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# Intel TXT Initial Boot Block
The Initial Boot Block (IBB) consists out of one or more files in the CBFS.
## Constraints
The IBB must follow the following constrains:
* One IBB must contain the reset vector as well as the [FIT table].
* The IBB should be as small as possible.
* The IBBs must not overlap each other.
* The IBB might overlap with microcode.
* The IBB must not overlap the BIOS ACM.
* The IBB size must be a multiple of 16.
* Either one of the following:
* The IBB must be able to train the main system memory and clear all secrets.
* If the IBB cannot train the main system memory it must verify the code
that can train the main system memory and is able to clear all secrets.
## Identification
To add the IBBs to the [FIT], all CBFS files are added using the `cbfstool`
with the `--ibb` flag set.
The flags sets the CBFS file attribute tag to LE `' IBB'`.
The make system in turn adds all those files to the [FIT] as type 7.
## Intel TXT measurements
Each IBB is measured and extended into PCR0 by [Intel TXT], before the CPU
reset vector is executed.
The IBBs are measured in the order they are listed in the [FIT].
## FIT schematic
![][fit_ibb]
[fit_ibb]: fit_ibb.svg
[FIT]: ../../soc/intel/fit.md
[Intel TXT]: txt.md
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