doc/uImage.FIT/x86-fit-boot.txt

5 ----------
10 binary which contains all sorts of parameters and a compressed self-extracting
11 binary for the kernel itself, often with a small built-in serial driver to
15 understanding is that an x86 CPU (even a really new one) starts up in a 16-bit
16 'real' mode where only 1MB of memory is visible, moves to 32-bit 'protected'
18 then to 64-bit 'long' mode if 64-bit execution is required.
20 Partly the self-extracting nature of Linux was introduced to cope with boot
21 loaders that were barely capable of loading anything. Even changing to 32-bit
25 Bit by bit more and more logic has been added to this post-boot pre-Linux
28 - Changing to 32-bit mode
29 - Decompression
30 - Serial output (with drivers for various chips)
31 - Load address randomisation
32 - Elf loader complete with relocation (for the above)
33 - Random number generator via 3 methods (again for the above)
34 - Some sort of EFI mini-loader (1000+ glorious lines of code)
35 - Locating and tacking on a device tree and ramdisk
38 doesn't make a huge amount of sense. Any boot loader worth its salts already
39 has most of the above features and more besides. The boot loader already knows
41 ELF loader and supports device tree and ramdisks. The decision to duplicate
43 denominator: a boot loader which consists of a BIOS call to load something off
46 (Aside: On ARM systems, we worry that the boot loader won't know where to load
48 or in the boot loader rather than adding a self-relocator to put it in the
51 As a result, the x86 kernel boot process is needlessly complex. The file
58 This document describes an alternative boot process which uses simple raw
59 images which are loaded into the right place by the boot loader and then
64 ----------------
66 Note: these instructions assume a 32-bit kernel. U-Boot also supports directly
67 booting a 64-bit kernel by jumping into 64-bit mode first (see below).
72 $ objdump -h vmlinux
74 vmlinux:     file format elf32-i386
152 arch/x86/boot/setup.bin and is about 12KB in size. It includes the command
153 line and various settings need by the kernel. Arguably the boot loader should
160 U-Boot although I suppose you could mostly script it. This would permit the
163 All we need to boot is the vmlinux file and the setup.bin file.
167 ------------
171 the 'arch' value for booting 64-bit kernels if desired. Put this into a file
177 address so that U-Boot doesn't overwrite it when decompressing. Something like
184    objcopy -O binary vmlinux vmlinux.bin
190    mkimage -f image.its image.fit
192 (be careful to run the mkimage from your U-Boot tools directory since it
197 $ dumpimage -l image.fit
222   Description:  Boot Linux kernel
227 ---------------
229 To make it boot you need to load it and then use 'bootm' to boot it. A
236 This will load the image from the network and boot it. The command line (from
240 tree then x86 doesn't normally use those - it has ACPI instead.
244 -----------
247 2. It allows use of the standard U-Boot boot file format
248 3. It allows U-Boot to perform decompression - problems will provide an error
249 message and you are still in the boot loader. It is possible to investigate.
250 4. It avoids all the pre-loader code in the kernel which is quite complex to
252 5. You can use verified/secure boot and other features which haven't yet been
253 added to the pre-Linux
260 ----------
262 In the Linux kernel, Documentation/x86/boot.txt defines the boot protocol for
263 the kernel including the setup.bin format. This is handled in U-Boot in
269 --
272 7-Oct-2014