From f1d29a9d12fa6d89015735ea010ac4b02f1b80b5 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Wed, 17 Feb 2021 11:19:50 +0100
Subject: games/snes: implement DSP noise

I'm not sure if this LFSR is the same one used by the hardware or is
arbitrary, but it matches the noise sequence used by all other snes
emulators I looked at.
---
 sys/src/games/snes/dsp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/sys/src/games/snes/dsp.c b/sys/src/games/snes/dsp.c
index 13406d216..4b5c0cfd2 100644
--- a/sys/src/games/snes/dsp.c
+++ b/sys/src/games/snes/dsp.c
@@ -6,7 +6,7 @@
 #include "fns.h"
 
 u8int dsp[256], dspstate;
-u16int dspcounter, noise;
+u16int dspcounter, noise = 0x8000;
 static s16int samp[2], echoin[2];
 
 enum {
@@ -517,6 +517,8 @@ dspstep(void)
 		}
 		if(dspcounter-- == 0)
 			dspcounter = 0x77ff;
+		if(envyes(dsp[FLG] & 0x1f))
+			noise = (noise << 13 ^ noise << 14) & 0x8000 | noise >> 1 & ~1;
 		break;
 	case 31: voice(0, 4); voice(2, 1); break;
 	}
-- 
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