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     On the EGA and MCGA, the bits of BL are used as follows:
 
     Bits     Significance
 
     0-1      Character block selected by attribute bytes with bit 3 = 0
     2-3      Character block selected by attribute bytes with bit 3 = 1
     4-7      Not used (should be 0)
 
     On the VGA, the bits of BL are used as follows:
 
     Bits      Significance
 
     0, 1, 4   Character block selected by attribute bytes with bit 3 = 0
     2, 3, 5   Character block selected by attribute bytes with bit 3 = 1
     6-7       Not used (should be 0)
 
     When using a 256-character set, both fields of BL should select
     the same character block. In such cases, character attribute bit 3
     controls the foreground intensity.
 
     When using 512-character sets, the fields of BL designate the
     blocks holding each half of the character set, and bit 3 of the
     character attribute selects the upper or lower half of the
     character set.
 
     When using a 512-character set, a call to Int 10h Function 10h
     Subfunction 00h with BX = 0712h is recommended to set the color
     planes to eight consistent colors.
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