INT 10h AH=12h: alternate select: EGA and VGA information, settings — 8086 assembly guide

A collection of EGA and VGA settings chosen by BL. BL = 10h is the classic test for an EGA or better: if BL comes back unchanged, the card is a CGA or MDA.

IBM called function 12h "alternate select"; in practice it is a grab bag of subfunctions chosen by BL, not by AL as elsewhere. The EGA BIOS brought BL = 10h and 20h, and the VGA added a row of switches. Those return AL = 12h when they are supported.

BL	What it does
10h	return EGA information (below)
20h	install the card's own print-screen routine
30h	text scan lines for the next mode set: AL = 0: 200, 1: 350, 2: 400 (VGA)
31h	AL = 0: load the default palette at mode sets, 1: keep the current one
32h	AL = 0: enable access to the card, 1: disable it
33h	AL = 0: grey-scale summing on, 1: off
34h	AL = 0: cursor emulation on, 1: off
35h	switch between two displays (PS/2)
36h	AL = 0: screen on, 1: screen off (memory stays usable)

Is there an EGA or VGA?

With BL = 10h the BIOS of an EGA or later card returns BH = 0 for a colour display or 1 for a monochrome one, BL = the memory on the card in units of 64K, minus one (3 means 256K or more, and VGAs say 3), CH the feature connector bits and CL the card's configuration switches. A CGA or MDA BIOS does not know the function and leaves BL = 10h, and that is the test. To tell a VGA from an EGA, use INT 10h AH=1Ah, which the EGA lacks.

Prints "colour display, memory code 3": DOSBox-X reports a colour VGA with 256K.

MOV     AH, 12h           ; alternate select
MOV     BL, 10h           ; get EGA information
INT     10h
CMP     BL, 10h           ; unchanged: no EGA or VGA BIOS
JE      NONE1
MOV     DX, OFFSET COL1   ; BH = 0: colour
OR      BH, BH
JZ      SHOW1
MOV     DX, OFFSET MONO1  ; BH = 1: monochrome
SHOW1:
MOV     AH, 09h
INT     21h               ; BL survives the DOS call
MOV     DL, BL
ADD     DL, "0"           ; the memory code, 0..3
MOV     AH, 02h
INT     21h
MOV     DX, OFFSET CRLF1
JMP     LAST1
NONE1:
MOV     DX, OFFSET NONE2
LAST1:
MOV     AH, 09h
INT     21h
 
COL1    DB      "colour display, memory code $"
MONO1   DB      "mono display, memory code $"
NONE2   DB      "no EGA or VGA", 13, 10, "$"
CRLF1   DB      13, 10, "$"

Pitfalls

BL = 30h only takes effect at the next mode set: for a 43-line screen on a VGA the sequence is BL = 30h with AL = 1 (350 lines), then mode 03h, then INT 10h AH=11h with AL = 12h for the 8×8 font. Turning the screen off with BL = 36h hides drawing until the picture is complete, but turn it on again before exiting, or the user sees nothing at all. The settings stay until changed or until the machine restarts, so restore what you change.