# OpenBeOS x86 boot process specification

###### Creation Date: November 23, 2002
Version: 1.0 (Nov 25, 2002)
Status: preliminary proposal
Author(s): Axel Dörfler

OpenBeOS will use a boot loader process with 3 different stages. Since the second stage is bound tightly to both other stages (which are independent from each other), is referred to as stage 1.5, whereas the other stages are referred to as stage 1 and 2.

The following will explain all stages in detail. Note that this document is not necessarily complete and a work in progress - it doesn't describe a situation as-is, but one that very likely will be.

### Stage 1

The first stage is responsible for loading the real boot loader from a BFS disk. It will be loaded by the Master Boot Record (MBR) and executed in the x86 real mode. It is only used if the system will be booted directly from a BFS partition, it won't be used at all if it is booted from a floppy disk or CD-ROM (in this case, stage 1.5 is in charge immediately).

It resides in the first first 1024 bytes of a BFS disk which usually refers to the first two sectors of the partition in question. Since the BFS super block is located at byte offset 512, and about 170 bytes large, this section is already reserved, and thus cannot be used by the loader itself.
The MBR only loads the first sector of a partition into memory, so it has to load the super block (and the rest of its implementation) by itself.

The loader must be able to load the real boot loader from a certain path, and execute it. In BeOS this boot loader would be in "/boot/beos/system/zbeos" - this name will likely change for OpenBeOS, though.
Theoretically, it is enough to load the first few blocks from the loader, and let the next stage then load the whole thing (which it has to do anyway if it has been written on a floppy). This would be one possible optimization if the 850 bytes of space are filled too early, but would require that "zbeos" is written in one sequential block (which should be always the case anyway).

### zbeos (or whatever it will be called)

Contains both, the stage 1.5 boot loader and the compressed stage 2 loader. It's not an ELF executable file; i.e. it can be directly written to a floppy disk which would cause the BIOS to load the first 512 bytes of that file and execute it.

Therefore, it will start with the stage 1.5 boot loader which will be loaded either by the BIOS when it directly resides on the disk (for example when loaded from a floppy disk), or the stage 1 boot loader, although this one could have a different entry point than the BIOS.

### Stage 1.5

Will have to load the rest of "zbeos" into memory (if not already done by the stage 1 loader in case it has been loaded from a BFS disk), set up the global descriptor table, switch to x86 protected mode, uncompress stage 2, and execute it.

This part is very similar to the current stage 1 boot loader from NewOS.

### Stage 2

This is the most complex part of the boot loader. In short, it has to load any modules and devices the kernel needs to access the boot device, set up the system, load the kernel, and execute it.

The kernel, and the modules and drivers needed are loaded from the boot disk - therefore the loader has to be able to access BFS disks. It also has to be able to load and parse the settings of these drivers (and the kernel) from the boot disk, some of them are already important for the boot loader itself (like "don't call the BIOS"). Since this stage is already executed in protected mode, it has to use the virtual-86 mode to call the BIOS and access any disk.

Before loading those files from the boot disk, it should look for additional files located on a specific disk location after the "zbeos" file (on floppy disk or CD-ROM). This way, it could access disks that cannot be accessed by the BIOS itself.

Setting up the system for the kernel also means initalizing PCI devices needed during the boot process before the kernel is up. It must be able to do so since the BIOS might not have set up those devices correctly or at all.

It also must calculate a check sum for the boot device which the kernel can then use to identify the boot volume and partition with - there is no other reliable way to map BIOS disk IDs to the /dev/disk/... tree the system itself is using.

After having loaded and relocated the kernel, it executes it by passing a special structure which tells the kernel things like the boot device check sum, which modules are already loaded and where they are.

The stage 2 boot loader also includes user interaction. If the user presses a special key during the boot process (like the space key, or some others as well), a menu will be presented where the user can select the boot device (if several, the loader has to scan for options), safe mode options, VESA mode, etc.

This menu may also come up if an error occured during the execution of the stage 2 loader.