1013 lines
34 KiB
Plaintext
1013 lines
34 KiB
Plaintext
THE LINUX/x86 BOOT PROTOCOL
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---------------------------
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On the x86 platform, the Linux kernel uses a rather complicated boot
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convention. This has evolved partially due to historical aspects, as
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well as the desire in the early days to have the kernel itself be a
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bootable image, the complicated PC memory model and due to changed
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expectations in the PC industry caused by the effective demise of
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real-mode DOS as a mainstream operating system.
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Currently, the following versions of the Linux/x86 boot protocol exist.
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Old kernels: zImage/Image support only. Some very early kernels
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may not even support a command line.
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Protocol 2.00: (Kernel 1.3.73) Added bzImage and initrd support, as
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well as a formalized way to communicate between the
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boot loader and the kernel. setup.S made relocatable,
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although the traditional setup area still assumed
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writable.
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Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
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Protocol 2.02: (Kernel 2.4.0-test3-pre3) New command line protocol.
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Lower the conventional memory ceiling. No overwrite
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of the traditional setup area, thus making booting
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safe for systems which use the EBDA from SMM or 32-bit
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BIOS entry points. zImage deprecated but still
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supported.
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Protocol 2.03: (Kernel 2.4.18-pre1) Explicitly makes the highest possible
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initrd address available to the bootloader.
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Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
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Protocol 2.05: (Kernel 2.6.20) Make protected mode kernel relocatable.
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Introduce relocatable_kernel and kernel_alignment fields.
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Protocol 2.06: (Kernel 2.6.22) Added a field that contains the size of
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the boot command line.
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Protocol 2.07: (Kernel 2.6.24) Added paravirtualised boot protocol.
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Introduced hardware_subarch and hardware_subarch_data
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and KEEP_SEGMENTS flag in load_flags.
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Protocol 2.08: (Kernel 2.6.26) Added crc32 checksum and ELF format
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payload. Introduced payload_offset and payload_length
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fields to aid in locating the payload.
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Protocol 2.09: (Kernel 2.6.26) Added a field of 64-bit physical
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pointer to single linked list of struct setup_data.
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Protocol 2.10: (Kernel 2.6.31) Added a protocol for relaxed alignment
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beyond the kernel_alignment added, new init_size and
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pref_address fields. Added extended boot loader IDs.
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**** MEMORY LAYOUT
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The traditional memory map for the kernel loader, used for Image or
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zImage kernels, typically looks like:
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0A0000 +------------------------+
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| Reserved for BIOS | Do not use. Reserved for BIOS EBDA.
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09A000 +------------------------+
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| Command line |
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| Stack/heap | For use by the kernel real-mode code.
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098000 +------------------------+
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| Kernel setup | The kernel real-mode code.
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090200 +------------------------+
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| Kernel boot sector | The kernel legacy boot sector.
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090000 +------------------------+
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| Protected-mode kernel | The bulk of the kernel image.
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010000 +------------------------+
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| Boot loader | <- Boot sector entry point 0000:7C00
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001000 +------------------------+
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| Reserved for MBR/BIOS |
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000800 +------------------------+
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| Typically used by MBR |
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000600 +------------------------+
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| BIOS use only |
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000000 +------------------------+
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When using bzImage, the protected-mode kernel was relocated to
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0x100000 ("high memory"), and the kernel real-mode block (boot sector,
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setup, and stack/heap) was made relocatable to any address between
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0x10000 and end of low memory. Unfortunately, in protocols 2.00 and
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2.01 the 0x90000+ memory range is still used internally by the kernel;
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the 2.02 protocol resolves that problem.
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It is desirable to keep the "memory ceiling" -- the highest point in
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low memory touched by the boot loader -- as low as possible, since
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some newer BIOSes have begun to allocate some rather large amounts of
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memory, called the Extended BIOS Data Area, near the top of low
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memory. The boot loader should use the "INT 12h" BIOS call to verify
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how much low memory is available.
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Unfortunately, if INT 12h reports that the amount of memory is too
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low, there is usually nothing the boot loader can do but to report an
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error to the user. The boot loader should therefore be designed to
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take up as little space in low memory as it reasonably can. For
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zImage or old bzImage kernels, which need data written into the
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0x90000 segment, the boot loader should make sure not to use memory
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above the 0x9A000 point; too many BIOSes will break above that point.
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For a modern bzImage kernel with boot protocol version >= 2.02, a
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memory layout like the following is suggested:
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~ ~
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| Protected-mode kernel |
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100000 +------------------------+
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| I/O memory hole |
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0A0000 +------------------------+
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| Reserved for BIOS | Leave as much as possible unused
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~ ~
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| Command line | (Can also be below the X+10000 mark)
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X+10000 +------------------------+
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| Stack/heap | For use by the kernel real-mode code.
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X+08000 +------------------------+
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| Kernel setup | The kernel real-mode code.
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| Kernel boot sector | The kernel legacy boot sector.
