Re: LiveCD bootloader

[Brendan Trotter]

Hi,

On Wed, Sep 8, 2010 at 12:12 PM, Teresa e Junior
<address@hidden> wrote:
> But due to some features I need (not the eyecandy), I noticed the CD
> only boots without gfx support (which is the most important for me).

You can't really safely use graphics support in a Live-CD anyway.

GRUB and Linux use "video mode numbers" to request a video mode. These
video mode numbers can be different for different video cards. Early
versions of VBE did define some standard mode numbers, but these
became obsolete in VBE version 2.0 (back in 1994). Software written
after 1994 shouldn't rely on these obsolete video mode numbers. For
example, if you're lucky mode 0x118 might be 1024*768 with 24-BPP, but
nothing guarantees that anymore, and it could easily be any other
video mode. There's also no guarantee that the the video card supports
the video mode (e.g. a lot of video cards only support 32-BPP video
modes and don't support any 24-BPP video modes; and some only support
24-BPP video modes and not 32-BPP video modes; and the same is true
for 15-BPP vs. 16-BPP).

Finally, even if you're lucky and VBE does support the video mode (and
it's the right one, not something else); there's no guarantee that the
monitor also supports the video mode (neither VBE nor GRUB nor Linux
bother the check the monitors EDID information to make sure). This can
be worse than "user can't see anything. For some (very old) "VGA only"
monitors exceeding the monitor's maximum timing frequencies can cause
the monitor to blow up (but these are quite rare now, for obvious
reasons). I've also heard rumours that for some LCDs built into
cheaper notebook type devices, exceeding the screen's maximum timing
frequencies can cause permanent damage.

Basically, the entire thing only works when the user tells GRUB/Linux
which video mode to use (and then it's the user's fault if they choose
something that doesn't work). It fails for a Live-CD which is meant to
work "as is" and isn't pre-configured by the user.

The only "least likely to fail" option would be to use standard VGA
video modes instead - e..g mode 0x12 (320*200*256 colours) or mode
0x13 (640*480*16 colours). These mode numbers are "guaranteed" (by VGA
compatibility) to be what you asked for, and are also "guaranteed" to
work with the monitor (by VGA compatibility for older monitors and by
VESA's "Safe Mode Timing" specification for newer monitors).
Unfortunately they also suck - very low resolution and/or poor colour
depth.

Of course, I'm not saying that you can't unsafely use graphics support
in a Live-CD - I'm sure plenty of people have done worse.



Cheers,

Brendan