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Update "boot process specs" documentation

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Quite a lot has happened since this was initially written.

Change-Id: Iad68ea821733ab7489d2f9713857d2752b80356d
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3674
Reviewed-by: Adrien Destugues <[email protected]>
This commit is contained in:
Adrien Destugues
2021-01-27 19:51:07 +00:00
committed by Adrien Destugues
parent e73999aa57
commit 4dcdff4fe3
1 changed files with 67 additions and 20 deletions
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docs/develop/kernel/boot/boot_process_specs_x86.html → docs/develop/kernel/boot/boot_process_specs.html
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@@ -1,22 +1,31 @@
<body bgcolor=white> <body bgcolor=white>
<h1>OpenBeOS x86 boot process specification</h1> <h1>Haiku boot process specification</h1>
<h6> <h6>
Creation Date: November 23, 2002<br> Creation Date: November 23, 2002<br>
Version: 1.0 (Nov 25, 2002)<br> Version: 2.0 (Jan 22, 2021)<br>
Status: preliminary proposal<br> Status: documenting the current state of things<br>
Author(s): Axel D&ouml;rfler Author(s): Axel D&ouml;rfler, Adrien Destugues
</h6> </h6>
<h2>Overview</h2>
<p>Unlike other systems, Haiku comes with its own user-friendly bootloader. The main task of
the bootloader is to load and start the kernel. We don't have a concept of an initramfs as
Linux does, instead our bootloader is able to find the kernel and modules in a BFS partition,
and even extract them from packages as needed. It also provides an early boot menu that can
be used to change settings, boot older versions of Haiku that were snapshotted by the package
system, and write boot logs to USB mass storage.</p>
<h2>Booting from BIOS</h2>
<p> <p>
OpenBeOS will use a boot loader process with 3 different stages. Since the second Haiku BIOS boot loader process is split into 3 different stages. Since the second
stage is bound tightly to both other stages (which are independent from each other), 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 it is referred to as stage 1.5, whereas the other stages are referred to as stage 1
and 2. and 2. This architecture is used because the BIOS booting process only loads a very
</p> small piece of code from disk for booting, insufficient for the needs outlined above.</p>
<p>
The following will explain all stages in detail. Note that this document is not <p>The following will explain all stages in detail.</p>
necessarily complete and a work in progress - it doesn't describe a situation
as-is, but one that very likely will be.
</p>
<h3>Stage 1</h3> <h3>Stage 1</h3>
<p> <p>
@@ -27,7 +36,7 @@
1.5 is in charge immediately). 1.5 is in charge immediately).
</p> </p>
<p> <p>
It resides in the first first 1024 bytes of a BFS disk which usually refers to the It resides in the first 1024 bytes of a BFS disk which usually refers to the
first two sectors of the partition in question. Since the BFS superblock is located first two sectors of the partition in question. Since the BFS superblock is located
at byte offset 512, and about 170 bytes large, this section is already reserved, at byte offset 512, and about 170 bytes large, this section is already reserved,
and thus cannot be used by the loader itself.<br> and thus cannot be used by the loader itself.<br>
@@ -36,8 +45,8 @@
</p> </p>
<p> <p>
The loader must be able to load the real boot loader from a certain path, and 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" - execute it. In BeOS this boot loader would be in "/boot/beos/system/zbeos",
this name will likely change for OpenBeOS, though.<br> in Haiku this is haiku_loader.bios_ia32 found in the haiku_loader package.<br>
Theoretically, it is enough to load the first few blocks from the loader, and 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 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 has been written on a floppy). This would be one possible optimization
@@ -45,9 +54,9 @@
is written in one sequential block (which should be always the case anyway). is written in one sequential block (which should be always the case anyway).
</p> </p>
<h3>zbeos (or whatever it will be called)</h3> <h3>haiku_loader.bios_ia32</h3>
<p> <p>
Contains both, the stage 1.5 boot loader and the compressed stage 2 loader. 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 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 disk which would cause the BIOS to load the first 512 bytes of that file and
execute it. execute it.
@@ -61,12 +70,12 @@
<h3>Stage 1.5</h3> <h3>Stage 1.5</h3>
<p> <p>
Will have to load the rest of "zbeos" into memory (if not already done by the Will have to load the rest of haiku_loader 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 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. descriptor table, switch to x86 protected mode, uncompress stage 2, and execute it.
</p> </p>
<p> <p>
This part is very similar to the current stage 1 boot loader from NewOS. This part is very similar to the stage 1 boot loader from NewOS.
</p> </p>
<h3>Stage 2</h3> <h3>Stage 2</h3>
@@ -115,4 +124,42 @@
This menu may also come up if an error occured during the execution of the stage This menu may also come up if an error occured during the execution of the stage
2 loader. 2 loader.
</p> </p>
<h2>Open Firmware</h2>
<p>On Open Firmware based systems, there is no need for a stage 1.5 because the firmware
does not give us as many constraints. Instead, the stage 2 is loaded directly by the firmware.
This requires converting the haiku_loader executable to the appropriate executable format
(a.out on sparc, pef on powerpc). The conversion is done using custom tools because binutils
does not support these formats anymore.</p>
<p>There is no notion of real and protected mode on non-x86 architectures, and the bootloader
is able to easily call Open Firmware methods to perform most tasks (disk access, network booting,
setting up the framebuffer) in a largely hardware-independent way.</p>
<h2>U-Boot</h2>
<p>U-Boot is able to load the stage2 loader directly from an ELF file. However, it does not
provide any other features. It is not possible for the bootloader to call into U-Boot APIs
for disk access, displaying messages on screen etc (while possible in theory, these features
are often disabled in U-Boot). This means haiku_loader would need to parse the FDT (describing
the available hardware) and bundle its own drivers for using the hardware. This approach is
not easy to set up, and it is recommended to instead use the UEFI support in U-Boot where
possible.</p>
<h2>EFI</h2>
<p>On EFI systems, there is no need for a stage1 loader as there is for BIOS. Instead, our stage2
loader (haiku_loader) can be executed directly from the EFI firmware.</p>
<p>The EFI firmware only knows how to run executables in the PE format
(as used by Windows) because Microsoft was involved in specifying it.
On x86_64, we can use binutils to output a PE file directly. But on other platforms, this is not
supported by binutils. So, what we do is generate a "fake" PE header and wrap our elf file inside
it. The bootloader then parses the embedded ELF header and relocates itself, so the other parts
of the code can be run.</p>
<p>After this initial loading phase, the process is very similar to the Open Firmware one. EFI
provides us with all the tools we need to do disk access and both text mode and framebuffer
output.</p>
</body> </body>