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X +------------------------+
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| Boot loader | <- Boot sector entry point 0000:7C00
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001000 +------------------------+
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| Reserved for MBR/BIOS |
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000800 +------------------------+
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| Typically used by MBR |
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000600 +------------------------+
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| BIOS use only |
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000000 +------------------------+
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... where the address X is as low as the design of the boot loader
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permits.
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**** THE REAL-MODE KERNEL HEADER
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In the following text, and anywhere in the kernel boot sequence, "a
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sector" refers to 512 bytes. It is independent of the actual sector
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size of the underlying medium.
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The first step in loading a Linux kernel should be to load the
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real-mode code (boot sector and setup code) and then examine the
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following header at offset 0x01f1. The real-mode code can total up to
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32K, although the boot loader may choose to load only the first two
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sectors (1K) and then examine the bootup sector size.
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The header looks like:
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Offset Proto Name Meaning
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/Size
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01F1/1 ALL(1 setup_sects The size of the setup in sectors
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01F2/2 ALL root_flags If set, the root is mounted readonly
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01F4/4 2.04+(2 syssize The size of the 32-bit code in 16-byte paras
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01F8/2 ALL ram_size DO NOT USE - for bootsect.S use only
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01FA/2 ALL vid_mode Video mode control
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01FC/2 ALL root_dev Default root device number
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01FE/2 ALL boot_flag 0xAA55 magic number
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0200/2 2.00+ jump Jump instruction
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0202/4 2.00+ header Magic signature "HdrS"
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0206/2 2.00+ version Boot protocol version supported
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0208/4 2.00+ realmode_swtch Boot loader hook (see below)
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020C/2 2.00+ start_sys_seg The load-low segment (0x1000) (obsolete)
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020E/2 2.00+ kernel_version Pointer to kernel version string
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0210/1 2.00+ type_of_loader Boot loader identifier
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0211/1 2.00+ loadflags Boot protocol option flags
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0212/2 2.00+ setup_move_size Move to high memory size (used with hooks)
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0214/4 2.00+ code32_start Boot loader hook (see below)
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0218/4 2.00+ ramdisk_image initrd load address (set by boot loader)
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021C/4 2.00+ ramdisk_size initrd size (set by boot loader)
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0220/4 2.00+ bootsect_kludge DO NOT USE - for bootsect.S use only
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0224/2 2.01+ heap_end_ptr Free memory after setup end
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0226/1 2.02+(3 ext_loader_ver Extended boot loader version
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0227/1 2.02+(3 ext_loader_type Extended boot loader ID
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0228/4 2.02+ cmd_line_ptr 32-bit pointer to the kernel command line
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022C/4 2.03+ ramdisk_max Highest legal initrd address
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0230/4 2.05+ kernel_alignment Physical addr alignment required for kernel
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0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
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0235/1 2.10+ min_alignment Minimum alignment, as a power of two
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0236/2 N/A pad3 Unused
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0238/4 2.06+ cmdline_size Maximum size of the kernel command line
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023C/4 2.07+ hardware_subarch Hardware subarchitecture
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0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
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0248/4 2.08+ payload_offset Offset of kernel payload
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024C/4 2.08+ payload_length Length of kernel payload
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0250/8 2.09+ setup_data 64-bit physical pointer to linked list
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of struct setup_data
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0258/8 2.10+ pref_address Preferred loading address
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0260/4 2.10+ init_size Linear memory required during initialization
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(1) For backwards compatibility, if the setup_sects field contains 0, the
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real value is 4.
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(2) For boot protocol prior to 2.04, the upper two bytes of the syssize
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field are unusable, which means the size of a bzImage kernel
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cannot be determined.
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(3) Ignored, but safe to set, for boot protocols 2.02-2.09.
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If the "HdrS" (0x53726448) magic number is not found at offset 0x202,
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the boot protocol version is "old". Loading an old kernel, the
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following parameters should be assumed:
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Image type = zImage
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initrd not supported
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Real-mode kernel must be located at 0x90000.
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Otherwise, the "version" field contains the protocol version,
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e.g. protocol version 2.01 will contain 0x0201 in this field. When
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setting fields in the header, you must make sure only to set fields
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supported by the protocol version in use.
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**** DETAILS OF HEADER FIELDS
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For each field, some are information from the kernel to the bootloader
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("read"), some are expected to be filled out by the bootloader
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("write"), and some are expected to be read and modified by the
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bootloader ("modify").
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All general purpose boot loaders should write the fields marked
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(obligatory). Boot loaders who want to load the kernel at a
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nonstandard address should fill in the fields marked (reloc); other
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boot loaders can ignore those fields.
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The byte order of all fields is littleendian (this is x86, after all.)
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Field name: setup_sects
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Type: read
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Offset/size: 0x1f1/1
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Protocol: ALL
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The size of the setup code in 512-byte sectors. If this field is
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0, the real value is 4. The real-mode code consists of the boot
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sector (always one 512-byte sector) plus the setup code.
