Glossary
PCR (Platform Configuration Register)
A TPM register that can only be extended with hashes, recording the sequence of firmware, bootloader and configuration measurements during boot.
A Platform Configuration Register holds a hash value that cannot be written directly, only extended: the new value is the hash of the old value concatenated with a new measurement. The final value therefore depends on every measurement and its order, which makes it a compact fingerprint of the boot chain.
On PCs, low-numbered PCRs record firmware (PCR 0), option ROMs (PCR 2), the boot manager (PCR 4) and Secure Boot policy (PCR 7). Disk-encryption keys can be sealed to chosen PCR values, and attestation quotes report them to a remote verifier.