When Is the N (Negative) Flag Set in Assembly?
Say We Have the Addition of the Following Two Registers: R0 = 6Dee 8765 R1 = 3458 Ffdd R0 + R1 = A2478742 Assuming We Are Using a 2'S Complement Sign...
Say we have the addition of the following two registers:
R0 = 6DEE 8765
R1 = 3458 FFDD
R0 + R1 = A2478742
Assuming we are using a 2's complement sign convention, what flags would be set? I know that overthrow would be set, but would "N" as well?
Regards
1 Answer
In all architectures that support flags, the N or Negative flag simply matches the most significant bit of the last number computed.
In addition many architectures have a O or Overflow flag which tells you if the most significant bit was changed by the last operation.
If a register went from positive to negative both O and N will be set.
If the value in the register was negative before and after N will be set, but not O. etc.
The overflow flag is only useful when using signed arithmetic. When using unsigned arithmetic you use the C (Carry) flag, which tells you when the result exceeded the largest result the register can hold (0xFFFF FFFF in 32-bit).
Finally there is the Z zero flag which tells you if the result is zero or not.
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