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Field name: root_flags
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Type: modify (optional)
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Offset/size: 0x1f2/2
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Protocol: ALL
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If this field is nonzero, the root defaults to readonly. The use of
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this field is deprecated; use the "ro" or "rw" options on the
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command line instead.
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Field name: syssize
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Type: read
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Offset/size: 0x1f4/4 (protocol 2.04+) 0x1f4/2 (protocol ALL)
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Protocol: 2.04+
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The size of the protected-mode code in units of 16-byte paragraphs.
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For protocol versions older than 2.04 this field is only two bytes
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wide, and therefore cannot be trusted for the size of a kernel if
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the LOAD_HIGH flag is set.
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Field name: ram_size
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Type: kernel internal
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Offset/size: 0x1f8/2
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Protocol: ALL
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This field is obsolete.
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Field name: vid_mode
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Type: modify (obligatory)
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Offset/size: 0x1fa/2
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Please see the section on SPECIAL COMMAND LINE OPTIONS.
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Field name: root_dev
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Type: modify (optional)
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Offset/size: 0x1fc/2
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Protocol: ALL
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The default root device device number. The use of this field is
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deprecated, use the "root=" option on the command line instead.
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Field name: boot_flag
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Type: read
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Offset/size: 0x1fe/2
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Protocol: ALL
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Contains 0xAA55. This is the closest thing old Linux kernels have
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to a magic number.
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Field name: jump
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Type: read
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Offset/size: 0x200/2
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Protocol: 2.00+
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Contains an x86 jump instruction, 0xEB followed by a signed offset
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relative to byte 0x202. This can be used to determine the size of
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the header.
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Field name: header
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Type: read
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Offset/size: 0x202/4
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Protocol: 2.00+
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Contains the magic number "HdrS" (0x53726448).
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Field name: version
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Type: read
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Offset/size: 0x206/2
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Protocol: 2.00+
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Contains the boot protocol version, in (major << 8)+minor format,
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e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
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10.17.
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Field name: realmode_swtch
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Type: modify (optional)
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Offset/size: 0x208/4
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Protocol: 2.00+
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Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
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Field name: start_sys_seg
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Type: read
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Offset/size: 0x20c/2
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Protocol: 2.00+
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The load low segment (0x1000). Obsolete.
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Field name: kernel_version
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Type: read
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Offset/size: 0x20e/2
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Protocol: 2.00+
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If set to a nonzero value, contains a pointer to a NUL-terminated
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human-readable kernel version number string, less 0x200. This can
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be used to display the kernel version to the user. This value
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should be less than (0x200*setup_sects).
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For example, if this value is set to 0x1c00, the kernel version
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number string can be found at offset 0x1e00 in the kernel file.
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This is a valid value if and only if the "setup_sects" field
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contains the value 15 or higher, as:
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0x1c00 < 15*0x200 (= 0x1e00) but
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0x1c00 >= 14*0x200 (= 0x1c00)
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0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
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Field name: type_of_loader
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Type: write (obligatory)
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Offset/size: 0x210/1
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Protocol: 2.00+
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If your boot loader has an assigned id (see table below), enter
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0xTV here, where T is an identifier for the boot loader and V is
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a version number. Otherwise, enter 0xFF here.
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For boot loader IDs above T = 0xD, write T = 0xE to this field and
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write the extended ID minus 0x10 to the ext_loader_type field.
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Similarly, the ext_loader_ver field can be used to provide more than
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four bits for the bootloader version.
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For example, for T = 0x15, V = 0x234, write:
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type_of_loader <- 0xE4
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ext_loader_type <- 0x05
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ext_loader_ver <- 0x23
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Assigned boot loader ids:
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0 LILO (0x00 reserved for pre-2.00 bootloader)
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1 Loadlin
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2 bootsect-loader (0x20, all other values reserved)
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3 Syslinux
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4 Etherboot/gPXE
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5 ELILO
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7 GRUB
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8 U-Boot
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9 Xen
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A Gujin
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B Qemu
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C Arcturus Networks uCbootloader
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E Extended (see ext_loader_type)
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F Special (0xFF = undefined)
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Please contact <hpa@zytor.com> if you need a bootloader ID
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value assigned.
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Field name: loadflags
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Type: modify (obligatory)
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Offset/size: 0x211/1
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Protocol: 2.00+
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This field is a bitmask.
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Bit 0 (read): LOADED_HIGH
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- If 0, the protected-mode code is loaded at 0x10000.
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- If 1, the protected-mode code is loaded at 0x100000.
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Bit 5 (write): QUIET_FLAG
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- If 0, print early messages.
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- If 1, suppress early messages.
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This requests to the kernel (decompressor and early
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kernel) to not write early messages that require
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accessing the display hardware directly.
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Bit 6 (write): KEEP_SEGMENTS
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Protocol: 2.07+
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- If 0, reload the segment registers in the 32bit entry point.
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- If 1, do not reload the segment registers in the 32bit entry point.
