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+ THE LINUX/x86 BOOT PROTOCOL
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+ ---------------------------
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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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+
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+Currently, the following versions of the Linux/x86 boot protocol exist.
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+
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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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+
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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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+
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+Protocol 2.01: (Kernel 1.3.76) Added a heap overrun warning.
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+
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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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+
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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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+
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+Protocol 2.04: (Kernel 2.6.14) Extend the syssize field to four bytes.
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+**** MEMORY LAYOUT
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+
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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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+
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+ | |
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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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+
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+
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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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+
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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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+
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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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+
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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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+ ~ ~
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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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+
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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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+
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+
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+**** THE REAL-MODE KERNEL HEADER
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+
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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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+
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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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+
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+The header looks like:
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+
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+Offset Proto Name Meaning
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+/Size
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+
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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 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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+0226/2 N/A pad1 Unused
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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+ initrd_addr_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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+0235/3 N/A pad2 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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+
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+**** DETAILS OF HEADER FIELDS
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+
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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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+
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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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+
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+The byte order of all fields is littleendian (this is x86, after all.)
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+ This field is obsolete.
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+
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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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+
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+ Please see the section on SPECIAL COMMAND LINE OPTIONS.
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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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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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+
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+ Contains the magic number "HdrS" (0x53726448).
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+
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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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+
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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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+
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+Field name: readmode_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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+
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+ Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
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+
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+Field name: start_sys
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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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+
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+ The load low segment (0x1000). Obsolete.
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+
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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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+
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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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+
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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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+
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+ 0x1c00 < 15*0x200 (= 0x1e00) but
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+ 0x1c00 >= 14*0x200 (= 0x1c00)
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+
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+ 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
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+
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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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+
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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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+
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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
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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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+
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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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+
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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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+
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+ This field is a bitmask.
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+
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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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+
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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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+
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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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+
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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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+
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+Field name: setup_move_size
|
|
|
+Type: modify (obligatory)
|
|
|
+Offset/size: 0x212/2
|
|
|
+Protocol: 2.00-2.01
|
|
|
+
|
|
|
+ When using protocol 2.00 or 2.01, if the real mode kernel is not
|
|
|
+ loaded at 0x90000, it gets moved there later in the loading
|
|
|
+ sequence. Fill in this field if you want additional data (such as
|
|
|
+ the kernel command line) moved in addition to the real-mode kernel
|
|
|
+ itself.
|
|
|
+
|
|
|
+ The unit is bytes starting with the beginning of the boot sector.
|
|
|
+
|
|
|
+ This field is can be ignored when the protocol is 2.02 or higher, or
|
|
|
+ if the real-mode code is loaded at 0x90000.
|
|
|
+
|
|
|
+Field name: code32_start
|
|
|
+Type: modify (optional, reloc)
|
|
|
+Offset/size: 0x214/4
|
|
|
+Protocol: 2.00+
|
|
|
+
|
|
|
+ The address to jump to in protected mode. This defaults to the load
|
|
|
+ address of the kernel, and can be used by the boot loader to
|
|
|
+ determine the proper load address.
|
|
|
+
|
|
|
+ This field can be modified for two purposes:
|
|
|
+
|
|
|
+ 1. as a boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
|
|
|
+
|
|
|
+ 2. if a bootloader which does not install a hook loads a
|
|
|
+ relocatable kernel at a nonstandard address it will have to modify
|
|
|
+ this field to point to the load address.
|
|
|
+
|
|
|
+Field name: ramdisk_image
|
|
|
+Type: write (obligatory)
|
|
|
+Offset/size: 0x218/4
|
|
|
+Protocol: 2.00+
|
|
|
+
|
|
|
+ The 32-bit linear address of the initial ramdisk or ramfs. Leave at
|
|
|
+ zero if there is no initial ramdisk/ramfs.
|
|
|
+
|
|
|
+Field name: ramdisk_size
|
|
|
+Type: write (obligatory)
|
|
|
+Offset/size: 0x21c/4
|
|
|
+Protocol: 2.00+
|
|
|
+
|
|
|
+ Size of the initial ramdisk or ramfs. Leave at zero if there is no
|
|
|
+ initial ramdisk/ramfs.
|
|
|
+
|
|
|
+Field name: bootsect_kludge
|
|
|
+Type: kernel internal
|
|
|
+Offset/size: 0x220/4
|
|
|
+Protocol: 2.00+
|
|
|
+
|
|
|
+ This field is obsolete.
|
|
|
+
|
|
|
+Field name: heap_end_ptr
|
|
|
+Type: write (obligatory)
|
|
|
+Offset/size: 0x224/2
|
|
|
+Protocol: 2.01+
|
|
|
+
|
|
|
+ Set this field to the offset (from the beginning of the real-mode
|
|
|
+ code) of the end of the setup stack/heap, minus 0x0200.
|
|
|
+
|
|
|
+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: initrd_addr_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 (reloc)
|
|
|
+Offset/size: 0x230/4
|
|
|
+Protocol: 2.05+
|
|
|
+
|
|
|
+ Alignment unit required by the kernel (if relocatable_kernel is true.)
|
|
|
+
|
|
|
+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: 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
|
|
|
+
|
|
|
+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 end of the
|
|
|
+ real-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. Currently only gzip compressed ELF is used.
|
|
|
+
|
|
|
+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.
|
|
|
+
|
|
|
+
|
|
|
+**** 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
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|
+
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+The kernel is started by jumping to the kernel entry point, which is
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|
+located at *segment* offset 0x20 from the start of the real mode
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|
+kernel. This means that if you loaded your real-mode kernel code at
|
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|
+0x90000, the kernel entry point is 9020:0000.
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|
+
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+At entry, ds = es = ss should point to the start of the real-mode
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|
+kernel code (0x9000 if the code is loaded at 0x90000), sp should be
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|
+set up properly, normally pointing to the top of the heap, and
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|
|
+interrupts should be disabled. Furthermore, to guard against bugs in
|
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|
+the kernel, it is recommended that the boot loader sets fs = gs = ds =
|
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|
+es = ss.
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|
+
|
|
|
+In our example from above, we would do:
|
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|
+
|
|
|
+ /* Note: in the case of the "old" kernel protocol, base_ptr must
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|
|
+ be == 0x90000 at this point; see the previous sample code */
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|
|
+
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|
|
+ seg = base_ptr >> 4;
|
|
|
+
|
|
|
+ cli(); /* Enter with interrupts disabled! */
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|
+
|
|
|
+ /* Set up the real-mode kernel stack */
|
|
|
+ _SS = seg;
|
|
|
+ _SP = heap_end;
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|
|
+
|
|
|
+ _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
|
|
|
+__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
|
|
|
+segment; __BOOS_CS must have execute/read permission, and __BOOT_DS
|
|
|
+must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
|
|
|
+must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
|
|
|
+address of the struct boot_params; %ebp, %edi and %ebx must be zero.
|