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A typical PROM comes with all bits reading as "1". Burning a fuse bit during programming causes the bit to be read as "0" by "blowing" the fuses, which is an irreversible process. Some devices can be "reprogrammed" if the new data replaces "1"s with "0"s. Some CPU instruction sets (e.g. [[MOS Technology 6502#Bugs and quirks|6502]]) took advantage of this by defining a break (BRK) instruction with the operation code of '00'. In cases where there was an incorrect instruction, it could be "reprogrammed" to a BRK causing the CPU to transfer control to a patch. This would execute the correct instruction and return to the instruction after the BRK.
The bit cell is programmed by applying a high-voltage pulse not encountered during a normal operation across the gate and substrate of the thin oxide transistor (around 6{{nbsp}}V for a 2 nm thick oxide, or 30{{nbsp}}MV/cm) to break down the oxide between gate and substrate. The positive voltage on the transistor's gate forms an inversion channel in the substrate below the gate, causing a tunneling current to flow through the oxide. The current produces additional traps in the oxide, increasing the current through the oxide and ultimately melting the oxide and forming a conductive channel from gate to substrate. The current required to form the conductive channel is around 100{{nbsp}}
==See also==
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