Int 10 Fn 4FFF  - Diamond Stealth 24 - Set/reset Dual Display Mode         [V]

   AX = 4FFFh
   BX = dual display mode
       00h reset
       01h set dual display, 32KB VGA test
       02h set dual display, 64KB VGA test

Return: AX = 4F00h if successful
       BX = number of scanlines off screen for test mode