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base:16bit_xorshift_random_generator [2019-05-10 02:37] – vsariolabase:16bit_xorshift_random_generator [2019-08-12 15:41] (current) – Standardized format of the heading ftc
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-=== 16-bit "798" Xorshift ===+====== 16-bit "798" Xorshift ====== 
   * original idea: [[https://www.jstatsoft.org/article/view/v008i14|George Marsaglia]]   * original idea: [[https://www.jstatsoft.org/article/view/v008i14|George Marsaglia]]
   * idea for fast 8-bit implementation: [[http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html|John Metcalf]]   * idea for fast 8-bit implementation: [[http://www.retroprogramming.com/2017/07/xorshift-pseudorandom-numbers-in-z80.html|John Metcalf]]
   * ported by: Veikko Sariola   * 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> <code c>
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         STA rng_zp_high         STA rng_zp_high
         ...         ...
-        ; the RNG, you can get 8-bit random numbers in A, leaves X/Y unchanged+        ; the RNG. You can get 8-bit random numbers in A or 16-bit numbers 
 +        ; from the zero page addresses. Leaves X/Y unchanged.
 random  LDA rng_zp_high random  LDA rng_zp_high
         LSR         LSR
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         ROR             ; A has now x >> 9 and high bit comes from low byte         ROR             ; A has now x >> 9 and high bit comes from low byte
         EOR rng_zp_low         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          EOR rng_zp_high 
         STA rng_zp_high ; x ^= x << 8 done         STA rng_zp_high ; x ^= x << 8 done
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 {{:base:xorshift_798_results.png?400|}} {{:base:xorshift_798_results.png?400|}}
- 
  
base/16bit_xorshift_random_generator.1557448664.txt.gz · Last modified: 2019-05-10 02:37 by vsariola