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--- base:16bit_xorshift_random_generator [2019-05-09 23:20] – created vsariola
+++ base:16bit_xorshift_random_generator [2019-05-10 02:45] – vsariola
@@ Line 9: / Line 9: @@
 /* 16-bit xorshift PRNG */
-unsigned xs = 1;
+unsigned x = 1;
 unsigned xorshift( )
 {
-    xs ^= xs << 7;
+    x ^= x << 7;
-    xs ^= xs >> 9;
+    x ^= x >> 9;
-    xs ^= xs << 8;
+    x ^= x << 8;
-    return xs;
+    return x;
 }
 </code>
-Here is an implementation for the C64. 38 cycles if branch not taken (without the RTS), 31 if branch is taken, which should be about half of the time, so on average 34.5 cycles.
+Here is an implementation for the C64. 30 cycles without the RTS.
 <code asm>
@@ Line 32: / Line 32: @@
         ...
         ; the RNG, you can get 8-bit random numbers in A, leaves X/Y unchanged
-random  LDA rng_zp_low ; x ^= x << 7
+random  LDA rng_zp_high
         LSR
+        LDA rng_zp_low
+        ROR
         EOR rng_zp_high
-        STA rng_zp_high
+        STA rng_zp_high ; high part of x ^= x << 7 done
-        BCC @skip      ; If the lowest bit was 0, there's no need to EOR
+        ROR             ; A has now x >> 9 and high bit comes from low byte
-        LDA #%10000000 ; the highest bit of the low byte.
-        EOR rng_zp_low ; Carry was set, EOR highest bit.
-        STA rng_zp_low
-@skip   LDA rng_zp_high ; x ^= x >> 9
-        LSR
         EOR rng_zp_low
-        STA rng_zp_low
+        STA rng_zp_low  ; x ^= x >> 9 and the low part of x ^= x << 7 done
-        EOR rng_zp_high ; x ^= x << 8
+        EOR rng_zp_high
-        STA rng_zp_high
+        STA rng_zp_high ; x ^= x << 8 done
         RTS
 </code>