Editing Two's complement (section)

===Comparison (ordering)===
[[Comparison (computer programming)|Comparison]] is often implemented with a dummy subtraction, where the flags in the computer's [[status register]] are checked, but the main result is ignored. The [[zero flag]] indicates if two values compared equal. If the exclusive-or of the [[Sign flag|sign]] and [[Overflow flag|overflow]] flags is 1, the subtraction result was less than zero, otherwise the result was zero or greater. These checks are often implemented in computers in [[conditional branch]] instructions.

Unsigned binary numbers can be ordered by a simple [[lexicographic ordering]], where the bit value 0 is defined as less than the bit value 1. For two's complement values, the meaning of the most significant bit is reversed (i.e. 1 is less than 0).

The following algorithm (for an {{mvar|n}}-bit two's complement architecture) sets the result register R to −1 if A < B, to +1 if A > B, and to 0 if A and B are equal:

<syntaxhighlight lang="pascal">
// reversed comparison of the sign bit

if A(n-1) = 0 and B(n-1) = 1 then
    return +1;
else if A(n-1) = 1 and B(n-1) = 0 then
    return -1
end;
 
// comparison of remaining bits

for i := n-2 downto 0 do begin
    if A(i) = 0 and B(i) = 1 then
        return -1
    else if A(i) = 1 and B(i) = 0 then
        return +1 
    end
end
 
return 0
</syntaxhighlight>