Bitfields for block specifier:
Bit(s)	Description	)
---EGA/MCGA---
 0,1	block selected by characters with attribute bit 3 clear
 2,3	block selected by characters with attribute bit 3 set
---VGA---
 0,1,4	block selected by characters with attribute bit 3 clear
 2,3,5	block selected by characters with attribute bit 3 set