Figure 1: Memory organization in mode 13h (ASCII version)
	  by Robert Schmidt
          (C) 1993 Ztiff Zox Softwear

a. Imagine that the top of the screen looks like this (pixel values are
   represented by color digits 0-9 for simplicity - actual colors may
   range from 0 to 255) - a screen width of 320 pixels is assumed:

    address:  0         10                310      319
             ----------------------------------------
             |0123456789012345    .....   0123456789|
             |                                      |
             |                                      |
             |

b. In VGA memory, the screen is represented as follows (question marks
   represent unused bytes):

   Plane 0:

    address:  0         10                310      319
             ----------------------------------------
             |0???4???8???2???    .....   ??2???6???|
             |                                      |
             |                                      |

   Plane 1:

    address:  0         10                310      319
             ----------------------------------------
             |?1???5???9???3??    .....   ???3???7??|
             |                                      |
             |                                      |

   Plane 2:

    address:  0         10                310      319
             ----------------------------------------
             |??2???6???0???4?    .....   0???4???8?|
             |                                      |
             |                                      |

   Plane 3:

    address:  0         10                310      319
             ----------------------------------------
             |???3???7???1???5    .....   ?1???5???9|
             |                                      |
             |                                      |

   I.e. a plane is selected automatically by the two least significant
   bits of the address of the byte being read from or written two.
   This renders 3/4 of the video memory unavailable and useless, but
   all visible pixels are easily accessed, as each address in the video
   segment provides access to one and ONLY ONE pixel.