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boot: Rewrite README

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K. Lange 2021-09-12 20:56:45 +09:00
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# ToaruOS Bootloader
This is a simple, limited BIOS and EFI bootloader for ToaruOS kernels.
This is version 4.0 of the ToaruOS Live CD bootloader.
It provides an implementation of the Multiboot bootloader standard.
The bootloader targets both BIOS and EFI. The BIOS loader is limited to El Torito CD boot, while the EFI loader should work in most configurations.
The BIOS loader includes an ISO 9660 filesystem driver, and is suitable for use as an El Torito no-emulation boot image for use on a CD.
The bootloader provides a menu for selecting boot options, a simple editor for further customization of the kernel command line, and a 32-bit ELF loader for loading multiboot-compatible builds of Misaka.
The EFI loader is built as both a 64-bit and 32-bit EFI executable and uses the EFI interfaces for file system access, and as such is suitable for a wider variety of environments.
While much of the codebase is shared between the two platforms, the BIOS loader works very differently from the EFI loader. It includes a minimal ISO 9660 filesystem implementation for locating boot files, and also loads the entire contents of the boot medium into memory before entering protected mode and displaying the menu.
Both loaders are based on the same codebase and implement the same menu system, which can be found `cstuff.c`.
The EFI loader, meanwhile, is built for x86-64 ("x64" in MS/EFI terms), and runs as a normal EFI application in long mode to display the menu, load the kernel and ramdisk through EFI filesystem access APIs, and load it into memory. It then downgrades to protected mode to allow the kernel's multiboot entrypoint to execute (which then returns to long mode again).
The EFI loader is built using GNU-EFI, but does not use any of its convenience library functions.
While the loader implements a subset of Multiboot functionality, it is likely not suited for general use by other multiboot kernels and is tailored specifically for loading Misaka.