ChromeOS Device Firmware

The firmware used by ChromeOS devices is built around serveral open-source projects, and consists (mainly) of a hardware init component (coreboot) and one or more payloads (depthcharge for Verified Boot and ChromeOS, SeaBIOS for Legacy Boot Mode) which are subsequently executed. The firmware resides on an SPI flash chip and uses the layout shown below. The shaded sections at the bottom are read-only, which is enforced by the firmware write-protect screw on the main board (sometimes with a sticker as well on some newer models). When we talk about updating the firmware on ChromeOS devices, we're referring to either the RW_LEGACY or (RO) BOOT_STUB sections, or to the entire firmware image (often referred to as a "full ROM"). These three update types are described in more detail below.