Glossary

Common terms and acronyms used in ChromeOS firmware and the MrChromebox project.

A

AltFw (Alternative Firmware) : The newer term for Legacy Boot Mode on recent ChromeOS devices. Accessed via the boot menu in Developer Mode, it allows booting alternative operating systems using firmware from the RW_LEGACY region.

AUE (Auto Update Expiration) : The date when a ChromeOS device stops receiving automatic updates from Google. Also called End of Life (EOL). Devices past AUE have restrictions on RW_LEGACY firmware updates and stock firmware restoration.

B

BIOS (Basic Input/Output System) : Traditional PC firmware interface. ChromeOS devices don't use traditional BIOS - they use either stock ChromeOS firmware or can be flashed with custom firmware (like MrChromebox) which supports UEFI booting.

Boot Stub : (Deprecated) A minimal firmware payload used on older ChromeOS devices to chainload an alternative bootloader.

C

cbfstool : Command-line utility for manipulating the coreboot File System (CBFS). Used to extract, add, or modify components in coreboot firmware images.

cbmem : Command-line utility for reading the coreboot memory console log. Essential for debugging firmware issues.

CBFS (Coreboot File System) : The file system used within coreboot firmware images to store payloads, configuration data, and other components.

CCD (Closed Case Debugging) : A Google Security Chip (GSC) feature (CR50/Ti50) that lets you change debug and write-protect controls after opening CCD with gsctool. A SuzyQable talks to the GSC/AP/EC consoles; it is not required to open CCD or to disable WP on Ti50.

coreboot : Open-source firmware project that initializes hardware during boot. All ChromeOS devices (2013+) use coreboot as their firmware base. MrChromebox firmware uses upstream coreboot with (extensive) modifications.

CR50 : Google Security Chip (GSC) found in ChromeOS devices from 2017-2022. Controls hardware write protection and provides closed-case debugging features.

crossystem : ChromeOS utility for reading and setting system properties and boot flags (e.g., enabling Legacy Boot Mode).

D

depthcharge : Google's custom firmware payload used in stock ChromeOS firmware to boot ChromeOS. It performs verified boot and loads the ChromeOS kernel.

Developer Mode : A special boot mode that disables verified boot and allows root access to ChromeOS, booting alternative operating systems, and running the Firmware Utility Script. Required for firmware modifications.

E

EC (Embedded Controller) : A separate microcontroller in ChromeOS devices that manages keyboard, battery, thermal, and other low-level hardware functions. Has its own firmware separate from the main system firmware.

edk2 : Open-source UEFI firmware implementation from the TianoCore project. Used as the payload in MrChromebox UEFI Full ROM firmware and newer RW_LEGACY firmware.

EOL (End of Life) : See AUE (Auto Update Expiration).

F

Firmware Utility Script : MrChromebox's bash script (firmware-util.sh) for installing, updating, and managing custom firmware on ChromeOS devices.

flashrom : Open-source utility for reading, writing, verifying, and erasing flash ROM chips. Used by the Firmware Utility Script to flash firmware.

Full ROM : See UEFI Full ROM Firmware.

G

GBB (Google Binary Block) : A data structure in ChromeOS firmware containing hardware ID, recovery keys, and boot flags. GBB Flags control boot behavior in Developer Mode.

GBB Flags : Firmware-level settings that modify ChromeOS boot behavior in Developer Mode, such as boot timeout, default boot target, and Developer Mode protection.

GSC (Google Security Chip) : Generic term for Google's security chips (CR50 or Ti50) that control hardware write protection and provide security features.

gsctool : ChromeOS command-line utility for talking to the Google Security Chip (CR50/Ti50). Used to open CCD (gsctool -a -o), query WP (-w), and on Ti50 to set AllowUnverifiedRo and disable hardware WP. Does not run after UEFI Full ROM is installed.

H

HWID (Hardware ID) : A unique identifier for each ChromeOS device model (e.g., "BANSHEE A2A-B3C-D4E"). Also called the board name. Used by ChromeOS for device identification and by the Firmware Utility Script for device detection.

I

IFD (Intel Flash Descriptor) : Data structure in Intel firmware that defines the layout of the SPI flash chip, including regions for BIOS, ME, and other components.