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Assume that %cs %ds %ss %es are all set to flat segments with
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a base of 0 (or the equivalent for their environment).
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Bit 7 (write): CAN_USE_HEAP
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Set this bit to 1 to indicate that the value entered in the
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heap_end_ptr is valid. If this field is clear, some setup code
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functionality will be disabled.
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Field name: setup_move_size
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Type: modify (obligatory)
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Offset/size: 0x212/2
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Protocol: 2.00-2.01
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When using protocol 2.00 or 2.01, if the real mode kernel is not
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loaded at 0x90000, it gets moved there later in the loading
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sequence. Fill in this field if you want additional data (such as
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the kernel command line) moved in addition to the real-mode kernel
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itself.
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The unit is bytes starting with the beginning of the boot sector.
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This field is can be ignored when the protocol is 2.02 or higher, or
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if the real-mode code is loaded at 0x90000.
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Field name: code32_start
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Type: modify (optional, reloc)
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Offset/size: 0x214/4
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Protocol: 2.00+
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The address to jump to in protected mode. This defaults to the load
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address of the kernel, and can be used by the boot loader to
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determine the proper load address.
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This field can be modified for two purposes:
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1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
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2. if a bootloader which does not install a hook loads a
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relocatable kernel at a nonstandard address it will have to modify
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this field to point to the load address.
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Field name: ramdisk_image
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Type: write (obligatory)
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Offset/size: 0x218/4
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Protocol: 2.00+
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The 32-bit linear address of the initial ramdisk or ramfs. Leave at
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zero if there is no initial ramdisk/ramfs.
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Field name: ramdisk_size
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Type: write (obligatory)
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Offset/size: 0x21c/4
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Protocol: 2.00+
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Size of the initial ramdisk or ramfs. Leave at zero if there is no
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initial ramdisk/ramfs.
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Field name: bootsect_kludge
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Type: kernel internal
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Offset/size: 0x220/4
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Protocol: 2.00+
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This field is obsolete.
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Field name: heap_end_ptr
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Type: write (obligatory)
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Offset/size: 0x224/2
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Protocol: 2.01+
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Set this field to the offset (from the beginning of the real-mode
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code) of the end of the setup stack/heap, minus 0x0200.
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Field name: ext_loader_ver
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Type: write (optional)
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Offset/size: 0x226/1
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Protocol: 2.02+
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This field is used as an extension of the version number in the
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type_of_loader field. The total version number is considered to be
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(type_of_loader & 0x0f) + (ext_loader_ver << 4).
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The use of this field is boot loader specific. If not written, it
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is zero.
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Kernels prior to 2.6.31 did not recognize this field, but it is safe
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to write for protocol version 2.02 or higher.
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Field name: ext_loader_type
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Type: write (obligatory if (type_of_loader & 0xf0) == 0xe0)
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|
Offset/size: 0x227/1
|
|
Protocol: 2.02+
|
|
|
|
This field is used as an extension of the type number in
|
|
type_of_loader field. If the type in type_of_loader is 0xE, then
|
|
the actual type is (ext_loader_type + 0x10).
|
|
|
|
This field is ignored if the type in type_of_loader is not 0xE.
|
|
|
|
Kernels prior to 2.6.31 did not recognize this field, but it is safe
|
|
to write for protocol version 2.02 or higher.
|
|
|
|
Field name: cmd_line_ptr
|
|
Type: write (obligatory)
|
|
Offset/size: 0x228/4
|
|
Protocol: 2.02+
|
|
|
|
Set this field to the linear address of the kernel command line.
|
|
The kernel command line can be located anywhere between the end of
|
|
the setup heap and 0xA0000; it does not have to be located in the
|
|
same 64K segment as the real-mode code itself.
|
|
|
|
Fill in this field even if your boot loader does not support a
|
|
command line, in which case you can point this to an empty string
|
|
(or better yet, to the string "auto".) If this field is left at
|
|
zero, the kernel will assume that your boot loader does not support
|
|
the 2.02+ protocol.
|
|
|
|
Field name: ramdisk_max
|
|
Type: read
|
|
Offset/size: 0x22c/4
|
|
Protocol: 2.03+
|
|
|
|
The maximum address that may be occupied by the initial
|
|
ramdisk/ramfs contents. For boot protocols 2.02 or earlier, this
|
|
field is not present, and the maximum address is 0x37FFFFFF. (This
|
|
address is defined as the address of the highest safe byte, so if
|
|
your ramdisk is exactly 131072 bytes long and this field is
|
|
0x37FFFFFF, you can start your ramdisk at 0x37FE0000.)
|
|
|
|
Field name: kernel_alignment
|
|
Type: read/modify (reloc)
|
|
Offset/size: 0x230/4
|
|
Protocol: 2.05+ (read), 2.10+ (modify)
|
|
|
|
Alignment unit required by the kernel (if relocatable_kernel is
|
|
true.) A relocatable kernel that is loaded at an alignment
|
|
incompatible with the value in this field will be realigned during
|
|
kernel initialization.
