[stella] Pitfall! LFSR

Subject: [stella] Pitfall! LFSR
From: Thomas Jentzsch <tjentzsch@xxxxxx>
Date: Mon, 3 Sep 2001 20:35:48 +0200
I'm currently disassembling Pitfall! and found
something (i think) quite interesting:
The game uses a one byte (what I'd call) bidirectional
LFSR for random scene generation.

Bidirectional means, that the process of generating the
next random number is reversible.

I haven't seen that type of LFSR before and thought it
would not be easy or possible to do so. (The two byte
LFSR used in River Raid for the same purpose seems to
be different, so Carol needs some extra RAM to store
the previous random number, which is necessary to
restart a scene when the player dies.)

The Pitfall! LFSR generates 255 values with one byte,
if somebody knows a good bidirectional two byte LFSR,
please let me know.

I think this could be useful for new homebrew games
too. You could generate huge worlds with that.

Have fun!
Thomas Jentzsch         | *** Every bit is sacred ! ***
tjentzsch at web dot de |

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