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

Question

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! :)

Kentigera

They control the DAC on the video hardware. – Splasher

@MichaelPetch Thanks! – Kentigera

More info: http://www.osdever.net/FreeVGA/vga/vga.htm. Particularly here and here. – Madeira

Outputting to 0x03c9 will make it automatically to advance, so you don't need to output the index of colour to 0x03c8 ahead of each one. Unfortunately you never know what is the current state, that's why it starts by outputting at least the 0 index. ("unfortunately" if you are working on 256B intro for DOS, where saving that one out of index would help a lot, but it will start from 0 colour only on some PCs+DOS combinations). – Deformity

Answer 1

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

Donetta