|
|
|
|
Starting with protocol version 2.10, this reflects the kernel
|
|
alignment preferred for optimal performance; it is possible for the
|
|
loader to modify this field to permit a lesser alignment. See the
|
|
min_alignment and pref_address field below.
|
|
|
|
Field name: relocatable_kernel
|
|
Type: read (reloc)
|
|
Offset/size: 0x234/1
|
|
Protocol: 2.05+
|
|
|
|
If this field is nonzero, the protected-mode part of the kernel can
|
|
be loaded at any address that satisfies the kernel_alignment field.
|
|
After loading, the boot loader must set the code32_start field to
|
|
point to the loaded code, or to a boot loader hook.
|
|
|
|
Field name: min_alignment
|
|
Type: read (reloc)
|
|
Offset/size: 0x235/1
|
|
Protocol: 2.10+
|
|
|
|
This field, if nonzero, indicates as a power of two the minimum
|
|
alignment required, as opposed to preferred, by the kernel to boot.
|
|
If a boot loader makes use of this field, it should update the
|
|
kernel_alignment field with the alignment unit desired; typically:
|
|
|
|
kernel_alignment = 1 << min_alignment
|
|
|
|
There may be a considerable performance cost with an excessively
|
|
misaligned kernel. Therefore, a loader should typically try each
|
|
power-of-two alignment from kernel_alignment down to this alignment.
|
|
|
|
Field name: cmdline_size
|
|
Type: read
|
|
Offset/size: 0x238/4
|
|
Protocol: 2.06+
|
|
|
|
The maximum size of the command line without the terminating
|
|
zero. This means that the command line can contain at most
|
|
cmdline_size characters. With protocol version 2.05 and earlier, the
|
|
maximum size was 255.
|
|
|
|
Field name: hardware_subarch
|
|
Type: write (optional, defaults to x86/PC)
|
|
Offset/size: 0x23c/4
|
|
Protocol: 2.07+
|
|
|
|
In a paravirtualized environment the hardware low level architectural
|
|
pieces such as interrupt handling, page table handling, and
|
|
accessing process control registers needs to be done differently.
|
|
|
|
This field allows the bootloader to inform the kernel we are in one
|
|
one of those environments.
|
|
|
|
0x00000000 The default x86/PC environment
|
|
0x00000001 lguest
|
|
0x00000002 Xen
|
|
0x00000003 Moorestown MID
|
|
0x00000004 CE4100 TV Platform
|
|
|
|
Field name: hardware_subarch_data
|
|
Type: write (subarch-dependent)
|
|
Offset/size: 0x240/8
|
|
Protocol: 2.07+
|
|
|
|
A pointer to data that is specific to hardware subarch
|
|
This field is currently unused for the default x86/PC environment,
|
|
do not modify.
|
|
|
|
Field name: payload_offset
|
|
Type: read
|
|
Offset/size: 0x248/4
|
|
Protocol: 2.08+
|
|
|
|
If non-zero then this field contains the offset from the beginning
|
|
of the protected-mode code to the payload.
|
|
|
|
The payload may be compressed. The format of both the compressed and
|
|
uncompressed data should be determined using the standard magic
|
|
numbers. The currently supported compression formats are gzip
|
|
(magic numbers 1F 8B or 1F 9E), bzip2 (magic number 42 5A), LZMA
|
|
(magic number 5D 00), and XZ (magic number FD 37). The uncompressed
|
|
payload is currently always ELF (magic number 7F 45 4C 46).
|
|
|
|
Field name: payload_length
|
|
Type: read
|
|
Offset/size: 0x24c/4
|
|
Protocol: 2.08+
|
|
|
|
The length of the payload.
|
|
|
|
Field name: setup_data
|
|
Type: write (special)
|
|
Offset/size: 0x250/8
|
|
Protocol: 2.09+
|
|
|
|
The 64-bit physical pointer to NULL terminated single linked list of
|
|
struct setup_data. This is used to define a more extensible boot
|
|
parameters passing mechanism. The definition of struct setup_data is
|
|
as follow:
|
|
|
|
struct setup_data {
|
|
u64 next;
|
|
u32 type;
|
|
u32 len;
|
|
u8 data[0];
|
|
};
|
|
|
|
Where, the next is a 64-bit physical pointer to the next node of
|
|
linked list, the next field of the last node is 0; the type is used
|
|
to identify the contents of data; the len is the length of data
|
|
field; the data holds the real payload.
|
|
|
|
This list may be modified at a number of points during the bootup
|
|
process. Therefore, when modifying this list one should always make
|
|
sure to consider the case where the linked list already contains
|
|
entries.
|
|
|
|
Field name: pref_address
|
|
Type: read (reloc)
|
|
Offset/size: 0x258/8
|
|
Protocol: 2.10+
|
|
|
|
This field, if nonzero, represents a preferred load address for the
|
|
kernel. A relocating bootloader should attempt to load at this
|
|
address if possible.
