When you boot your system, the CPU is reset, and it executes whatever is at its reset vector. This is usually a BIOS on x86 boxes, but on other platforms it might be a ROM monitor, or it might be a direct jump into some IPL code for that board. After a ROM monitor runs, it generally jumps to the IPL, and a BIOS might do this as well—or it might jump directly to the start of the OS image.
Figure 1. Booting a Neutrino system.The IPL copies the boot image into memory and jumps to the startup. The startup code initializes the hardware, fills the system page with information about the hardware, loads callout routines that the kernel uses for interacting with the hardware, and then loads and starts the microkernel and process manager, procnto (which, starting with release 6.3.0, also manages named semaphores). IPL and startup for a board are generally part of a Board Support Package (BSP) for a particular board.
After procnto has completed its initialization, it runs the commands supplied in the boot script, which might start further customization of the runtime environment either through a shell script or through some program written in C, C++, or a combination of the two.
On a non-x86 disk-booted system, that's pretty well how it happens: most customization is done in the boot script or in a shell script that it calls. For more details, see Making an OS Image in Building Embedded Systems.
For an x86 BIOS boot, this becomes more complex:
Figure 2. Booting a Neutrino system with an x86 BIOS.After gaining control, the BIOS configures the hardware, and then it scans for BIOS extension signatures (0x55AA). It calls each BIOS extension (e.g., a network card with a boot ROM or hard disk controller) until one of them boots the system. If none of the BIOS extensions boots the system, the BIOS presents some (usually strange) failure message.
For the network boot case, the boot ROM (usually bootp) downloads an image from a server, copies it into memory, then jumps to the start of the image. The boot image generally needs to run a network stack, and starts some sort of network filesystem to retrieve or access additional programs and files.
You can use the mkifs utility to create the OS image. For a sample buildfile for this sort of image, see the Examples appendix.
For a disk-based boot of a Neutrino desktop system, the process of booting, and especially system initialization, is more complex. After the BIOS has chosen to boot from the disk, the primary boot loader (sometimes called the partition loader) is called. This loader is "OS-agnostic;" it can load any OS. The one installed by Neutrino installations displays the message:
Press F1-F4 to select drive or select partition 1,2,3? 1
After a short timeout, it boots whatever OS system is in the partition prompted for. This loader is /boot/sys/ipl-diskpc1. You can write a loader onto a disk by using dloader.