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In computer processors the carry flag is a single bit in a system status register/flag register used to indicate when an arithmetic carry or borrow has been generated out of the most significant arithmetic logic unit bit position. The carry flag enables numbers larger than a single ALU width to be added/subtracted by carrying a binary digit from a partial addition/subtraction to the least significant bit position of a more significant word. This is typically programmed by the user of the processor on the assembly or machine code level, but can also happen internally in certain processors, via digital logic or microcode, where some processors have wider registers and arithmetic instructions than ALU. It is also used to extend bit shifts and rotates in a similar manner on many processors. For subtractive operations, two conventions are employed as most machines set the carry flag on borrow while some machines instead reset the carry flag on borrow.