INT 10h AH=1Ah

The PS/2 and VGA BIOS keep a display combination code: a number for the active adapter and monitor, and one for a second, alternate display. Function 1Ah with AL = 00h returns them in BL and BH and sets AL to 1Ah as proof that it understood the call. An EGA, CGA or MDA BIOS does not have the function and leaves AL as it was, so CMP AL, 1Ah after the call is the standard test for a VGA.

AL = 01h sets the codes, which only display drivers that switch monitors need. Programs read them:

MOV     AX, 1A00h              ; get the display combination code
INT     10h
CMP     AL, 1Ah                ; does the BIOS know the function?
JNE     OLD1
MOV     [DCC1], BX             ; BL = active, BH = alternate
MOV     AH, 09h
MOV     DX, OFFSET TXT1
INT     21h
MOV     AL, BYTE PTR [DCC1]    ; the active display
CALL    PHEX2
MOV     AH, 09h
MOV     DX, OFFSET TXT2
INT     21h
MOV     AL, BYTE PTR [DCC1+1]  ; the alternate
CALL    PHEX2
MOV     DX, OFFSET TXT3
JMP     LAST1
OLD1:
MOV     DX, OFFSET TXT4        ; an EGA or older
LAST1:
MOV     AH, 09h
INT     21h
 
DCC1    DW      0
TXT1    DB      "active $"
TXT2    DB      "h, alternate $"
TXT3    DB      "h", 13, 10, "$"
TXT4    DB      "no VGA BIOS", 13, 10, "$"
PHEX2:                         ; print AL as two hex digits; changes AX, CX, DX
PUSH    AX
MOV     CL, 4
SHR     AL, CL                 ; the high nibble first
CALL    PDIG1
POP     AX
AND     AL, 0Fh                ; then the low one, falling into PDIG1
PDIG1:
ADD     AL, "0"
CMP     AL, "9"
JBE     PDIG2
ADD     AL, 7                  ; "A".."F"
PDIG2:
MOV     DL, AL
MOV     AH, 02h                ; DOS: print DL
INT     21h
RET

Finding out what card is there

The usual ladder: try INT 10h AH=1Ah (VGA or MCGA); if AL did not come back as 1Ah, try INT 10h AH=12h with BL = 10h (EGA); if BL stays 10h, look at the mode from INT 10h AH=0Fh: 07h means a monochrome card, anything else a CGA. Telling a Hercules card from a plain MDA needs a hardware trick: bit 7 of port 3BAh changes with each frame only on a Hercules.

Pitfalls

An SVGA card reports 08h like any VGA; learning more about it takes the VESA functions, starting with AX = 4F00h. The codes say what the BIOS believes is attached, which on old machines came from switches set by hand. DOSBox-X configured as an EGA or CGA machine does not answer, exactly like the real cards.