9 Advanced topics
Additions automatically provide the reduced video modes. Additionally, the VESA BIOS has been adjusted to duplicate its standard mode table with adjusted resolutions. The adjusted mode IDs can be calculated using the following formula:
reduced_modeid = modeid + 0x30
For example, in order to start Linux with 1024 x 748 x 16, the standard mode 0x117 (1024 x 768 x 16) is used as a base. The Linux video mode kernel parameter can then be calculated using:
vga = 0x200 0x117 + 0x30 vga = 839
The reason for duplicating the standard modes instead of only supplying the ad- justed modes is that most guest operating systems require the standard VESA modes to be fixed and refuse to start with different modes.
When using the X.org VESA driver, custom modelines have to be calculated and added to the configuration (usually in /etc/X11/xorg.conf. A handy tool to determine modeline entries can be found at http://www.tkk.fi/Misc/Electronics/ faq/vga2rgb/calc.html.)
9.6 Custom VESA resolutions
Apart from the standard VESA resolutions, the VirtualBox VESA BIOS allows you to add up to 16 custom video modes which will be reported to the guest operating system. When using Windows guests with the VirtualBox Guest Additions, a custom graphics driver will be used instead of the fallback VESA solution so this information does not apply.
Additional video modes can be configured for each VM using the extra data facility. The extra data key is called CustomVideoMode<x> with x being a number from 1 to
16.Please note that modes will be read from 1 until either the following number is not defined or 16 is reached. The following example adds a video mode that corresponds to the native display resolution of many notebook computers:
VBoxManage setextradata VMNAME "CustomVideoMode1" "1400x1050x16"
The VESA mode IDs for custom video modes start at 0x160. In order to use the above defined custom video mode, the following command line has be supplied to Linux:
vga = 0x200 0x160 vga = 864
For guest operating systems with VirtualBox Guest Additions, a custom video mode can be set using the video mode hint feature.
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