|
|
|
|
A non-relocatable kernel will unconditionally move itself and to run
|
|
at this address.
|
|
|
|
Field name: init_size
|
|
Type: read
|
|
Offset/size: 0x25c/4
|
|
|
|
This field indicates the amount of linear contiguous memory starting
|
|
at the kernel runtime start address that the kernel needs before it
|
|
is capable of examining its memory map. This is not the same thing
|
|
as the total amount of memory the kernel needs to boot, but it can
|
|
be used by a relocating boot loader to help select a safe load
|
|
address for the kernel.
|
|
|
|
The kernel runtime start address is determined by the following algorithm:
|
|
|
|
if (relocatable_kernel)
|
|
runtime_start = align_up(load_address, kernel_alignment)
|
|
else
|
|
runtime_start = pref_address
|
|
|
|
|
|
**** THE IMAGE CHECKSUM
|
|
|
|
From boot protocol version 2.08 onwards the CRC-32 is calculated over
|
|
the entire file using the characteristic polynomial 0x04C11DB7 and an
|
|
initial remainder of 0xffffffff. The checksum is appended to the
|
|
file; therefore the CRC of the file up to the limit specified in the
|
|
syssize field of the header is always 0.
|
|
|
|
|
|
**** THE KERNEL COMMAND LINE
|
|
|
|
The kernel command line has become an important way for the boot
|
|
loader to communicate with the kernel. Some of its options are also
|
|
relevant to the boot loader itself, see "special command line options"
|
|
below.
|
|
|
|
The kernel command line is a null-terminated string. The maximum
|
|
length can be retrieved from the field cmdline_size. Before protocol
|
|
version 2.06, the maximum was 255 characters. A string that is too
|
|
long will be automatically truncated by the kernel.
|
|
|
|
If the boot protocol version is 2.02 or later, the address of the
|
|
kernel command line is given by the header field cmd_line_ptr (see
|
|
above.) This address can be anywhere between the end of the setup
|
|
heap and 0xA0000.
|
|
|
|
If the protocol version is *not* 2.02 or higher, the kernel
|
|
command line is entered using the following protocol:
|
|
|
|
At offset 0x0020 (word), "cmd_line_magic", enter the magic
|
|
number 0xA33F.
|
|
|
|
At offset 0x0022 (word), "cmd_line_offset", enter the offset
|
|
of the kernel command line (relative to the start of the
|
|
real-mode kernel).
|
|
|
|
The kernel command line *must* be within the memory region
|
|
covered by setup_move_size, so you may need to adjust this
|
|
field.
|
|
|
|
|
|
**** MEMORY LAYOUT OF THE REAL-MODE CODE
|
|
|
|
The real-mode code requires a stack/heap to be set up, as well as
|
|
memory allocated for the kernel command line. This needs to be done
|
|
in the real-mode accessible memory in bottom megabyte.
|
|
|
|
It should be noted that modern machines often have a sizable Extended
|
|
BIOS Data Area (EBDA). As a result, it is advisable to use as little
|
|
of the low megabyte as possible.
|
|
|
|
Unfortunately, under the following circumstances the 0x90000 memory
|
|
segment has to be used:
|
|
|
|
- When loading a zImage kernel ((loadflags & 0x01) == 0).
|
|
- When loading a 2.01 or earlier boot protocol kernel.
|
|
|
|
-> For the 2.00 and 2.01 boot protocols, the real-mode code
|
|
can be loaded at another address, but it is internally
|
|
relocated to 0x90000. For the "old" protocol, the
|
|
real-mode code must be loaded at 0x90000.
|
|
|
|
When loading at 0x90000, avoid using memory above 0x9a000.
|
|
|
|
For boot protocol 2.02 or higher, the command line does not have to be
|
|
located in the same 64K segment as the real-mode setup code; it is
|
|
thus permitted to give the stack/heap the full 64K segment and locate
|
|
the command line above it.
|
|
|
|
The kernel command line should not be located below the real-mode
|
|
code, nor should it be located in high memory.
