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.
+ +- 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), - 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. -
+ it is referred to as stage 1.5, whereas the other stages are referred to as stage 1 + and 2. This architecture is used because the BIOS booting process only loads a very + small piece of code from disk for booting, insufficient for the needs outlined above. + +The following will explain all stages in detail.
@@ -27,7 +36,7 @@ 1.5 is in charge immediately).
- 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
at byte offset 512, and about 170 bytes large, this section is already reserved,
and thus cannot be used by the loader itself.
@@ -36,8 +45,8 @@
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.
+ execute it. In BeOS this boot loader would be in "/boot/beos/system/zbeos",
+ in Haiku this is haiku_loader.bios_ia32 found in the haiku_loader package.
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
@@ -45,9 +54,9 @@
is written in one sequential block (which should be always the case anyway).
- 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 disk which would cause the BIOS to load the first 512 bytes of that file and execute it. @@ -61,12 +70,12 @@
- 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 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. + This part is very similar to the stage 1 boot loader from NewOS.
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.
+ +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.
+ +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.
+ +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.
+ +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.
+ +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.