Question

I’m a newbie in assembly and I have a question about how to represent negative numbers
I have three DWORDS variable, let say:

result DWORD 0
i DWORD 3
j DWORD 5

and I want to calculate this formula: result = i – j + 8
but, when i do the i-j, the result will be a very high number because of the sign
so how do I make the result ok in the end?

Answer 1

For 32 bit DWORD the integer range is from –2147483648 to 2147483647 or in hex -0x80000000 to 0x7FFFFFFF.

So the number -1 is present like 0xFFFFFFFF. (Like counter underflow)

If the high (31) bit is set then the number is negative. To make positive number from negative (negation) you must make compement of number and add 1.

Example:

    0xFFFFFFFE   //-2
xor 0xFFFFFFFF   //binary complement 
---------------
    0x00000001   //result of complement
+   0x00000001   //add 1
---------------
    0x00000002   //Result of negation is 2