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--- base:16bit_xorshift_random_generator [2019-05-10 02:37] – vsariola
+++ base:16bit_xorshift_random_generator [2019-05-10 21:43] – vsariola
@@ Line 4: / Line 4: @@
   * ported by: Veikko Sariola
-Xorshift is a fast pseudorandom generator algorithm originally developed by [[https://www.jstatsoft.org/article/view/v008i14|George Marsaglia]]. [[http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html|John Metcalf]] found a 16-bit version of the algorithm that is fast on 8-bit platforms with only single bit shifts available. It has a period of 65535 and passes reasonable tests for randomness, unlike most of the LFSRs on this site. His pseudocode is reprinted here:
+Xorshift is a fast pseudorandom generator algorithm originally developed by [[https://www.jstatsoft.org/article/view/v008i14|George Marsaglia]]. [[http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html|John Metcalf]] found a 16-bit version of the algorithm that is fast on 8-bit platforms with only single bit shifts available. It has a period of 65535 and passes reasonable tests for randomness. His pseudocode is reprinted here:
 <code c>
@@ Line 40: / Line 40: @@
         ROR             ; A has now x >> 9 and high bit comes from low byte
         EOR rng_zp_low
-        STA rng_zp_low  ; x ^= x >> 9 done
+        STA rng_zp_low  ; x ^= x >> 9 and the low part of x ^= x << 7 done
         EOR rng_zp_high
         STA rng_zp_high ; x ^= x << 8 done