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OS on the wearable

The bitsy has on-board flash memory which contains the operating system and user applications. On start-up, some file systems are mounted, and some programs are executed. The command set is not GNU, it is a minimal command set provided by BusyBox. This is a single binary that support multiple functions. Each of the standard commands (eg touch,ls etc) are symbolic links in /bin to the /bin/busybox binary. The name of the symbolic link is used by the binary to decide which function to carry out.

File systems

The bitsy is configured to have the following file systems: On startup, the system will check for a file 'autostart' in /flash. If it is present, and it is executable, then this file will be executed. This will allow you to install a stand-alone application on the Bitsy. ***IS THIS TRUE???***

If necessary, one can make an extra ramdisk by first creating the file system "mke2fs /dev/ram1", and subsequently mount it somewhere "mkdir /blah ; mount /dev/ram1 /blah".

Disk partitioning

The flash memory is divided in four partitions.
  1. The first partition is the boot-loader of the bitsy. This partition is not to be erased at any time!
  2. The second partition stores the gzipped kernel. This partition is loaded by the boot-loader of the bitsy on startup. Creating and updating kernels is documented in the page on playing God.
  3. The third partition stores the gzipped root-device. On startup of the kernel, this data is unzipped onto a ram disk, and mounted on /. Note that any changes made to files on / are not stored on flash. Note that it is impossible to change just one file on this device, because it is stored compressed; you must load an entire new file system. See also the utility Loadflash
  4. The fourth partition stores the user partition. On startup of the system, this partition is mounted read-only on /flash. Note that if it is mounted read-write, this partition can be changed, and changes are persistent.
The kernel hardly ever needs modifying. It should be in a fit state to run any user application.

The root partition needs to be modified only if the startup sequence is changed. It maybe a good idea to invoke /flash/rc.local at the end of the sequence; if present. This way, we can change the start sequence without modifying the second partition.

The third partition contains root data. Note that data stored on the second partition is copied to ram disk and hence occupies RAM; while data stored on the third partition stays there and therefore only occupies flash memory. All files on this partition are owned by the user `cyber' (to be done), so that they can be updated with rsync.

Resetting the disk partitions

The disk partitions can be reset to safe values by popping the rescue flash card in the PCMCIA slot, and powering up the Bitsy. After the reset, wait a couple of minutes until all flash memory is written, and then take the card out, put the 802.11 back in and reset again.

Linux

The card runs linux with the 2.4.9 kernel. It does not run any compilers; only a core operating system. with some features like a minimal shell and the like. The base patch-tree for the kernel is 2.4.9-ac10-rmk2-np1-ads2. There are some custom patches to be applied to the tree once it is in this state.

Networking

On startup, the rc scripts will launch pump on eth1, in order to discover its IP address etc. Pump is not started on eth0; this must be enabled manually in /etc/rc.d/init.d/network; if required. Pump will find the IP address for the buffalo card (assuming that a DHCP server is contactable over the wireless network), and a network connection is established.

The DHCP server on waihona knows the mac-addresses of all Buffalo cards, and gives them static IP addresses; it could be made dynamic, but this way we have a bit of control over which machine has which name. Pele runs a local name-daemon who maintains a network 'wear.cs.bris.ac.uk' with in it machines 'pele', 'm001', 'm002', 'm003', etc. All have IP addresses in the range 192.168.250.XXX; this is a set of private addresses, not visible to the outside world. All traffic is masqueraded by pele. ***IS THIS TRUE???***


This page last updated February 24, 2003
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