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.

diff --git a/sys/src/games/snes/dsp.c b/sys/src/games/snes/dsp.c
index 13406d216394e3c172078bc7760f7520d5f01cc3..4b5c0cfd2343dfc43e57fb6f8c779f3fc6d191e1 100644 (file)
--- 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;
        }