Re: 8 Bit Random Numbers

On Feb 2, 2:05 pm, Nobody <nob...@xxxxxxxxxxx> wrote:
On Mon, 02 Feb 2009 07:58:02 -0600, John Fields wrote:
Well, we could argue forever about what constitutes "[E]XOR-based", but
it's not an LFSR, and I thought it was clear that's what "nospam" was
referring to by "XOR-based PRNG".
---
He was, and you're both wrong.

What you're saying, in effect, is that if an 8 cylinder ICE fitted with
a 7 cylinder ignition system was fitted with a system winch allowed all
8 cylinders to work it would no longer be an ICE, which is total
nonsense.

No, I'm saying that if you modify an LFSR so that its input is no longer
an N-input XOR of (some of) its outputs, it's no longer an LFSR.

I don't think that anone would contest that there exist *other* circuits
which will cycle through all 256 8-bit values, or at least don't have the
lock-up state.

---
Your point being???

There are circuits which don't have the lockup state, but an LFSR isn't
one of them.

The output of the 8 bit LFSR that Fields did is exactly the same as
the standard version using 4 taps. The only addition is the zero state
appears between the values hex 80 and 01. The new sequence goes
80,00,01.....

Another solution (with a processor) might be to use a lookup table
where the desired values are read in order. And if you did that, why
not just add the extra forbidden zero into the table? And if you did
add it, where should it be located?

Seems logical to me to add the forbidden zero state where it might
normally occur, which is right after the 10000000 state and just
before the 00000001 state, so the sequence is 10000000, 00000000,
00000001..... And that's exactly what the circuit does, without the
lookup table.

-Bill
.

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