[SOLVED] ASM: What does port 3c8h & 3c9h do?

ASM: What does port 3c8h & 3c9h do?

I'm trying to create ASM code which will load and print an 256 color BMP file.
I saw several codes that do this job, and they first load 0 to port 3c8h, and then load the palette to port 3c9h.
What does the load to those ports do?
Thanks in addition! :)

Solution

I remember using those ports to set up VGA color palette. You out the color number on 3c8 and R, G, B values on 3c9 consecutively, IIRC:

mov al, 1    ; set color index 0's rgb value
mov dx, 3c8h
out dx, al
inc dx       ; now 3c9h
mov al, 11h
out dx, al   ; set R = 11h
mov al, 22h
out dx, al   ; set G = 22h
mov al, 33h
out dx, al   ; set B = 33h

so whenever VGA hardware encounters the value "1" in video memory it would emit a pixel with an RGB value of #112233.

Because the color index register is automatically incremented by VGA chip, you could also make use of OUTS instructions. to change the whole palette of the VGA card according to a memory block, you could simply do a:

xor al, al      ; zero al register
mov dx, 3c8h
out dx, al      ; start with color zero
inc dx          ; dx = 3c9h
lds si, palette ; ds:si points to color palette data
mov cx, 300h    ; 3 bytes rgb x 256 colors
rep outsb