Why is ~4 -3?

Question

Can someone explain me why it works this way? If I flip all the bits, It seems like I would get a very big number. My questions are:
1. What is 1's and 2's complement?
2. How does the computer know which one is used?
3. Does the amount of bits give me another answer of ~4 (or any other number)?

Answer 1

take a look at this lesson
and the code

the key to your answer lies in how numbers are represented by a computer and what is the MSB (research it ;) )

https://www.sololearn.com/learn/4076/?ref=app
https://code.sololearn.com/WBOdrCZZT0a7/?ref=app

Answer 2

nope

the left most bit (MSB) determines the sign of the number (1 for negative, 0 for positive)

and numbers with a minus sign numbers it works the opposite (almost)

-1 for example is a composed of only 1's

-1
1111 1111 1111 1111 1111 1111 1111 1111

-2
1111 1111 1111 1111 1111 1111 1111 1110

-3
1111 1111 1111 1111 1111 1111 1111 1101

and so on

Answer 3

Look into twos complement if you want to know more

Answer 4

Lucas Knook you are correct. All modern processors use twos complement, it hardwired in their ALU. Only legacy systems use ones complement. See this for more info https://superuser.com/questions/1137182/is-there-any-existing-cpu-implementation-which-uses-ones-complement

Answer 5

yes, I have seen these, and I understand what MSB means, but still, I don't understand.

Answer 6

when I put in 5, I get this as result:
1111 1111 1111 1111 1111 1111 1111 1010
Shouldn't this be a very big number?

Answer 7

Am I correct if I say when the msb is 1, and the number is negative, I have to look for the zero's instead of the one's and then substract 1 from what I get? Also, how does the computer know it uses one's complement or two's complement? Or can a computer only use one of them?

Answer 8

I know complementing 0 and 1 is its opposite. 1 become 0 and 0 become 1.

Answer 9

So, I can say that when I use bitwise not, it flips all the ones and zero's including the MSB, which tells the computer that it is a negative number and then it reads 0 and 1 the opposite. after that it substracts one.

Answer 10

Yeah

Answer 11

I only have a little question left. Why does it substract one? can't 1111 1111 just be zero?

Answer 12

Cause then you have two zeros
0000 0000 and
1111 1111

Answer 13

ah, that makes sense. I understand now, thanks!