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Security Fusings
A common misconception is that all iDevices enforce certain flags in their bootloaders when certain "fuses" are "blown".
Processor Security
Processor security flags are enforced by pulling several pins on the BGA low or high depending on the state of security needed. The same is done for board configuration. For example, the device security state is set to 0000 by removing all resistors. This is the PVT configuration.
Board Identifiers
Board identifiers are enforced by pulling several pins on the BGA low or high depending on the state of security needed. For example, the device configuration n94ap is set by pulling the pins to set the board ID: "0b1000" or 0x8. This measure saves costs by allowing Apple to reuse the same processor die with a specified ECID, but they can change the motherboard if necessary.
"Fuses"
These so-called "fuses" are actually fixed resistors on the motherboard.
