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+# IFD Layout
+
+A coreboot image for an Intel SoC contains two separate definitions of the
+layout of the flash. The Intel Flash Descriptor (IFD) which defines offsets and
+sizes of various regions of flash and the [coreboot FMAP](../lib/flashmap.md).
+
+The FMAP should define all of the of the regions defined by the IFD to ensure
+that those regions are accounted for by coreboot and will not be accidentally
+modified.
+
+## IFD mapping
+
+The names of the IFD regions in the FMAP should follow the convention of
+starting with the prefix `SI_` which stands for `silicon initialization` as a
+way to categorize anything required by the SoC but not provided by coreboot.
+
+|IFD Region index|IFD Region name|FMAP Name|Notes|
+|---|---|---|---|
+|0|Flash Descriptor|SI_DESC|Always the top 4KB of flash|
+|1|BIOS|SI_BIOS|This is the region that contains coreboot|
+|2|Intel ME|SI_ME||
+|3|Gigabit Ethernet|SI_GBE||
+|4|Platform Data|SI_PDR||
+|8|EC Firmware|SI_EC|Most Chrome OS devices do not use this region; EC firmware is stored BIOS region of flash|
+
+## Validation
+
+The ifdtool can be used to manipulate a firmware image with a IFD. This tool
+will not take into account the FMAP while modifying the image which can lead to
+unexpected and hard to debug issues with the firmware image. For example if the
+ME region is defined at 6 MB in the IFD but the FMAP only allocates 4 MB for the
+ME, then when the ME is added by the ifdtool 6 MB will be written which could
+overwrite 2 MB of the BIOS.
+
+In order to validate that the FMAP and the IFD are compatible the ifdtool
+provides --validate (-t) option. `ifdtool -t` will read both the IFD and the
+FMAP in the image and for every non empty region in the IFD if that region is
+defined in the FMAP but the offset or size is different then the tool will
+return an error.
+
+Example:
+
+```console
+foo@bar:~$ ifdtool -t bad_image.bin
+Region mismatch between bios and SI_BIOS
+ Descriptor region bios:
+  offset: 0x00400000
+  length: 0x01c00000
+ FMAP area SI_BIOS:
+  offset: 0x00800000
+  length: 0x01800000
+Region mismatch between me and SI_ME
+ Descriptor region me:
+  offset: 0x00103000
+  length: 0x002f9000
+ FMAP area SI_ME:
+  offset: 0x00103000
+  length: 0x006f9000
+Region mismatch between pd and SI_PDR
+ Descriptor region pd:
+  offset: 0x003fc000
+  length: 0x00004000
+ FMAP area SI_PDR:
+  offset: 0x007fc000
+  length: 0x00004000
+```
+\ No newline at end of file