|
|
|
|
|
|
**** SAMPLE BOOT CONFIGURATION
|
|
|
|
As a sample configuration, assume the following layout of the real
|
|
mode segment:
|
|
|
|
When loading below 0x90000, use the entire segment:
|
|
|
|
0x0000-0x7fff Real mode kernel
|
|
0x8000-0xdfff Stack and heap
|
|
0xe000-0xffff Kernel command line
|
|
|
|
When loading at 0x90000 OR the protocol version is 2.01 or earlier:
|
|
|
|
0x0000-0x7fff Real mode kernel
|
|
0x8000-0x97ff Stack and heap
|
|
0x9800-0x9fff Kernel command line
|
|
|
|
Such a boot loader should enter the following fields in the header:
|
|
|
|
unsigned long base_ptr; /* base address for real-mode segment */
|
|
|
|
if ( setup_sects == 0 ) {
|
|
setup_sects = 4;
|
|
}
|
|
|
|
if ( protocol >= 0x0200 ) {
|
|
type_of_loader = <type code>;
|
|
if ( loading_initrd ) {
|
|
ramdisk_image = <initrd_address>;
|
|
ramdisk_size = <initrd_size>;
|
|
}
|
|
|
|
if ( protocol >= 0x0202 && loadflags & 0x01 )
|
|
heap_end = 0xe000;
|
|
else
|
|
heap_end = 0x9800;
|
|
|
|
if ( protocol >= 0x0201 ) {
|
|
heap_end_ptr = heap_end - 0x200;
|
|
loadflags |= 0x80; /* CAN_USE_HEAP */
|
|
}
|
|
|
|
if ( protocol >= 0x0202 ) {
|
|
cmd_line_ptr = base_ptr + heap_end;
|
|
strcpy(cmd_line_ptr, cmdline);
|
|
} else {
|
|
cmd_line_magic = 0xA33F;
|
|
cmd_line_offset = heap_end;
|
|
setup_move_size = heap_end + strlen(cmdline)+1;
|
|
strcpy(base_ptr+cmd_line_offset, cmdline);
|
|
}
|
|
} else {
|
|
/* Very old kernel */
|
|
|
|
heap_end = 0x9800;
|
|
|
|
cmd_line_magic = 0xA33F;
|
|
cmd_line_offset = heap_end;
|
|
|
|
/* A very old kernel MUST have its real-mode code
|
|
loaded at 0x90000 */
|
|
|
|
if ( base_ptr != 0x90000 ) {
|
|
/* Copy the real-mode kernel */
|
|
memcpy(0x90000, base_ptr, (setup_sects+1)*512);
|
|
base_ptr = 0x90000; /* Relocated */
|
|
}
|
|
|
|
strcpy(0x90000+cmd_line_offset, cmdline);
|
|
|
|
/* It is recommended to clear memory up to the 32K mark */
|
|
memset(0x90000 + (setup_sects+1)*512, 0,
|
|
(64-(setup_sects+1))*512);
|
|
}
|
|
|
|
|
|
**** LOADING THE REST OF THE KERNEL
|
|
|
|
The 32-bit (non-real-mode) kernel starts at offset (setup_sects+1)*512
|
|
in the kernel file (again, if setup_sects == 0 the real value is 4.)
|
|
It should be loaded at address 0x10000 for Image/zImage kernels and
|
|
0x100000 for bzImage kernels.
|
|
|
|
The kernel is a bzImage kernel if the protocol >= 2.00 and the 0x01
|
|
bit (LOAD_HIGH) in the loadflags field is set:
|
|
|
|
is_bzImage = (protocol >= 0x0200) && (loadflags & 0x01);
|
|
load_address = is_bzImage ? 0x100000 : 0x10000;
|
|
|
|
Note that Image/zImage kernels can be up to 512K in size, and thus use
|
|
the entire 0x10000-0x90000 range of memory. This means it is pretty
|
|
much a requirement for these kernels to load the real-mode part at
|
|
0x90000. bzImage kernels allow much more flexibility.
|
|
|
|
|
|
**** SPECIAL COMMAND LINE OPTIONS
|
|
|
|
If the command line provided by the boot loader is entered by the
|
|
user, the user may expect the following command line options to work.
|
|
They should normally not be deleted from the kernel command line even
|
|
though not all of them are actually meaningful to the kernel. Boot
|
|
loader authors who need additional command line options for the boot
|
|
loader itself should get them registered in
|
|
Documentation/kernel-parameters.txt to make sure they will not
|
|
conflict with actual kernel options now or in the future.
|
|
|
|
vga=<mode>
|
|
<mode> here is either an integer (in C notation, either
|
|
decimal, octal, or hexadecimal) or one of the strings
|
|
"normal" (meaning 0xFFFF), "ext" (meaning 0xFFFE) or "ask"
|
|
(meaning 0xFFFD). This value should be entered into the
|
|
vid_mode field, as it is used by the kernel before the command
|
|
line is parsed.
|
|
|
|
mem=<size>
|
|
<size> is an integer in C notation optionally followed by
|
|
(case insensitive) K, M, G, T, P or E (meaning << 10, << 20,
|
|
<< 30, << 40, << 50 or << 60). This specifies the end of
|
|
memory to the kernel. This affects the possible placement of
|
|
an initrd, since an initrd should be placed near end of
|
|
memory. Note that this is an option to *both* the kernel and
|
|
the bootloader!
|
|
|
|
initrd=<file>
|
|
An initrd should be loaded. The meaning of <file> is
|
|
obviously bootloader-dependent, and some boot loaders
|
|
(e.g. LILO) do not have such a command.
|
|
|
|
In addition, some boot loaders add the following options to the
|
|
user-specified command line:
|
|
|
|
BOOT_IMAGE=<file>
|
|
The boot image which was loaded. Again, the meaning of <file>
|
|
is obviously bootloader-dependent.
