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
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
height
depth