"Implement register mirroring"
https://github.com/mupen64plus/mupen64plus-core/pull/1119

Index: src/device/rcp/si/si_controller.h
--- src/device/rcp/si/si_controller.h.orig
+++ src/device/rcp/si/si_controller.h
@@ -72,7 +72,7 @@ struct si_controller
 
 static osal_inline uint32_t si_reg(uint32_t address)
 {
-    return (address & 0xffff) >> 2;
+    return (address & 0x1f) >> 2;
 }
 
 
