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If your kernel is multiboot-compliant, you can simply extend your multiboot header to specify a graphics mode for GRUB to switch into:

And 1 other question. If I setup an ISO image of my kernel with GRUB as bootloader, how should I tell it to use VGA or SVGA modules and what screen resolution should it setup VBE with?

.long 0, 0, 0, 0, 0
.long 0 # 0 = set graphics mode
.long 1024, 768, 32 # Width, height, depth

If your kernel is multiboot-compliant, you can simply extend your multiboot header to specify a graphics mode for GRUB to switch into:

Code:
.set ALIGN, 1 << 0
.set MEMINFO, 1 << 1
.set VIDINFO, 1 << 2
.set FLAGS, ALIGN | MEMINFO | VIDINFO
.set MAGIC, 0x1BADB002
.set CHECKSUM, -(MAGIC + FLAGS)

.section .multiboot
.long MAGIC
.long FLAGS
.long CHECKSUM
.long 0, 0, 0, 0, 0
.long 0 # 0 = set graphics mode
.long 1024, 768, 32 # Width, height, depth

(See this file for some more detailed explanations)

You will probably also need to access the multiboot structure that GRUB gives you a pointer to in EBX (it's a physical address!) to get the VBE info structure, in which you can locate where in memory (physical, again, so you'll need to map this to your virtual address space) the linear framebuffer is. From there, if you have either 16bpp or 32bpp modes, it's as simple as writing an RGB value to that memory location.

MadZarx wrote:So there should be no special parameters or other stuff when creating the iso image with grub. Right?
Then how can I change the resolution later?

tristanseifert wrote: You would use the VBE graphics BIOS calls. You could drop back into real mode, use V8086 mode and write a BIOS monitor to let you access other BIOS functions.

Brendan wrote:Hi,

For multi-boot 1 (which is supported by GRUB, GRUB2, and a few other things); you can put the "preferred video mode" in your file's multiboot header. This "preferred video mode" is only a recommendation and the boot loader is free to ignore it or choose a similar video mode. For example, if kernel doesn't support 15-bpp modes at all and asks for "800*600 with 32 bits per pixel", then you might get 800*600 with 15 bits per pixel (there's no way to prevent the boot loader from choosing video modes you don't support). Also note that the code in most boot loaders is crap and won't (e.g.) use the monitor's EDID to ensure that the video mode you've requested is actually supported by the monitor; which means that even if you do get what you asked for it may be unusable.

For multi-boot 1, the specification says that the boot loader has to "leave the environment usable" but does not say what that environment might be, and there's no way to determine what the environment was. If you switch back to real mode and use BIOS functions then it will work if the boot loader was booted from a BIOS environment; but it will crash if the boot loader was booted from anything else (e.g. UEFI) and there's no way to prevent that.

Finally, for some cases (e.g. a decent UEFI system) the environment supports multiple monitors. Neither of these options will ever be able to support this.


Cheers,

Brendan

MadZarx wrote:I need at least the 800x640 resolution

So what is the best way to get it working??

Combuster wrote:qemu -kernel is ugly. It also doesn't work that way on real hardware. Therefore, don't use it.