Mike_Benna@mindlink.UUCP (Mike Benna) (02/13/91)
For those of you who wanted more information about forcing VGA cards
into 16-bit operation, here it is:
Note that all bits are numbered from 0 to 7 ($01 to $80).
Paradise/Western Digital chips (PVGA1A and PVGA1B):
Port 3CEh/3CFh, index 0Bh, bit 2. Here's an example code fragment:
mov dx,3ceh
mov ax,50fh ;turn off write protect on VGA
registers
out dx,ax
mov al,0Bh
out dx,al
inc dx
in al,dx
or al,00000100b ;enable 16-bit display RAM
out dx,al
Ahead V5000 (perhaps only works with revision B):
First, enable extended reg access with:
mov dx,3ceh
mov ax,200fh ;enable extended register access
out dx,ax
Then, port 3CEh/3CFh, index 0Ch, bit 4 is the 16-bit enable reg.
ATI VGAWonder (18800):
mov ax,0C000h ;video BIOS segment
mov es,ax
mov dx,word ptr es:[10h] ;fetch address of ext. regs
mov al,0B3h
out dx,al
inc dx
in al,dx
or al,00100000b ;enable 16-bit operation
out dx,al
Video 7:
mov dx,3c4h
mov al,06h
out dx,al
inc dx
mov al,0EAh
out dx,al ;enable extended regs $80-$FF
dec dx
mov al,0FFh
out dx,al
inc dx
in al,dx
or al,00000001b ;enable 16-bit memory
out dx,al
In addition to this information, there is a very good book about this
kind of stuff called 'Advanced Programmer's Guide to SuperVGAs' by
George Sutty and Steve Blair (Brady, ISBN 0-13-010455-8). If nothing
else the book provides register-level information on several of the
major SVGA chip sets.
---> Mike
--
---> Mike Benna, Vancouver, B.C., Canada
MindSpan Technologies Corp - Video Game Design and Development
UUCP: Mike_Benna@mindlink.UUCP or uunet!van-bc!rsoft!mindlink!Mike_Benna