INT 10h AH=10h: palette registers and the DAC — 8086 assembly guide

EGA and VGA colour: the 16 palette registers that map the attribute colours, the blink or bright-background switch, and on a VGA the 256 DAC registers that hold the real red, green and blue.

On a VGA a colour goes through two tables. A text or 16-colour pixel value 0–15 first selects one of 16 palette registers in the attribute controller, inherited from the EGA. The value found there (0–63) selects one of 256 DAC registers, and each DAC register holds a red, a green and a blue intensity of 6 bits, 0–63. In mode 13h the pixel value effectively selects the DAC register directly.

In mode 03h the palette registers send colours 0–5 and 7 to the DAC registers of the same numbers, brown (6) to 14h, and the bright colours 8–15 to 38h–3Fh. Changing DAC register 1 therefore changes every blue on the screen at once, without touching video memory: fades, palette animation and the colour cycling of old games all work this way.

AL	What it does
00h	set palette register BL to the value BH
01h	set the border (overscan) colour to BH
02h	set all 16 palette registers and the border from 17 bytes at ES:DX
03h	BL = 0: bit 7 of the attribute means bright background; BL = 1: it means blink
07h	read palette register BL into BH (VGA)
08h	read the border colour into BH (VGA)
09h	read the 16 palette registers and the border into 17 bytes at ES:DX (VGA)
10h	set DAC register BX to red DH, green CH, blue CL
12h	set CX DAC registers from BX on, from 3 bytes each at ES:DX
13h	colour paging: BL = 0 selects the paging mode in BH, BL = 1 the page in BH
15h	read DAC register BX into DH, CH, CL
17h	read CX DAC registers from BX on into 3 bytes each at ES:DX
1Ah	read the colour paging state: BL = mode, BH = page
1Bh	turn CX DAC registers from BX on into shades of grey

The example reads DAC register 1, shows a blue bar, makes register 1 orange for a second and then puts the old value back:

Prints "DAC 1 was 00 00 2A" (red 0, green 0, blue 42 of 63: the standard blue). On the screen a blue bar shows for a second, turns orange for a second, together with anything else blue, and is blue again.

MOV     AX, 1015h  ; read one DAC register
MOV     BX, 1      ; register 1: what text colour 1, blue, shows
INT     10h        ; DH = red, CH = green, CL = blue, 0..63
MOV     [RED1], DH
MOV     [GRN1], CH
MOV     [BLU1], CL
MOV     AX, 0920h  ; a bar of 20 blanks...
MOV     BX, 001Fh  ; ...on page 0, white on blue
MOV     CX, 20
INT     10h
MOV     BH, 0      ; page 0
MOV     AX, 0E0Dh  ; BIOS teletype: CR...
INT     10h
MOV     AX, 0E0Ah  ; ...and LF: on to the next line
INT     10h
CALL    WAIT1      ; a second of blue
MOV     AX, 1010h  ; set one DAC register
MOV     BX, 1
MOV     DH, 63     ; red
MOV     CH, 32     ; green
MOV     CL, 0      ; blue: together, orange
INT     10h
CALL    WAIT1      ; a second of orange
MOV     AX, 1010h  ; the old colour back
MOV     BX, 1
MOV     DH, [RED1]
MOV     CH, [GRN1]
MOV     CL, [BLU1]
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG1
INT     21h
MOV     AL, [RED1]
CALL    PHEX2
MOV     DL, " "
MOV     AH, 02h
INT     21h
MOV     AL, [GRN1]
CALL    PHEX2
MOV     DL, " "
MOV     AH, 02h
INT     21h
MOV     AL, [BLU1]
CALL    PHEX2
MOV     AH, 09h
MOV     DX, OFFSET CRLF1
INT     21h
 
RED1    DB      0
GRN1    DB      0
BLU1    DB      0
MSG1    DB      "DAC 1 was $"
CRLF1   DB      13, 10, "$"
WAIT1:             ; wait 1 s: CX:DX = 0F4240h microseconds
MOV     CX, 000Fh
MOV     DX, 4240h
MOV     AH, 86h
INT     15h
RET
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

Bright backgrounds

AX = 1003h with BL = 00h switches blinking off: attribute bit 7 then selects the bright colours 8–15 as backgrounds, so all 16 backgrounds become available, as most text-mode user interfaces of the 1990s used. BL = 01h turns blinking back on, and a mode set does so too.

MOV     AX, 1003h  ; blink / intensity
MOV     BL, 0      ; 0: bright backgrounds, 1: blink
INT     10h

Pitfalls

DAC values have 6 bits: 63 is full intensity, and larger values do not make a brighter colour (the top bits are ignored). The registers belong to the whole screen, so restore what you change: read it first with 15h or 17h, or set mode 03h again, which reloads the defaults. Changing many DAC registers while the screen is being drawn can flicker on real cards, so programs wait for the vertical retrace (bit 3 of port 3DAh) first. An EGA has only subfunctions 00h–03h. The block functions take ES:DX, and in a .COM program ES already equals DS.