Overview

Booting a CapROS system involves the following steps, each of which is discussed in detail below.

1. The kernel and preloaded objects are loaded into RAM.
2. The kernel runs and initializes itself.
3. The kernel runs the nonpersistent IPL process, which in turn starts the other initially running nonpersistent processes.
4. The kernel executes the restart sequence, which initializes the persistent objects.

Loading the kernel and preloaded objects

A CapROS system consists of the kernel and objects. Preloaded objects are those that are loaded at the same time as the kernel. Non-preloaded objects are loaded from disk by the CapROS pager. Preloaded objects include all nonpersistent objects, and may include the persistent objects that define a big bang. The build tutorial has more information on the big bang.

The kernel and preloaded objects are loaded by a boot loader from some nonvolatile medium. The details are machine-specific. Here are some examples:

• On the PC, we use GRUB to load a system image containing the kernel and preloaded objects. The kernel is Multiboot compliant. See the build tutorial for details of building the system image. For development, we use NFS to copy the system image from the development system to the test system, temporarily running Linux on the test system. See the build instructions for details.
• On the ARM, for development we use Redboot to load the kernel code, the kernel data, and the preloaded objects. Redboot receives these files from the development system over Ethernet. See the build instructions for details.
• An embedded system might load a system image from ROM.

Kernel initialization

Kernel initialization is not yet documented.

IPL process

The kernel starts the system by running the single nonpersistent IPL process. The IPL process is identified by an OID that is defined in the system image, using the mkimage "ipl" directive. The IPL process goes through a list of nonpersistent processes to be started, and starts them. This list is built using the mkimage "run" and "threadlist" directives. In a fully nonpersistent system, this list of processes includes all the initial processes, such as device drivers.

Big Bang Theory

In a persistent system, the initial state of the persistent objects must be defined using mkimage and then set up in the system. There are two ways to do this.

• Sysgen takes a file produced by mkimage and writes it to a disk volume so the objects can be loaded from the disk as though they were checkpointed there. During development, this method requires moving the initialized volume from the development system to the test system.
• Npgen takes a file produced by mkimage and includes those objects in the preloaded objects of a system image. These preloaded persistent objects will then be saved to disk on the first checkpoint. This method limits the big bang objects to what will fit in RAM, but it is more convenient for development.

Restart Sequence

There are several different scenarios for the restart sequence.

• Big Bang from Preloaded Objects

  In this scenario, the initial (big bang) persistent objects are preloaded into RAM. The system image of the persistent objects should contain an IPL process. The IPL process is started in the restart sequence, and it in turn starts the other persistent processes. If there is any checkpoint on disk, it is ignored.

  To build such a system, npgen is run twice, once to create an image file of the nonpersistent objects, and once to create an image file of the persistent objects. When building the nonpersistent objects, the -p option must be given to npgen.

  After the system is initialized and running, it can be checkpointed to disk. Note that it takes at least two checkpoints to overwrite any old checkpoint that may already be on the disk.

• Restart from Disk

  If there are no preloaded persistent objects, but there is a disk with a checkpoint, the restart sequence loads the state of the persistent world from the checkpoint. See the Checkpoint Mechanism for details. The checkpoint on disk may be either the previous state of the system saved by a checkpoint, or a big bang (including an IPL process) written to disk by sysgen.

  When building the nonpersistent objects, the -p option must be given to npgen so the nonpersistent world can find the persistent world.

• Nonpersistent system

  If there are no preloaded persistent objects and no disk with a checkpoint, the system runs without any persistent objects; it is nonpersistent only. In this case the -p option must not be given to npgen.

As an implementation detail, the restart sequence is driven by a kernel process.

Copyright 2007, 2008 by Strawberry Development Group. All rights reserved. For terms of redistribution, see the GNU General Public License This material is based upon work supported by the US Defense Advanced Research Projects Agency under Contract No. W31P4Q-07-C-0070. Approved for public release, distribution unlimited.