INT 10h AH=0Bh: set the border, background or CGA palette — 8086 assembly guide

A CGA-era function: the border colour in text modes, and in CGA graphics the background colour and the choice between the two 4-colour palettes.

Function 0Bh comes from the CGA, whose colour select register controlled three things at once, and the EGA and VGA BIOSes keep its meaning. With BH = 0, BL is a colour 0–15: in text modes it sets the border, the overscan area around the text; in CGA graphics it sets the background, colour 0, together with the border. With BH = 1, BL chooses between the CGA's two sets of colours 1–3 for 320×200 graphics:

Palette	Colour 1	Colour 2	Colour 3
0	green	red	brown
1	cyan	magenta	light grey / white

Each palette also exists in a bright version, chosen on the CGA by an intensity bit that goes with the background colour. Mode 04h starts with palette 1. Here three bars are drawn in colours 1, 2 and 3 on a blue background, then the palette is switched under them:

Prints "Palette 1, then palette 0"; on the screen three bars on a blue background for a second, which then change colour as palette 0 replaces palette 1; after another second the text screen comes back cleared.

MOV     AX, 0004h  ; mode 04h: CGA 320x200, 4 colours
INT     10h
MOV     AH, 0Bh
MOV     BX, 0001h  ; BH = 0: background; BL = 1: blue
INT     10h
MOV     AH, 0Bh
MOV     BX, 0101h  ; BH = 1: palette; BL = 1
INT     10h
MOV     BX, 0001h  ; BH = page 0, BL = colour of the first bar
MOV     DX, 70     ; first row
BAND1:
MOV     SI, 10     ; 10 rows per bar
LINE1:
MOV     CX, 80     ; x from 80 to 239
DOT1:
MOV     AH, 0Ch    ; write a pixel: AL = colour, CX = x, DX = y
MOV     AL, BL
INT     10h
INC     CX
CMP     CX, 240
JB      DOT1
INC     DX
DEC     SI
JNZ     LINE1
ADD     DX, 10     ; a gap of 10 rows
INC     BL         ; the next colour
CMP     BL, 4
JB      BAND1
CALL    WAIT1      ; a second with palette 1
MOV     AH, 0Bh
MOV     BX, 0100h  ; BH = 1, BL = 0: palette 0
INT     10h
CALL    WAIT1      ; a second with palette 0
MOV     AX, 0003h  ; mode 03h again: 80x25 text, clears the screen
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG1
INT     21h
 
MSG1    DB      "Palette 1, then palette 0", 13, 10, "$"
WAIT1:             ; wait 1 s: CX:DX = 0F4240h microseconds
MOV     CX, 000Fh
MOV     DX, 4240h
MOV     AH, 86h
INT     15h
RET

Nothing is redrawn when the palette changes: the pixels still hold the numbers 1, 2 and 3, and only their meaning changes. That is why the switch is instant.

On EGA and VGA

The general way to change colours on these cards is INT 10h AH=10h, the palette registers and the DAC; 0Bh remains for the CGA modes and the border. In the 16-colour EGA and VGA graphics modes, BH = 0 sets the background colour. The text-mode border is a narrow strip on a VGA, and many monitors and emulators show little or nothing of it, depending on their settings.