Recently I came across 4-bit signed integers. While I just wanted to make sense of them somehow in the first place, I thought that’s a nice opportunity to dig deeper.

Signed, but positive

So, let’s say we’ve got a four-bit signed number, 0111 in binary and, let’s assume this value comes from some peripheral register. In addition, we have a datasheet that does not clearly describe the binary representation of this number.

They do define, however, the position of the sign bit: it’s the most significant one (the zero on the left, in this case). It’s fair to assume that the number is represented as either ones’ or twos’ complement. If that’s true, a value of zero for the sign bit means positive. Therefore, we can interpret this number simply as it is: 7. We may assign it to an int8_t right away.

Our job is certainly going to be trickier if the number is negative. Let’s now assume a binary value of 1111.

What representation is it?

First of all, the exact format is now really something we want to know: the value would translate to -0 in ones’ complement but would mean -1 in twos’ complement representation (see the last row of the table below).

After consulting the datasheet again, let’s assume we come across the allowed range for that register. It reads: (-8) through (+7).

This is a great clue if you compare it to the table above (focus on the first two columns): twos’ complement is the only representation out of the three that doesn’t have two different values for zero, and therefore allows a negative limit of (-8), instead of (-7). That’s a strong indication that we are dealing with twos’ complement.

You might be wondering what the third column is for. I’ve added it to show a somewhat “naïve” representation, where you turn positive numbers negative by simply putting a ‘1’ bit in front of them. I think I’ve seen this kind of representation used occasionally; but because of its lack of the complement feature it’s generally harder for computers to deal with them. Well, this was just a side note – back to twos’ complement.

Now that we know all that, we can deduce the actual value by applying the conversion rule for twos’ complement:

1111 | 'not'
0000 | +1
0001

By flipping each bit and adding one, we get the result: It’s a -1!

This is pretty good so far. However, to be able to do further calculations we would either need either native support for 4-bit signed integers or a way to convert the number to some type our platform supports.

How to convert to int8_t?

Assuming we obtain the original value as a uint8_t, we may try to cast it to a signed type:

uint8_t input_value = 0x0f;
int8_t output_value = (int8_t)input_value;

Printing this value shows this cannot be right: the number is interpreted as +15. This is certainly not what we expected.

It seems we need to think more about the twos’ complement representation. From Twos’ Complement (Wikipedia):

The two’s complement of an N-bit number is defined as its complement with respect to 2^N. For instance, for the three-bit number 010, the two’s complement is 110, because 010 + 110 = 1000

This applies to the 4-bit numbers in our example: Adding (+1) and (-1) yields 2⁴ (in binary: 0001 + 1111 = 10000).

So, if we want our negative number to be represented correctly as an 8-bit signed type, we need to adjust it in a way that it follows the rules: We know that it’s (-1), so adding it to (+1) should result in 2⁸ (1'0000'0000). Currently, this doesn’t hold true: 0000'0001 + 0000'1111 = 0001'0000.

Apparently there needs to be an overflow to bit #8 when adding (-1) and its complement. How could 0000'1111 be modified to make this happen? Well, if bits #7 through #4 were set before adding, this would work: 0000'0001 + 1111'1111 = 1'0000'0000.

That seems to be the (general) solution: Replicate the sign bit of the input value to all higher bit positions of the target type, up to the most-significant bit.

Conclusion

You can handle negative integers of ‘narrow’ (sub-8bit) types smoothly as int8_t if you adjust them so that they keep acting as a complement to their positive counterpart.

Let me know what you folks think (twitter link below)!