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I can find out which of the read/write firmware sections was used on a particular boot by checking fw_tried (or perhaps it is mainfw_act) under bios_info at chrome://system/. But, how can I tell which kernel-filesystem pair was booted?

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So, looking at: https://www.chromium.org/chromium-os/chromiumos-design-docs/disk-format

The "Google Chrome OS devices" section says, "The BIOS searches the first drive (only) for a GPT partition identified with our special ChromeOS Kernel Type GUID (fe3a2a5d-4f32-41a7-b725-accc3285a309). There should be two (image A and image B). Attribute bits within each partition table entry select which of the two is the most recent (or valid) one." And scrolling down to the "Selecting the kernel" section, there's a table that outlines the attributes.

So, I'm sure there's another way to dump these, but good ol' fdisk. Be careful with fdisk. After firing up 'fdisk /dev/sda', the 'i' command prints info about a partition, including the attributes. My KERN-A partition has attribute bits 49, 53 and 54 set, and KERN-B has 48 and 56 set. Looking back at the link above, it looks like I'm running B: bit 56 set for B for a successful boot and it has priority 1 (bit 48 set). KERN-A has priority 2 (bit 49) and six boot attempts left (bits 53, 54 and 52).

There might be a cleaner way to figure this out, but I didn't find anything obvious when I googled.

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  • Also, looks like 'rootdev -s' will print out the rootfs device. In my case, /dev/sda5, which is root-b.
    – jaf
    Apr 26, 2017 at 2:56
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it sounds like you have a x86 chromebook. Arm chromebook's do not have a (legacy) bios.

rootdev -s shows /dev/mmcblk0p3 and chrome://system/ shows:

CHROME VERSION = 59.0.3071.113
CHROMEOS_FIRMWARE_VERSION = Google_Veyron_Speedy.6588.237.0
CHROMEOS_RELEASE_BUILDER_PATH = veyron_speedy-release/R59-9460.67.0

Paul

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