Two's Complement for Unusual Integer Sizes

Introduction

Negative integers are generally represented as Two's complement in binary. For most programmers, this is nothing they have to worry about, as it's handled automatically, including the conversion when assigning to a larger type. But when you work with integers of unusual length - like 5, 7 or 10 bits - you have to handle the negative representation yourself. This tip gives you a generic function to do just that.

Background

A signed 8-bit integer with the decimal value -2 is stored as the binary 11111110, or 0xfe. When this is assigned to a 16-bit integer, the value stored is not 0x00fe, but 0xfffe, which preserves the two's complement representation. In contrast, when you assign 0xfe to an unsigned 16-bit integer, it stores 0x00fe, since unsigned types do not adjust for two's complement.

However, when working with custom hardware or writing device drivers, you occasionally come across non-standard integer lengths, since register space can be limited. I recently had to cope with both 6-bit and 10-bit signed integers. When I read those, I stored them as an 8-bit and 16-bit integer, respectively, which messed up negative numbers. A 10-bit integer storing -1 in two's complement will contain 0x3ff, but a 16-bit integer containing 0x03ff is taken to represent 1023 in decimal. It should be 0xffff, which is 16-bit two's complement for -1.

I needed a conversion function to make sure negative numbers were represented accurately.

ETA: Philippe Mori reminded me of bit fields, which handles this for you (provided your data fits in an int), so I have written a little template class (because bit fields must be members) to give a third option.

The Function

#include <cstdint>
#include <stdexcept>

int16_t UpscaleTwosComplement(int16_t value, size_t length)
{
    // Too large?
    if (length > 15)
    {
        throw std::out_of_range("UpscaleTwosComplement length too large");
    }
    uint16_t mask = (~0) << length;
    // Too small for complement?
    if (length < 2)
    {
        return (~mask & value);
    }
    // Two's complement?
    uint16_t msb = 1 << (length-1);
    if (value & msb)
    {
        return (mask | value);
    }
    else
    {
        return (~mask & value);
    }
}

After basic sanity-checking, it checks the most significant bit of value to see if it is a negative number. If it is, all the higher bits in the result are set to 1, which preserves the representation. If it isn't, all higher bits are filtered out.

...
// 10-bit sensor value, 2's complement
int16_t registerTemp; 
// Read...
...
int16_t temp = UpscaleTwosComplement(registerTemp, 10);
// registerTemp temp
//   -1 (0x3ff)    -1 (0xffffffff)          
//   10 (0x00a)    10 (0x0000000a)          
//  -10 (0x3f6)   -10 (0xfffffff6)

I also wrote a templated version, for more control of the target type, and slightly fewer runtime operations:

template<typename Target, size_t Length>
Target UpscaleTwosComplement(Target value)
{
    // Template type sanity checks (C++11 and later)
    static_assert(sizeof(Target) * 8 > Length, "Length too large for Target");
    static_assert(Length > 1, "Too short for two's complement");
    Target mask = (~0) << Length;
    // Two's complement?
    Target msb = 1 << (Length-1);
    if (value & msb)
    {
        return (mask | value);
    }
    else
    {
        return (~mask & value);
    }
}

Bit Fields

I was reminded of bit fields, which is a mechanism in C and C++ to let you work on non-standard length integers. I cannot use them directly in my project, because of the way the hardware library is constructed, but you might be able to.

I can, however, use bitfields to do the conversion, so I have written the code to do that below as an alternative to my homespun conversions above. There are two limitations you need to be aware of, though. The first is that bit fields must be data members, of a struct or class. The second is that if you assign a bit field a value outside its range, the value it holds depends on the compiler implementation. For instance, a 10-bit signed bit field has a range of -512 (0x200) to 511 (0x1ff), but if it is assigned a 16-bit value of 512 (0x0200) will yield -512 in VC2008, and probably most compilers, but it is not guaranteed every compiler does the same thing - it is perfectly legal for a compiler to discard the sign bit, so you end up with 0, for instance. Check it works as you'd expect on your compiler before using it.

template <typename Target, size_t Length>
class TwosComplementUpscaler
{
    Target bitfield : Length;
    // Private constructors
    TwosComplementUpscaler(Target value)
    : bitfield(value)
    {}
    TwosComplementUpscaler();
public:

    static Target Convert(Target value)
    {
        // Template type sanity checks (C++11 and later)
        static_assert(sizeof(Target) * 8 > Length, "Length too large for Target");
        static_assert(Length > 1, "Too short for two's complement");

        Target mask = (~0) << Length;
        TwosComplementUpscaler temp(value & ~mask);
        return static_cast<Target>(temp.bitfield);
    }
}; 

You use it as a stand-alone template function:

int16_t tt = TwosComplementUpscaler<int16_t, 10>::Convert(0x3ff); // = -1
tt = TwosComplementUpscaler<int16_t, 10>::Convert(0x1ff); // = 511
tt = TwosComplementUpscaler<int16_t, 10>::Convert(0x200); // = -512

History

• 18^th Feb, 2016 - First posted
• 19^th Feb, 2016 - Updates after comments. Added section on bit fields and code for that, changed UpscaleTwosComplement to use size-specified types, and check length