[sdiy] Non maximal-length LFSR

[mskala at ansuz.sooke.bc.ca]

On Thu, 3 Mar 2016, Richie Burnett wrote:
> only using two taps in the LFSR, hence asking.)  The pseudo-code looks like
> this:
>
> mov    [pointer+1],accumulator
> xor      [pointer],accumulator,[pointer++]

I'm not convinced that this entire design is the right way to do what
you're trying to achieve, but I don't think you're forced to use 48-bit
registers; if you play with the indices, you should be able to make each
bit depend on the bits you generated 47 and 42 ticks ago (that is, use a
maximal-length 47-bit sequence), while still running in a 48-tick buffer
and recording the last 48 bits generated.  You just don't have to look
quite all the way back in the buffer.

The basic operation is:

z[i]  <-  z[i-47] XOR z[i-42]
(repeat for each z[i] in the buffer, any buffer size >=47, modulo indexing)

I don't know if that's going to cost you a third instruction, but I doubt
you can do better without more significant changes to your goals.

-- 
Matthew Skala
mskala at ansuz.sooke.bc.ca                 People before principles.
http://ansuz.sooke.bc.ca/