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(grub.info)Embedded data

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Embedded variables in GRUB
==========================

   Stage 1 and Stage 2 have embedded variables whose locations are
well-defined, so that the installation can patch the binary file
directly without recompilation of the stages.

   In Stage 1, these are defined:

`0x3E'
     The version number (not GRUB's, but the installation mechanism's).

`0x40'
     The boot drive. If it is 0xFF, use a drive passed by BIOS.

`0x41'
     The flag for if forcing LBA.

`0x42'
     The starting address of Stage 2.

`0x44'
     The first sector of Stage 2.

`0x48'
     The starting segment of Stage 2.

`0x1FE'
     The signature (`0xAA55').

   See the file `stage1/stage1.S', for more information.

   In the first sector of Stage 1.5 and Stage 2, the block lists are
recorded between `firstlist' and `lastlist'. The address of `lastlist'
is determined when assembling the file `stage2/start.S'.

   The trick here is that it is actually read backward, and the first
8-byte block list is not read here, but after the pointer is decremented
8 bytes, then after reading it, it decrements again, reads, and so on,
until it is finished. The terminating condition is when the number of
sectors to be read in the next block list is zero.

   The format of a block list can be seen from the example in the code
just before the `firstlist' label. Note that it is always from the
beginning of the disk, but _not_ relative to the partition boundaries.

   In the second sector of Stage 1.5 and Stage 2, these are defined:

`0x6'
     The version number (likewise, the installation mechanism's).

`0x8'
     The installed partition.

`0xC'
     The saved entry number.

`0x10'
     The identifier.

`0x11'
     The flag for if forcing LBA.

`0x12'
     The version string (GRUB's).

`0x12' + "the length of the version string"
     The name of a configuration file.

   See the file `stage2/asm.S', for more information.