Int 10 Fn 12  - Video - Alternate Function Select (ps, Ega, Vga, Mcga) -   [V]

   AH = 12h
   BL = 10h

Return: BH = video state
       00h color mode in effect (I/O port 3Dxh)
       01h mono mode in effect (I/O port 3Bxh)
   BL = installed memory (00h = 64K, 01h = 128K, 02h = 192K, 03h = 256K)
   CH = feature connector bits (see #0021)
   CL = switch settings (see #0022,#0023)
   AH destroyed (at least by Tseng ET4000 BIOS v8.00n)

Note:  one possible check for the presence of an EGA or later display card
     is to call this function with BH=FFh; if not present, BH will be
     unchanged on return
   another installation check is used by Athena Digital's HGCIBM, which
     sets CX to FFFFh on calling and checks whether it has been changed
     on return
Index: installation check;EGA

Bitfields for feature connector bits:
Bit(s) Description (Table 0021)
 0 FEAT 1 line, state 2
 1 FEAT 0 line, state 2
 2 FEAT 1 line, state 1
 3 FEAT 0 line, state 1
 4-7   unused (0)

Bitfields for switch settings:
Bit(s) Description (Table 0022)
 0 switch 1 OFF
 1 switch 2 OFF
 2 switch 3 OFF
 3 switch 4 OFF
 4-7   unused

(Table 0023)
Values for switch settings on original EGA/VGA:
 00h       primary MDA/HGC,    secondary EGA+ 40x25
 01h-03h   primary MDA/HGC,    secondary EGA+ 80x25
 04h       primary CGA 40x25,  secondary EGA+ 80x25 mono
 05h       primary CGA 80x25,  secondary EGA+ 80x25 mono
 06h       primary EGA+ 40x25, secondary MDA/HGC (optional)
 07h-09h   primary EGA+ 80x25, secondary MDA/HGC (optional)
 0Ah       primary EGA+ 80x25 mono,secondary CGA 40x25 (optional)
 0Bh       primary EGA+ 80x25 mono,secondary CGA 80x25 (optional)