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Index > Main > Writing a pixel in VGA mode 12h (and VESA)

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Joined: 10 Aug 2007
Posts: 789
Location: Adelaide
sinsi 20 Sep 2010, 10:39
This is the code I am using at the moment which seems to work OK, even with banked VESA modes.
        mov ax,12h
        int 10h
        mov dx,3ceh
        mov ax,205h     ;5=mode register
        out dx,ax       ;2=write mode 2

        push es
        mov bx,ax        ;save AL (colour)
        and bl,15
        mov ax,[bpline]  ;bytes per line
        mov di,cx        ;column
        mul dx           ;row*bytes per line = byte address
        shr di,3         ;column/8 = bit address
        add di,ax        ;byte that has the pixel (1 of 8 )
        ;here you would check for carry and switch banks for VESA
        mov ax,0a000h
        mov es,ax
        not cl           ;bits reversed, column<>bit
        and cl,7         ;bit position of pixel
        mov dx,3ceh
        mov ax,108h      ;8=bitmask register
        shl ah,cl        ;correct bit
        out dx,ax        ;?=bitmask
        mov al,[es:di]   ;load latch and ignore
        mov [es:di],bl   ;set the colour planes
        pop es

I set the mode and then set write mode 2 because that's all I am using, since it seems
to make it as easy to write a character (aligned at 8 bits) a line at once.

My thoughts are:
1. Is it correct? It seems to work (both bochs and my real computer).
2. It even seems to work with a VESA banked mode like 1024x768 even though my 8800GT says the mode is not VGA compatible.

I only want to support 16-colour graphics to make my VESA routines complete, but after reading parts of the "graphics black book" I'm not so sure what I'm doing...
Post 20 Sep 2010, 10:39
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Joined: 19 Mar 2008
Posts: 1651
baldr 20 Sep 2010, 17:29

VBE core functions standard 3.0 specifically warns against this (in Developing for Maximum Compatibility section). While it may work on your 8800GT (and many others), this can probably introduce occasional malfunctions and/or lock-ups.
Post 20 Sep 2010, 17:29
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