From 216105f6df0f24ee656de3cf3abca4591ed3723d Mon Sep 17 00:00:00 2001
From: LRFLEW <LRFLEW@aol.com>
Date: Wed, 9 Oct 2019 01:42:50 -0500
Subject: [PATCH 058/170] Update comments in rand.s
---
libsrc/common/rand.s | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/libsrc/common/rand.s b/libsrc/common/rand.s
index fc23b6ee..a272af80 100644
--- a/libsrc/common/rand.s
+++ b/libsrc/common/rand.s
@@ -3,6 +3,7 @@
;
; Written and donated by Sidney Cadot - sidney@ch.twi.tudelft.nl
; 2016-11-07, modified by Brad Smith
+; 2019-10-07, modified by Lewis "LRFLEW" Fox
;
; May be distributed with the cc65 runtime using the same license.
;
@@ -22,6 +23,17 @@
; The best 8 bits, 24-31 are returned in the
; low byte A to provide the best entropy in the
; most commonly used part of the return value.
+;
+; Uses the following LCG values for ax + c (mod m)
+; a = $01010101
+; c = $B3B3B3B3
+; m = $100000000 (32-bit truncation)
+;
+; The multiplier was carefully chosen such that it can
+; be computed with 3 adc instructions, and the increment
+; was chosen to have the same value in each byte to allow
+; the addition to be performed in conjunction with the
+; multiplication, adding only 1 additional adc instruction.
;
.export _rand, _srand
@@ -35,10 +47,10 @@ rand: .dword 1
.code
_rand: clc
- lda rand+0 ; SEED += $B3
+ lda rand+0
adc #$B3
sta rand+0
- adc rand+1 ; SEED *= $01010101
+ adc rand+1
sta rand+1
adc rand+2
sta rand+2
--
2.26.0