GRUB won't set video mode above 640x480x32 [Solved]

codyd51 · **Posted:** Wed Jan 18, 2017 11:45 am

Hello,

I'm trying to use GRUB to set a video mode instead of querying available modes myself. This is my multiboot header:

Code:

mboot:
   dd MBOOT_HEADER_MAGIC   ; header value for GRUB
   dd MBOOT_HEADER_FLAGS   ; grub settings
   dd MBOOT_CHECKSUM   ; ensure above values are correct

   dd 0 ; these flags are unused
   dd 0
   dd 0
   dd 0
   dd 0 

   dd 0 ; set video mode
   ;1024 x 768 x 24
   dw 1024
   dw 768
   dw 24

No matter what resolution/bpp I set there, GRUB always sets 640x480x32. I'm running my OS in QEMU.

Has anyone encountered a similar issue?

glauxosdever · **Posted:** Wed Jan 18, 2017 12:18 pm

Hi,

If GRUB can't find a video mode of the specified properties, it will set whatever best it can find.

It would be useful to also provide the qemu command.

Hope this helps. :-)

Regards,
glauxosdever

matt11235 · **Posted:** Wed Jan 18, 2017 12:19 pm

What do you have for MBOOT_HEADER_FLAGS? I think that you need to set the second bit to 1.

hannah · **Posted:** Wed Jan 18, 2017 12:33 pm

Try putting -vga std on your QEMU command line.

codyd51 · **Posted:** Wed Jan 18, 2017 1:01 pm

These are my multiboot flags:

Code:

MBOOT_PAGE_ALIGN   equ 1<<0   ; load kernel and modules on page boundary
MBOOT_MEM_INFO      equ 1<<1   ; provide kernel. with memory info
MBOOT_VBE_MODE      equ 1<<2   ; have GRUB set video mode
MBOOT_HEADER_MAGIC   equ 0x1BADB002   ; multiboot magic value
MBOOT_HEADER_FLAGS   equ MBOOT_PAGE_ALIGN | MBOOT_MEM_INFO | MBOOT_VBE_MODE
MBOOT_CHECKSUM      equ -(MBOOT_HEADER_MAGIC + MBOOT_HEADER_FLAGS)

And this is the command I'm using to launch QEMU:

Code:

   $(EMULATOR) -vga std -net nic,model=ne2k_pci -d cpu_reset -D qemu.log -serial file:syslog.log -cdrom $^

Kazinsal · **Joined:** Wed Jul 13, 2011 7:38 pm **Posts:** 558

The mode parameters in the header should be stored as dd (32 bits), not dw (16 bits).

codyd51 · **Posted:** Wed Jan 18, 2017 2:54 pm

Kazinsal wrote:

The mode parameters in the header should be stored as dd (32 bits), not dw (16 bits).

Whoops! That's a goof. Thanks a lot! That solved the issue.

OSDev.org

GRUB won't set video mode above 640x480x32 [Solved]

Who is online