|
|
|
|
auto
|
|
The kernel was booted without explicit user intervention.
|
|
|
|
If these options are added by the boot loader, it is highly
|
|
recommended that they are located *first*, before the user-specified
|
|
or configuration-specified command line. Otherwise, "init=/bin/sh"
|
|
gets confused by the "auto" option.
|
|
|
|
|
|
**** RUNNING THE KERNEL
|
|
|
|
The kernel is started by jumping to the kernel entry point, which is
|
|
located at *segment* offset 0x20 from the start of the real mode
|
|
kernel. This means that if you loaded your real-mode kernel code at
|
|
0x90000, the kernel entry point is 9020:0000.
|
|
|
|
At entry, ds = es = ss should point to the start of the real-mode
|
|
kernel code (0x9000 if the code is loaded at 0x90000), sp should be
|
|
set up properly, normally pointing to the top of the heap, and
|
|
interrupts should be disabled. Furthermore, to guard against bugs in
|
|
the kernel, it is recommended that the boot loader sets fs = gs = ds =
|
|
es = ss.
|
|
|
|
In our example from above, we would do:
|
|
|
|
/* Note: in the case of the "old" kernel protocol, base_ptr must
|
|
be == 0x90000 at this point; see the previous sample code */
|
|
|
|
seg = base_ptr >> 4;
|
|
|
|
cli(); /* Enter with interrupts disabled! */
|
|
|
|
/* Set up the real-mode kernel stack */
|
|
_SS = seg;
|
|
_SP = heap_end;
|
|
|
|
_DS = _ES = _FS = _GS = seg;
|
|
jmp_far(seg+0x20, 0); /* Run the kernel */
|
|
|
|
If your boot sector accesses a floppy drive, it is recommended to
|
|
switch off the floppy motor before running the kernel, since the
|
|
kernel boot leaves interrupts off and thus the motor will not be
|
|
switched off, especially if the loaded kernel has the floppy driver as
|
|
a demand-loaded module!
|
|
|
|
|
|
**** ADVANCED BOOT LOADER HOOKS
|
|
|
|
If the boot loader runs in a particularly hostile environment (such as
|
|
LOADLIN, which runs under DOS) it may be impossible to follow the
|
|
standard memory location requirements. Such a boot loader may use the
|
|
following hooks that, if set, are invoked by the kernel at the
|
|
appropriate time. The use of these hooks should probably be
|
|
considered an absolutely last resort!
|
|
|
|
IMPORTANT: All the hooks are required to preserve %esp, %ebp, %esi and
|
|
%edi across invocation.
|
|
|
|
realmode_swtch:
|
|
A 16-bit real mode far subroutine invoked immediately before
|
|
entering protected mode. The default routine disables NMI, so
|
|
your routine should probably do so, too.
|
|
|
|
code32_start:
|
|
A 32-bit flat-mode routine *jumped* to immediately after the
|
|
transition to protected mode, but before the kernel is
|
|
uncompressed. No segments, except CS, are guaranteed to be
|
|
set up (current kernels do, but older ones do not); you should
|
|
set them up to BOOT_DS (0x18) yourself.
|
|
|
|
After completing your hook, you should jump to the address
|
|
that was in this field before your boot loader overwrote it
|
|
(relocated, if appropriate.)
|
|
|
|
|
|
**** 32-bit BOOT PROTOCOL
|
|
|
|
For machine with some new BIOS other than legacy BIOS, such as EFI,
|
|
LinuxBIOS, etc, and kexec, the 16-bit real mode setup code in kernel
|
|
based on legacy BIOS can not be used, so a 32-bit boot protocol needs
|
|
to be defined.
|
|
|
|
In 32-bit boot protocol, the first step in loading a Linux kernel
|
|
should be to setup the boot parameters (struct boot_params,
|
|
traditionally known as "zero page"). The memory for struct boot_params
|
|
should be allocated and initialized to all zero. Then the setup header
|
|
from offset 0x01f1 of kernel image on should be loaded into struct
|
|
boot_params and examined. The end of setup header can be calculated as
|
|
follow:
|
|
|
|
0x0202 + byte value at offset 0x0201
|
|
|
|
In addition to read/modify/write the setup header of the struct
|
|
boot_params as that of 16-bit boot protocol, the boot loader should
|
|
also fill the additional fields of the struct boot_params as that
|
|
described in zero-page.txt.
|
|
|
|
After setupping the struct boot_params, the boot loader can load the
|
|
32/64-bit kernel in the same way as that of 16-bit boot protocol.
|
|
|
|
In 32-bit boot protocol, the kernel is started by jumping to the
|
|
32-bit kernel entry point, which is the start address of loaded
|
|
32/64-bit kernel.
|
|
|
|
At entry, the CPU must be in 32-bit protected mode with paging
|
|
disabled; a GDT must be loaded with the descriptors for selectors
|
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__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
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segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
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must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
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must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
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address of the struct boot_params; %ebp, %edi and %ebx must be zero.
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