Use the Calculator to reveal a Mac’s firmware password
A firmware password on Mac systems locks the hardware to prevent booting to alternative modes that could bypass OS X security, but in order to access some of these modes you first have to disable the firmware password. While this can be done using Apple’s Firmware Password Utility on the recovery drive or OS X installation disc, there may be times when you do not have access to this drive either because it is missing or corrupted, and therefore cannot use the password utility.
Unfortunately in these cases for newer Mac systems shipped in 2011 or later, you have to get the system serviced…
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