Subtracting binary

ILikeTofuuJoe · 2018-10-09 05:19:18

What is the easiest way to subtract binary in EE?

pwnzor · 2018-10-09 05:20:23

Just simply replace the gates with doors (or vice versa).

ILikeTofuuJoe

ILikeTofuuJoe · 2018-10-09 05:22:00

wait i put this topic in the wrong place help mods

Pingohits

Trytu · 2018-10-09 16:05:38

there is special complementary system, that makes adding works same as substracting - just add negative numbers
to add negative number you need to replace all 1 to 0, and all 0 to 1, and then add to result binary 1 (or substract 1, I don't remember)
everytime it'll give you right results

▼example

last carry has to be ignored
overflow can be easily found
if last carry and almost last carry isn't the same, there is an overflow

Slabdrill · 2018-10-10 17:49:55

In the same way you add

ILikeTofuuJoe · 2018-10-11 05:21:50

So 1-1 = 0, 1-0 = 1, 0-0 = 0, 0-1 = -1. How can we use that in EE?

LukeM · 2018-10-11 16:52:38

As Trytu said, the best way to do it is to use two's complement.

Two's complement allows you to represent negative numbers using binary, for example when using 8 bits, the largest bit is taken to mean -128 intead of 128, so rather than having the numbers 0 - 255 you have the numbers -128 - 127.

Converting from positive to negative is simple, you just invert all the bits and add one (or invert all the bits larger than the smallest 1 to do both at the same time)

e.g. 54 = 00110110, flipping all the bits gives you 11001001, adding 1 gives you 11001010 (or alternatively flipping all the bits past the 2s column also gives 11001010)
= 2 + 8 (= 10) + 64 (= 74) - 128 = -54

Once you have numbers in two's complement you can just do normal binary addition on them, and if you want to subtract a number then you just flip its sign then add it. This works because a negative two's complement number is 256 - the positive number, so when adding them they cause an overflow which wraps the result back to the answer you want (47 - 32 = 47 + (256 - 32) = 256 + 15, which overflows and wraps back around to 15)

PTU, Security-Drone

Official Everybody Edits Forums

#1 2018-10-09 05:19:18

Subtracting binary

#2 2018-10-09 05:20:23

Re: Subtracting binary

#3 2018-10-09 05:22:00

Re: Subtracting binary

#4 2018-10-09 16:05:38, last edited by Trytu (2018-10-09 16:07:11)

Re: Subtracting binary

#5 2018-10-10 17:49:55

Re: Subtracting binary

#6 2018-10-11 05:21:50

Re: Subtracting binary

#7 2018-10-11 16:52:38

Re: Subtracting binary

Board footer