Bitfields for HP Vectra extended control register:
Bit(s)	Description	)
 0	screen resolution (0 = 200 lines, 1 = 400 lines)
 1	underline enable (if set, 'blue' of fg color = underline)
 2	font (0 = Standard-8, 1 = HP-Roman-8)
 3	memory disabled for CPU access
 4	allow access to full 32K memory instead of wrapping at 16K
 5	select second 16K page instead of first
 6,7	unused