Carry Flag, Auxiliary Flag and Overflow Flag in Assembly

assembly x86 overflow eflags
user2322960 picture user2322960 · Oct 10, 2013 · Viewed 126.9k times · Source

I Cannot seem to tell the difference between the Carry Flag, Auxiliary Flag and Overflow Flag in Assembly. I'm currently studying it in school and the teacher didn't go into any details. Please help me to understand, I will be needing it for the exam. I will be really grateful!! From what I seem to know by now is that the Carry Flag is used when you try, let's say, 255+9 when you only have 8 bits for data, The Auxiliary Flag is the same, but only for the 4 last bits?? And The Overflow is used when you have 7 bits for the binary number and the 8(left-most ) is used for the sign???

Answer

dark_knight picture dark_knight · Sep 19, 2014

Carry Flag

The rules for turning on the carry flag in binary/integer math are two:

  1. The carry flag is set if the addition of two numbers causes a carry out of the most significant (leftmost) bits added. 1111 + 0001 = 0000 (carry flag is turned on)

  2. The carry (borrow) flag is also set if the subtraction of two numbers requires a borrow into the most significant (leftmost) bits subtracted. 0000 - 0001 = 1111 (carry flag is turned on) Otherwise, the carry flag is turned off (zero).

    • 0111 + 0001 = 1000 (carry flag is turned off [zero])
    • 1000 - 0001 = 0111 (carry flag is turned off [zero])

In unsigned arithmetic, watch the carry flag to detect errors.

In signed arithmetic, the carry flag tells you nothing interesting.

Overflow Flag

The rules for turning on the overflow flag in binary/integer math are two:

  1. If the sum of two numbers with the sign bits off yields a result number with the sign bit on, the "overflow" flag is turned on. 0100 + 0100 = 1000 (overflow flag is turned on)

  2. If the sum of two numbers with the sign bits on yields a result number with the sign bit off, the "overflow" flag is turned on. 1000 + 1000 = 0000 (overflow flag is turned on)

Otherwise the "overflow" flag is turned off

  • 0100 + 0001 = 0101 (overflow flag is turned off)
  • 0110 + 1001 = 1111 (overflow flag turned off)
  • 1000 + 0001 = 1001 (overflow flag turned off)
  • 1100 + 1100 = 1000 (overflow flag is turned off)

Note that you only need to look at the sign bits (leftmost) of the three numbers to decide if the overflow flag is turned on or off.

If you are doing two's complement (signed) arithmetic, overflow flag on means the answer is wrong - you added two positive numbers and got a negative, or you added two negative numbers and got a positive.

If you are doing unsigned arithmetic, the overflow flag means nothing and should be ignored.

For more clarification please refer: http://teaching.idallen.com/dat2343/10f/notes/040_overflow.txt

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