chunlee

hi there:

i am just learning more about data types and thought i will write a short code to see the bit patterns of signed variable, in this case a char.

i know that the first bit of signed data types is called sign bit, so i suspected to see something like:

decimal 1 -> 00000001
decimal -1 -> 10000001

the left most bit being the sign bit.

however, in the code i worte i got:

decimal value -> -1 :: bit pattern -> 11111111
decimal value -> 1 :: bit pattern -> 00000001

and it seems that -128 comes after +127 as:

decimal value -> 127 :: bit pattern -> 01111111
decimal value -> -128 :: bit pattern -> 10000000

my question is that, if this is the correct bit representation of signed char, how does negative numbers are convereted into bit patterns? of course, it could be something wrong in the code i wrote, my code are as below:

main()
{
char string[9];
int i;
char test;
unsigned char MASK;

for(test = -128; test<127; test++)
{
MASK = 1<<7;
for(i=0; i<8; i++)
{
sprintf(string+i, "%d", (test & MASK) ? 1 : 0);
MASK>>=1;
}
string[8] = (char)NULL;
printf("decimal value -> %d :: bit pattern -> %s\n", test, string);
}
}

many thanks

CHUN

hk_mp5kpdw

Negation is accomplished using something called two's complement. Do a search on it to find out more but basically to convert a value from positive to negative, you negate all the bits (zeros become ones and ones become zeros) and then you add 1. So, if 1 is 00000001 then to negate it we first takes its ones complement which is 11111110 and then add 1 to get the two's complement which is 11111111. 127 is 01111111 so -127 is 10000000 (ones complement) and add 1 to get the final answer of 10000001. -128 is 1 less than -127 so take 10000001 and subtract 1 and you get 10000000.

__________________
On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.
--Charles Babbage, 1792-1871

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

Thantos

Code Tags

The reason for the twos complement is so you can add signed numbers and get the correct result.

1 + -1 = 0

But you only take the least signficant bits (in this case 8) so you get 00000000

chunlee

thanks, i will look into it in more depth.

cheers

CHUN

Dave_Sinkula

There are three possible representations of signed integral types in C:

• sign and magnitude
• two's complement
• one's complement

A brief description is here. Use Google for more.

__________________
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*