Subtraction
Our new 16-bit adder looks all fancy and well. However, now you might be curious about how subtraction works in binary. But before we talk about subtraction, we need to talk about negative numbers. There are two reasons:
A bigger number - a smaller number = a negative number
a - b = a + (-b)
So how do we represent negative numbers in binary?
Option 1: Sign + Magnitude
Let's refer to the decimal system with "+" and "-" and encode "+" as 0 and "-" as 1.
n in decimal
n in binary
-n in decimal
-n in binary
+1
0001
-1
1001
+2
0010
-2
1010
+3
0011
-3
1011
+4
0100
-4
1100
+5
0101
-5
1101
+6
0110
-6
1110
+7
0111
-7
1111
What about 0? We unfortunately get two representations of 0: +0 and -0.
+0
-0
0000
1000
Two zeros are an extreme pain to deal with both in the hardware and software level. For example, a simple check whether a number is zero need to handle two cases - whether it's a positive or a negative zero.
This looks fine until we try to do a subtraction, say 7 - 3. Because subtraction is just the addition of the negated, 7 - 3 is just (+7) + (-3). If we look up the above table for +7 and -3 and add them together:
Notice that we get 010 which is 2 in decimals but we expect 7 - 3 to be 4. We don't even want to think about the messy carry of the sign bit. This system of tagging an extra sign bit in front of the magnitude is looks familiar and fine at first sight, but is very tricky to deal with.
Option 2: 1's Complement
In Mathematics, the method of complements is a technique to encode a symmetric range of positive and negative integers. For a given number of places half of the possible representations of numbers encode the positive numbers, the other half represents their respective negative numbers.
We obtain the 1's complement of a binary number by subtracting it from 1111. Let's say we have a binary number 0110. We get the 1's complement of 0110 like so:
Another example:
Do you notice any patterns?
Our Answer Yes, we simply flip each bit to get the 1's complement.
n in decimal
n in binary
-n in decimal
-n in binary
+1
0001
-1
1110
+2
0010
-2
1101
+3
0011
-3
1100
+4
0100
-4
1011
+5
0101
-5
1010
+6
0110
-6
1001
+7
0111
-7
1000
What about 0? We unfortunately still get two representations of 0: +0 and -0.
+0
-0
0000
1111
Let's try the same 7 - 3 by looking up the table for the binary representations of +7 and -3 and add them together:
We get back 0011 or 3 which is not the right answer. However, if we add the carry bit 1 to 0011:
Voila! We get back 0100 which is the correct answer +4 in binary!
This seems like a neat trick and works every time. However, as computer scientists, we want to understand exactly how things work instead of memorizing tricks. In this case, the explanation is simple with some clever tweaks:
Notice that 1111 - b is our 1's complement of b. Continue from the previous step:
Notice the result means the result of adding a and the 1's complement of b. This is analogous to the result 10011 we got previously when adding 0111 (+7) and 1100 (-3). Continue from the previous step:
We rewrote 1111 with 10000 - 1 and extract the -1 out of the parentheses to become a +1. Continue:
Subtracting any 4-bit number by 10000 does nothing to the 4-bits. Since we are dealing with 4-bit numbers only, we don't care what happens to the 1 on the fifth bit.
Now you should understand why we add the 1 from the carry bit to our result.
Option 3: 2's Complement
While 1's complement is much better than the sign + magnitude approach, we still need to deal with two zeros and had to add the carry bit when doing addition. Enter 2's complement which elegantly solves both issues. All you need to do is add 1 to a number's 1's complement to get its 2's complement.
n in decimal
n in binary
-n in decimal
-n in binary
+1
0001
-1
1111
+2
0010
-2
1110
+3
0011
-3
1101
+4
0100
-4
1100
+5
0101
-5
1011
+6
0110
-6
1010
+7
0111
-7
1001
-8
1000
Note that the numbers we can represent with 4 bits range from -8 to +7. The reason for the extra -8 is because we freed one slot by removing the double zeros!
Just in case you are not convinced, let's try to get the negative zero by taking the 2's complement of zero:
Since we only care about 4 bits, the result 10000 is effectively still 0.
Let's try the same 7 - 3 by looking up the table for the binary representations of +7 and -3 and add them together:
Voila! We got 7 - 3 = 4 as expected.
Let's see why 2's complement works:
Notice that (1111 - b + 1) is the 2's complement of b. It also matches our algorithm of getting the 1's complement of a number then add 1 to get its 2's complement. Continue:
After going through three viable options for representing negative numbers in binary, we and most computer scientists choose 2's complement for its simplicity. Enough theory, let's try to implement a subtractor! So what circuits do we need? Because a - b = a + (-b), we just need to create a negator and add the negated b to a as if we are subtracting b from a.
16-bit Negator
How do we negate a binary number? We just take its 2's complement:
Flip all bits
Add 1
Try implement the negator on your own.
Here's the header:
If you get stuck, click on "See Hints".
Additional Resource
Check out Computerphile's awesome explaination of how signed binary works:
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