[haiku-development] Re: vm86 Question
- From: Stephan Assmus <superstippi@xxxxxx>
- To: haiku-development@xxxxxxxxxxxxx
- Date: Tue, 30 Sep 2008 10:18:10 +0200
Gerald Zajac wrote:
> I'm working on an enhancement to the S3 video driver to use the vm86
> functions to obtain the EDID info from the video BIOS when the current S3
> EDID code is unable to get the EDID info. I've tried it with the Trio64
> chips and it works fine; however I have a question about how an address
> is selected for the buffer to receive the data from the BIOS. I used the
> following function from file
> src/add-ons/kernel/drivers/graphics/vesa/vesa.cpp as an example for how
> to set up my code:
>
> static status_t
> vbe_get_mode_info(struct vm86_state *vmState, uint16 mode,
> struct vbe_mode_info *modeInfo)
> {
> struct vbe_mode_info *vbeModeInfo = (struct vbe_mode_info *)0x1000;
>
> memset(vbeModeInfo, 0, sizeof(vbe_mode_info));
> vmState->regs.eax = 0x4f01;
> vmState->regs.ecx = mode;
> vmState->regs.es = 0x1000 >> 4;
> vmState->regs.edi = 0x0000;
>
> status_t status = vm86_do_int(vmState, 0x10);
>
> ...
> }
>
> My question: Is there any particular reason that 0x1000 is used as the
> address for vbeModeInfo which is then set in the es & edi regs.? From
> trial & error with the EDID code in the S3 driver, it appears that any
> address can be used as long as the address and the data are completely
> within the ram area set up by a previous call to vm86_prepare(). Is this
> correct? The ram area has a starting address of 0.
If my memory serves, this was done to still be able to recognize NULL
pointer access. Maybe someone else can chime in with more details.
Best regards,
-Stephan
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