----------------------------
H. Peter Anvin <hpa@zytor.com>
- Last update 2007-05-16
+ Last update 2007-05-23
On the i386 platform, the Linux kernel uses a rather complicated boot
convention. This has evolved partially due to historical aspects, as
0234/1 2.05+ relocatable_kernel Whether kernel is relocatable or not
0235/3 N/A pad2 Unused
0238/4 2.06+ cmdline_size Maximum size of the kernel command line
+023C/4 2.07+ hardware_subarch Hardware subarchitecture
+0240/8 2.07+ hardware_subarch_data Subarchitecture-specific data
+0248/4 2.08+ payload_offset Offset of kernel payload
+024C/4 2.08+ payload_length Length of kernel payload
(1) For backwards compatibility, if the setup_sects field contains 0, the
real value is 4.
nonstandard address should fill in the fields marked (reloc); other
boot loaders can ignore those fields.
-Field name: setup_secs
+The byte order of all fields is littleendian (this is x86, after all.)
+
+Field name: setup_sects
Type: read
Offset/size: 0x1f1/1
Protocol: ALL
Offset/size: 0x206/2
Protocol: 2.00+
- Contains the boot protocol version, e.g. 0x0204 for version 2.04.
+ Contains the boot protocol version, in (major << 8)+minor format,
+ e.g. 0x0204 for version 2.04, and 0x0a11 for a hypothetical version
+ 10.17.
Field name: readmode_swtch
Type: modify (optional)
Offset/size: 0x208/4
Protocol: 2.00+
- Boot loader hook (see separate chapter.)
+ Boot loader hook (see ADVANCED BOOT LOADER HOOKS below.)
Field name: start_sys
Type: read
If set to a nonzero value, contains a pointer to a NUL-terminated
human-readable kernel version number string, less 0x200. This can
be used to display the kernel version to the user. This value
- should be less than (0x200*setup_sects). For example, if this value
- is set to 0x1c00, the kernel version number string can be found at
- offset 0x1e00 in the kernel file. This is a valid value if and only
- if the "setup_sects" field contains the value 14 or higher.
+ should be less than (0x200*setup_sects).
+
+ For example, if this value is set to 0x1c00, the kernel version
+ number string can be found at offset 0x1e00 in the kernel file.
+ This is a valid value if and only if the "setup_sects" field
+ contains the value 15 or higher, as:
+
+ 0x1c00 < 15*0x200 (= 0x1e00) but
+ 0x1c00 >= 14*0x200 (= 0x1c00)
+
+ 0x1c00 >> 9 = 14, so the minimum value for setup_secs is 15.
Field name: type_of_loader
Type: write (obligatory)
- If 0, the protected-mode code is loaded at 0x10000.
- If 1, the protected-mode code is loaded at 0x100000.
+ Bit 6 (write): KEEP_SEGMENTS
+ Protocol: 2.07+
+ - if 0, reload the segment registers in the 32bit entry point.
+ - if 1, do not reload the segment registers in the 32bit entry point.
+ Assume that %cs %ds %ss %es are all set to flat segments with
+ a base of 0 (or the equivalent for their environment).
+
Bit 7 (write): CAN_USE_HEAP
Set this bit to 1 to indicate that the value entered in the
heap_end_ptr is valid. If this field is clear, some setup code
This field can be modified for two purposes:
- 1. as a boot loader hook (see separate chapter.)
+ 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
cmdline_size characters. With protocol version 2.05 and earlier, the
maximum size was 255.
+Field name: hardware_subarch
+Type: write
+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
+Offset/size: 0x240/8
+Protocol: 2.07+
+
+ A pointer to data that is specific to hardware subarch
+
+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.
+
+**** 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
a demand-loaded module!
-**** ADVANCED BOOT TIME HOOKS
+**** 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
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.
+ 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.
git.samba.org / sfrench / cifs-2.6.git / blobdiff