61 lines
3.2 KiB
Markdown
61 lines
3.2 KiB
Markdown
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# Intel Firmware Interface Table
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The FIT allows to run code before the actual IA32 reset vector is executed
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by the CPU. The FIT resides in the BIOS region (usually near the reset vector)
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and is pointed to by the FIT pointer residing at `0xFFFFFFC0`.
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## Table layout
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The table consists of blocks each 16 bytes in size.
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The first is called *FIT header* the other are called *FIT entry*.
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![FIT in x86 memory map][fit]
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[fit]: fit.svg
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## Fit types
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Each entry has a *type* that give the other bits in the entry a different
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meaning. The following types are known:
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```eval_rst
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+-----------+------------------------------------------------------------------+
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| no. | Description |
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+===========+==================================================================+
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| 0x0 | HEADER. |
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+-----------+------------------------------------------------------------------+
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| 0x1 | MICROCODE. |
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+-----------+------------------------------------------------------------------+
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| 0x2 | STARTUP_ACM. |
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+-----------+------------------------------------------------------------------+
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| 0x7 | BIOS_STARTUP_MODULE. |
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+-----------+------------------------------------------------------------------+
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| 0x8 | TPM_POLICY. |
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+-----------+------------------------------------------------------------------+
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| 0x9 | BIOS_POLICY. |
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+-----------+------------------------------------------------------------------+
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| 0xa | TXT_POLICY. |
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+-----------+------------------------------------------------------------------+
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| 0xb | KEY_MANIFEST. |
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+-----------+------------------------------------------------------------------+
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| 0xc | BOOT_POLICY_MANIFEST. |
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+-----------+------------------------------------------------------------------+
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| 0x10 | CSE_SECURE_BOOT. |
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+-----------+------------------------------------------------------------------+
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| 0x2d | TXTSX_POLICY. |
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+-----------+------------------------------------------------------------------+
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| 0x2f | JMP_DEBUG_POLICY. |
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+-----------+------------------------------------------------------------------+
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| 0x7f | SKIP. |
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+-----------+------------------------------------------------------------------+
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```
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## Usage in coreboot
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The most common usage of FIT is to use *Type1* to update microcode before
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execution of the IA32 reset vector happens.
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## References
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* [Intel TXT LAB handout](https://downloadmirror.intel.com/18931/eng/Intel%20TXT%20LAB%20Handout.pdf)
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* [FIT bios specification](https://www.intel.com/content/dam/www/public/us/en/documents/guides/fit-bios-specification.pdf)
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