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**ILikeTofuuJoe****Banned**- From: Obvervable Universe
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What is the easiest way to subtract binary in EE?

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**pwnzor****Member**- From: ca
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Just simply replace the gates with doors (or vice versa).

Friendship ended with now is my friend.

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**ILikeTofuuJoe****Banned**- From: Obvervable Universe
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wait i put this topic in the wrong place help mods

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#Joe Griffin

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**Trytu****Member**- From: Poland
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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

*Last edited by Trytu (2018-10-09 16:07:11)*

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**ILikeTofuuJoe****Banned**- From: Obvervable Universe
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So 1-1 = 0, 1-0 = 1, 0-0 = 0, 0-1 = -1. How can we use that in EE?

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#Joe Griffin

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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)

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