2's Complement...

[hughLg]

In 2's complement calculation system, the most significant bit is used to identify if a number is positive or negative. "signed int" in C is for this purpose.

Currently, I haven't go further inside this feature in C yet. But i want to know, how can I identify a binary number is a positive number or negative? I know how to convert decimal number into binary even negative number. But after i got the result, if I act as another person that want to know if this number is positive number or negative, how can i do that? Stop talking about theory! Show you a example:

/* Example... */

Decimal Binary
---------------------------------------------
255 = 1111 1111 (as usual)

-255 = 1111 1111
= 0000 0000 (negate all bits)
+ 1
--------------------
0000 0001 (end result)

/* ...Example */

Since the '0' for most significant bit indicating positive number and '1' indicating negative number, but if I treat the end result shown above as positive number, it's wrong.

Anybody tell me more about these?...

Thanks for your attention.

 

[MKuiper]

You need 1 bit for the sign, so if you have 8 bits minus 1 for the sign, there are 7 bits left for the number. That means the range of 8 bits is from -128 to 127 and NOT from -255 to 255.

127 = 0111 1111
64 = 0100 0000
32 = 0010 0000
16 = 0001 0000
8 = 0000 1000
4 = 0000 0100
2 = 0000 0010
1 = 0000 0001
0 = 0000 0000
-1 = 1111 1111
-2 = 1111 1110
-4 = 1111 1100
-8 = 1111 1000
-16 = 1111 0000
-32 = 1110 0000
-64 = 1100 0000
-128 = 1000 0000

With 16 bits, the range is from -32768 to 32767,
32767 = 0111 1111 1111 1111
1 = 0000 0000 0000 0001
0 = 0000 0000 0000 0000
-1 = 1111 1111 1111 1111
-32768 = 1000 0000 0000 0000

etc.

Marcel

 

[hughLg]

oh...i forget that -255 can only be expressed with higher bits...i just used 8 bits...

thanks a lot.