OS architecture

From AlwaysInnovating

The NAND storage is only used for the u-boot environment. Don't store anything on the NAND, it may be used for the upgrade mechanism in future releases.

Partitions

The 2011-03.a version uses three partitions on an 8GiB SD card:

Boot & Android datas

FAT32, around 2.3G, boot enabled

Root

ext3, around 4.6GB

Swap

swap, around 512MB

Boot

The filesystem on the first partition contains these files:

• mlo (the signed x-loader which is the first file executed by the OMAP processor)
• u-boot.bin (launched by mlo, and setting up all registers of the OMAP processor)
• uImage (the kernel)
• uImage2 (the kernel with initramfs)
• ai-ui.tbz2 (the UI for the multi-boot feature)
• ai-install.tbz2 (an archive used for option 7 of the multi-boot feature)

Multi-Boot options

Since 2010-01.b, the Touch Book comes with the ability to boot different OS with a simple finger press at startup.

See also 2011-03.a for more detailed informations on the multi-boot process and options.

Root base

This partition contains the read-only base of the root filesystem in a squashfs 4.0 format. You can browse the squashfs with the following commands:

$ sudo -s
# mkdir /tmp/ai_2/
# mount /dev/mmcblk0p2 /tmp/ai_2/
# mkdir /tmp/squashfs_mounted
# mount -t squashfs -o loop /tmp/ai_2/.images/ai-os-dev.squashfs /tmp/squashfs_mounted

Root overlay

This is the main filesystem where user data is stored. It's located in the .overlay-ai-os folder on the second partition.

Root filesystem

The root filesystem is a union (using AUFS) of the read-write "overlay" on top of the read-only "base". This means that write operations go straight to the overlay, but read operations are first checked against the overlay before optionally being performed on the base. If a filename exists in the overlay, it is used instead of the base (whether it exists there or not).

So when the overlay is empty, the root fs will appear to contain all the files in the base. But any changes made to the root fs will be performed on the overlay instead, which will then be visible in the composite root fs. This arrangement allows for the core files of the OS to be highly compressed in a read-only format, but for the whole root filesystem to be writeable.

To reset the device to its original state, simply delete all the files on the overlay. Visit the page Revert OS.