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

Index: src/device/rcp/si/si_controller.c
--- src/device/rcp/si/si_controller.c.orig
+++ src/device/rcp/si/si_controller.c
@@ -123,7 +123,10 @@ void read_si_regs(void* opaque, uint32_t address, uint
     struct si_controller* si = (struct si_controller*)opaque;
     uint32_t reg = si_reg(address);
 
-    *value = si->regs[reg];
+    if (reg < SI_REGS_COUNT)
+    {
+        *value = si->regs[reg];
+    }
 }
 
 void write_si_regs(void* opaque, uint32_t address, uint32_t value, uint32_t mask)
