Morwenn - 6 months ago 97

C Question

Is there any known way to compute the addition (and maybe the subtraction) of two Gray codes without having to convert the two Gray codes to regular binary, perform a binary addition then convert the result back to a Gray code? I managed to write increment and decrement functions, but the addition and subtraction seem even less documented and harder to write.

Answer

In this document under #6, there is an algorithm for serial Gray code addition (copied directly; note, that `⊕`

is `xor`

):

```
procedure add (n: integer; A,B:word; PA,PB:bit;
var S:word; var PS:bit; var CE, CF:bit);
var i: integer; E, F, T: bit;
begin
E := PA; F := PB;
for i:= 0 to n-1 do begin {in parallel, using previous inputs}
S[i] := (E and F) ⊕ A[i] ⊕ B[i];
E := (E and (not F)) ⊕ A[i];
F := ((not E) and F) ⊕ B[i];
end;
CE := E; CF := F;
end;
```

This adds the Gray code words A and B to form the Gray code word S. The operand parities are PA and PB, the sum parity is PS. This propagates two carry bits internally, E and F, and produces two external carry bits CE and CF

Unfortunately, it doesn't state anything about subtraction, but I assume, when you can encode negative numbers, you can use addition for that.