From mrchromebox.tech:
The ChromeOS firmware boot flags / Google Binary Block (GBB) flags are firmware level settings
stored directly in the firmware flash chip itself in a read-only (RO) area, and therefore require
the firmware write protect to be disabled before setting. The GBB flags control the behavior of
the firmware payload used to boot ChromeOS (depthcharge); they are used to set the default boot
mode (ChromeOS or Legacy Boot Mode), the Developer Mode boot screen timeout (2s or 30s), prevent
exiting Developer Mode via spacebar, and to enable Legacy Boot Mode regardless of crossystem
dev_boot_altfw/dev_boot_legacy setting, among other things.
Important: The GBB Flags are only valid when using the the stock firmware, or stock + RW_LEGACY
firmware. They have no effect (and cannot be set) when using the UEFI Full ROM firmware.
You must first disable firmware write protection.
Note that the following commands must be run as root
On modern versions of ChromeOS, you can use the futility tool to
set the GBB flags, for example:
futility gbb -s --flash --flags=0x4a9
On older versions, there is a script you can call to set the GBB flags:
/usr/share/vboot/bin/set_gbb_flags.sh 0x4a9
Conversely, you can read the current GBB flags with the following command:
futility gbb -g --flash --flags
Or for older versions:
/usr/share/vboot/bin/get_gbb_flags.sh
The current GBB flags are also shown on the recovery/developer boot screen, after
gbb.flags when you press TAB.
I will try to keep this site updated as frequently as possible, but you can find the official list in the ChromiumOS source code.
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