L

Legacy Boot Mode : Older term for booting alternative operating systems using the RW_LEGACY firmware region. Accessed via CTRL+L in Developer Mode on older devices. See also AltFw.

M

ME (Management Engine) : Intel's proprietary firmware subsystem that runs independently of the main processor. On ChromeOS devices with MrChromebox UEFI Full ROM firmware, the ME can be enabled/disabled from the Platform Setup menu.

MRC Cache (Memory Reference Code Cache) : Cached memory training data that speeds up boot times. Stored in the firmware flash chip and reused across boots to avoid re-training RAM.

N

Normal Mode : The default ChromeOS boot mode with full verified boot enabled. Only allows booting Google-signed ChromeOS images.

NVRAM (Non-Volatile Random Access Memory) : Memory that retains data when power is off. In UEFI firmware, stores boot configuration (boot order, UEFI settings). RW_LEGACY firmware lacks persistent NVRAM.

P

Payload : The firmware component executed by coreboot to boot the operating system. Examples: depthcharge (ChromeOS), SeaBIOS (Legacy BIOS), edk2 (UEFI).

PP (Physical Presence) : A security mechanism requiring physical interaction (typically pressing the power button) to confirm certain operations like opening CCD. Prevents remote attacks.

R

Recovery Mode : A special boot mode for restoring ChromeOS and RW firmware components from recovery media. Entered via ESC+Refresh+Power (Chromebooks) or recovery button (Chromeboxes).

RO (Read-Only) : Firmware regions protected from modification when write protection is enabled. Contains core boot code and recovery components.

RW (Read-Write) : Firmware regions that can be updated without disabling write protection. Contains updateable firmware components.

RW_LEGACY : A firmware region in stock ChromeOS firmware that can contain an alternative bootloader payload (SeaBIOS or edk2) for booting non-ChromeOS operating systems. Accessed via Legacy Boot Mode/AltFw.

S

SeaBIOS : Open-source legacy BIOS implementation. Used as the payload in older RW_LEGACY firmware implementations.

SMMSTORE : A firmware region used by UEFI firmware to store NVRAM variables (boot order, settings). Emulates UEFI NVRAM on systems without a dedicated NVRAM chip.

SPI (Serial Peripheral Interface) : Communication protocol used for firmware flash chips on ChromeOS devices.

Stock Firmware : The original ChromeOS firmware installed by Google/the manufacturer. Can be restored from a backup or extracted from a recovery image.

SuzyQable / SuzyQ Cable : A USB Type-C debug cable that exposes GSC, AP, and EC serial consoles on CR50/Ti50 devices. Used for persistent WP disable (wp disable atboot), ccd reset factory, and unbricking. Ti50 WP disable from ChromeOS uses gsctool and does not require this cable.

T

Ti50 : Google Security Chip (GSC) on most ChromeOS device families first sold in 2023 or later (Nissa, Skyrim, Brox, Rex, and similar). Successor to CR50. Adds AP RO firmware verification; there is no battery or jumper WP method. Disable WP and RO verification with gsctool from ChromeOS, or with a SuzyQable.

Tianocore : The project name for edk2. Sometimes used interchangeably with edk2.

U

UEFI (Unified Extensible Firmware Interface) : Modern firmware standard that replaced legacy BIOS. Provides features like secure boot, boot manager, and graphical setup menus.

UEFI Full ROM Firmware : MrChromebox's complete firmware replacement for ChromeOS devices. Completely replaces stock firmware with coreboot + edk2, enabling full UEFI boot with persistent NVRAM and boot manager.

V

Verified Boot : ChromeOS security feature that cryptographically verifies firmware and OS components before executing them. Prevents unauthorized modifications.

VPD (Vital Product Data) : Device-specific data stored in firmware including serial number, region, and other manufacturing information. Should be backed up before flashing custom firmware.

W

WP (Write Protection) : Prevents modification of firmware regions. Hardware WP is a screw, jumper, battery sense, or the GSC !WP pin; software WP is flash-chip registers. Must be disabled to flash UEFI Full ROM firmware. On Ti50 you must also set AllowUnverifiedRo=always.

WP Screw : A screw used on some ChromeOS devices to assert hardware write protection. Common on 2013–2016 boards (no GSC) and on a few later CR50 boards. Removing it disables hardware WP. Ti50 boards do not use a WP screw.

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