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AUXILIARY CARRY FLAG: This flag is set to a 1 by the instruction just ending if a carry occurred from bit 3 to bit 4 of the A Register during the instruction’s execution
Explanation from Quora
It indicates when a carry or borrow has been generated out of the least significant four bits of the accumulator register following the execution of an arithmetic instruction. It is primarily used in decimal (BCD) arithmetic instructions.
Explanation from Wikipedia
These two different definitions really confuses me. As I experimented this on Keil using 8051, and it showed that AC is set when the carry occurs out of four low bits. What is the precise definition?