Firmware UEFI mods for 2011 iMacs
Some patches may work on other same era Macs. Look at description inside patch for detailed information.
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Make a backup of your iMac firmware (bios).
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Depending on patch type, use UEFITool version 0.28 or UEFIPatch (part of UEFITool 0.28) to apply patches to your bios.
Create a GRML linux usb, boot from it and use flashrom command to dump your eeprom: flashrom -p internal -r my_bios.img
If reading fails with error "Multiple flash chip definitions..." add option -c "CHIP_MODEL" to flashrom.
Do multiple reads and make sure they are the same. Always keep a backup of this file.
Write back modified bios to eeprom chip using a hardware programmer like CH341A + SOIC clip.
If you have previously applied the eeprom unprotection patch you can instead use a software programming like flashrom from Linux or Intel Flash Programming Tool (FPT) from Windows or UEFI Shell.
You should be familiar with how to disassemble your system and hardware programming of the eeprom chip. While this patches have been extensively tested, there is always the risk of breaking some hardware parts.
PROCEED AT YOUR OWN RISK!