shithub: plan9front

Download patch

ref: f1d29a9d12fa6d89015735ea010ac4b02f1b80b5
parent: 52b54097bf23ed4f14e82ff21b24f39827055448
author: Michael Forney <mforney@mforney.org>
date: Wed Feb 17 05:19:50 EST 2021

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.

--- 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 @@
 		}
 		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;
 	}