I am using VirtualBox 4.3 (on Ubuntu) to take screenshots of various Ubuntu installer bootloader screens (eg. GRUB2, syslinux), and I would like to decrease the number of rows and columns that the bootloader renders to.
Since the OS isn't loaded at this point, I can neither install nor use the VirtualBox guest additions. Is there a way to change the resolution presented the virtual machine at this point?
If it makes a difference, I am only concerned with (virtual) EFI booting.
You need to configure the vga kernel startup parameter:
For example at boot you might use: vga=795. This would set your system to boot using the Live label with a screen resolution to 24bit 1280X1024. Here are some more examples of common vga boot values:
Depth | 800×600 | 1024×768 | 1152×864 | 1280×1024 | 1600×1200
8 bit | vga=771 | vga=773 | vga=353 | vga=775 | vga=796
16 bit | vga=788 | vga=791 | vga=355 | vga=794 | vga=798
24 bit | vga=789 | vga=792 | NA | vga=795 | vga=799
See this post from the Ubuntu forums for more details: http://ubuntuforums.org/showthread.php?t=258484
Edit:
If using GRUB2 and its graphical interface, you will need to edit
/etc/default/grub
find the line
#GRUB_GFXMODE=640x480
remove the # and change 640x480 for the preferred mode you wrote down.
e.g. GRUB_GFXMODE=1200x800
save, then type
sudo update-grub
NOTE: the preferred mode has to be among those listed by vbeinfo.
See post https://askubuntu.com/questions/54067/how-do-i-safely-change-grub2-screen-resolution for additional details
For rEFInd:
# Set the EFI text mode to be used for textual displays. This option
# takes a single digit that refers to a mode number. Mode 0 is normally
# 80x25, 1 is sometimes 80x50, and higher numbers are system-specific
# modes. Mode 1024 is a special code that tells rEFInd to not set the
# text mode; it uses whatever was in use when the program was launched.
# If you specify an invalid mode, rEFInd pauses during boot to inform
# you of valid modes.
# CAUTION: On VirtualBox, and perhaps on some real computers, specifying
# a text mode and uncommenting the "textonly" option while NOT specifying
# a resolution can result in an unusable display in the booted OS.
# Default is 1024 (no change)
#
#textmode 2
# Set the screen's video resolution. Pass this option either:
# * two values, corresponding to the X and Y resolutions
# * one value, corresponding to a GOP (UEFI) video mode
# Note that not all resolutions are supported. On UEFI systems, passing
# an incorrect value results in a message being shown on the screen to
# that effect, along with a list of supported modes. On EFI 1.x systems
# (e.g., Macintoshes), setting an incorrect mode silently fails. On both
# types of systems, setting an incorrect resolution results in the default
# resolution being used. A resolution of 1024x768 usually works, but higher
# values often don't.
# Default is "0 0" (use the system default resolution, usually 800x600).
#
#resolution 1024 768
#resolution 1440 900
#resolution 3
resolution 1920 1080
