Assembly - Carry flag VS overflow flag
Solution 1
In unsigned arithmetic, you have added 0xFB
to 0x84
, i.e. 251 + 132, which indeed is larger than 8-bit, and so the carry flag is set.
In the second case, you are adding +127 to 1, which indeed exceeds a signed 8-bit range, and so the overflow flag is set.
Solution 2
Overflow occurs when the result of adding two positive numbers is negative or
the result of adding two negative numbers is positive.
For instance:
+127+1=?
+127=0111 1111
+1=0000 0001
---------
1000 0000
As we look at the sign bits of the two operands and the sign bit of the result, we find out that Overflow occurred and the answer is incorrect.
Tom O
Updated on September 14, 2020Comments
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Tom O almost 4 years
I have the next code:
mov al, -5 add al, 132 add al, 1
As I check it, the overflow flag and the carry flag will set in the first operation, and in the second, only the overflow will set.
But I don't understand why:
- In unsigned number, the result is 143 (8FH), and for that is fit 8-bit unsigned number (is smaller than 255) => the carry flag shouldn't be set. In signed number, the result is 127, It's fit to 8-bit signed, and the overflow shouldn't be set.
Whats wrong? Thanks.
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Tom O over 12 yearsSo the overflow is set only when I do 'add al, 1'? If I will delete this line, the overflow will not set?
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Oliver Charlesworth over 12 years@Tom: The overflow will be set by both
add
instructions. -
Tom O over 12 yearsSo you can explain me please where the overflow set? because as I understood it, in the range of -128 to 127 is not affecting the overflow.
-
Oliver Charlesworth over 12 years@Tom O: Convert all your values to signed 8-bit. In the first
add
, you're doing -5 + -124, which exceeds -128. In the secondadd
, you're doing +127 + +1, which exceeds +127. -
Simon Z. about 5 yearsThe V flag also turns to 1 when we add two negative and the result is 0. If we
adds r0, r1, r0
, wherer0 = 0x80000000
,r1 = 0x80000000
, after execution,Z = 1
,C = 1
,V = 1