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Condition flags

The N, Z, C, and V condition flags are held in the APSR.

The condition flags are held in the APSR. They are set or cleared as follows:

N

Set to 1 when the result of the operation is negative, cleared to 0 otherwise.

Z

Set to 1 when the result of the operation is zero, cleared to 0 otherwise.

C

Set to 1 when the operation results in a carry, or when a subtraction results in no borrow, cleared to 0 otherwise.

V

Set to 1 when the operation causes overflow, cleared to 0 otherwise.

C is set in one of the following ways:

  • For an addition, including the comparison instruction CMN, C is set to 1 if the addition produced a carry (that is, an unsigned overflow), and to 0 otherwise.

  • For a subtraction, including the comparison instruction CMP, C is set to 0 if the subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise.

  • For non-addition/subtractions that incorporate a shift operation, C is set to the last bit shifted out of the value by the shifter.

  • For other non-addition/subtractions, C is normally left unchanged, but see the individual instruction descriptions for any special cases.

Overflow occurs if the result of a signed add, subtract, or compare is greater than or equal to 231, or less than -231.

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