I am absolutely not getting how you can just minus a binary (1 Viewer)

[Winston]

ok i've been seeing this in Sam other peoples working out for my questions how on earth can you just minus a binary number????


i don't get the logic, wat's the rules, i fort the only method of subtraction is changing the number to the two's comp and adding it.

 

[Winston]

ok i searched up on it


and i understand the basic


0 - 0 = 0
1 - 0 = 1
1 - 1 = 0
0 - 1 = 1 (borrow a 1 from the digit next to it )

but my big issue is what happens if u can't borrow a digit next to it because there's nothing to borrow for example


1000
1001

 

[Glide]

Is this for your option topic or SDD as a whole?

Havnt really focused much on binary, argh! not really keen on doing it in the exam either

 

[-X-]

lol MS calculator gives "111111111111111111111111....." as the answer.

 

[saves.the.day]

Originally posted by Glide
Is this for your option topic or SDD as a whole?

Havnt really focused much on binary, argh! not really keen on doing it in the exam either

No no, its for the option topic "Software Developers view of hardware" in the sub-section "binary arithmetic". If you're doing the other option of evolution of programming languages (assuming it has no dot point covering binary arithmetic) you should be fine.

 

[Rahul]

that would be a negative number would it not, i dont think that is in the syllabus, not sure but.

as long as you do something like

10000
- 1001

then you should be fine i think

 

[fatmuscle]

1000 - 1001 = you're pretty screwed in calculation by hand.

if you look at my notes, they'll confuse you even more if you were taught by australian schooling, learning how to subtract with the cross method.

 

[saves.the.day]

Couldn't you just convert the second element to a ones or twos complement then add the two numbers together? I'm using the logic:

x - y = x + (-y)
where -y is simply the ones complement or twos complement.

 

[fatmuscle]

or u could just switch the numbers around, then put a negative infront of your answer?

1000 - 1001 --> 1001 - 1000

= -1

*shrugs*

 

[SamD]

Originally posted by -X-
lol MS calculator gives "111111111111111111111111....." as the answer.

MS Calculator uses the twos complement system and 1111111111... is the twos complement representation of -1 in normal binary.

Everyone seems to be confusing normal binary numbers with twos complement binary, they are different systems!

1. Lets do 1000-1001 in normal binary:
This is the decimal subtraction 8 - 9, which you would not attempt directly in decimal, rather you'd do what fatmuscle said and do 9-8 then make the answer a negative. The same applies in binary, so the answer in binary is -1.

2. Now in twos complement:
In twos complement the range of numbers wraps around to form a circle. So with 8bits the range is -128 to 127 (10000000 to 01111111), however if you add 1 to 127 you end up at -128 (this is an example of overflow). This wrap around means that you can do subtractions like 1000-1001 with twos complement.

If its a 4bit twos complement system then using normal subtraction you get:
1000 -
1001
--------
1111
(Does this need more explanation!)

Sam

 

[Glide]

Originally posted by saves.the.day
No no, its for the option topic "Software Developers view of hardware" in the sub-section "binary arithmetic". If you're doing the other option of evolution of programming languages (assuming it has no dot point covering binary arithmetic) you should be fine.

Great, im doing evolution
Thanks for the heads up, I thought we didnt cover it in class!