3D4, 0x0C | ((PAGE_0_START >> 8) << 8));