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3.1.4 The graphics fields of Multiboot header

All of the graphics fields are enabled by flag bit 2. They specify the preferred graphics mode. Note that that is only a recommended mode by the OS image. If the mode exists, the boot loader should set it, when the user doesn't specify a mode explicitly. Otherwise, the boot loader should fall back to a similar mode, if available.

The meaning of each is as follows:

mode_type
Contains ‘0’ for linear graphics mode or ‘1’ for EGA-standard text mode. Everything else is reserved for future expansion. Note that the boot loader may set a text mode, even if this field contains ‘0’.
width
Contains the number of the columns. This is specified in pixels in a graphics mode, and in characters in a text mode. The value zero indicates that the OS image has no preference.
height
Contains the number of the lines. This is specified in pixels in a graphics mode, and in characters in a text mode. The value zero indicates that the OS image has no preference.
depth
Contains the number of bits per pixel in a graphics mode, and zero in a text mode. The value zero indicates that the OS image has no preference.