# Intel Firmware Interface Table

The FIT allows to run code before the actual IA32 reset vector is executed
by the CPU. The FIT resides in the BIOS region (usually near the reset vector)
and is pointed to by the FIT pointer residing at `0xFFFFFFC0`.

## Table layout
The table consists of blocks each 16 bytes in size.
The first is called *FIT header* the other are called *FIT entry*.

![FIT in x86 memory map][fit]

[fit]: fit.svg

## Fit types

Each entry has a *type* that give the other bits in the entry a different
meaning. The following types are known:

```{eval-rst}
+-----------+------------------------------------------------------------------+
| no.       | Description                                                      |
+===========+==================================================================+
|       0x0 | HEADER.                                                          |
+-----------+------------------------------------------------------------------+
|       0x1 | MICROCODE.                                                       |
+-----------+------------------------------------------------------------------+
|       0x2 | STARTUP_ACM.                                                     |
+-----------+------------------------------------------------------------------+
|       0x7 | BIOS_STARTUP_MODULE.                                             |
+-----------+------------------------------------------------------------------+
|       0x8 | TPM_POLICY.                                                      |
+-----------+------------------------------------------------------------------+
|       0x9 | BIOS_POLICY.                                                     |
+-----------+------------------------------------------------------------------+
|       0xa | TXT_POLICY.                                                      |
+-----------+------------------------------------------------------------------+
|       0xb | KEY_MANIFEST.                                                    |
+-----------+------------------------------------------------------------------+
|       0xc | BOOT_POLICY_MANIFEST.                                            |
+-----------+------------------------------------------------------------------+
|      0x10 | CSE_SECURE_BOOT.                                                 |
+-----------+------------------------------------------------------------------+
|      0x2d | TXTSX_POLICY.                                                    |
+-----------+------------------------------------------------------------------+
|      0x2f | JMP_DEBUG_POLICY.                                                |
+-----------+------------------------------------------------------------------+
|      0x7f | SKIP.                                                            |
+-----------+------------------------------------------------------------------+
```

## Usage in coreboot

The most common usage of FIT is to use *Type1* to update microcode before
execution of the IA32 reset vector happens.

## References

* [Intel TXT LAB handout](https://downloadmirror.intel.com/18931/eng/Intel%20TXT%20LAB%20Handout.pdf)
* [FIT BIOS specification](https://www.intel.com/content/dam/www/public/us/en/documents/guides/fit-bios-specification.pdf)