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

Index: src/device/rcp/rsp/rsp_core.c
--- src/device/rcp/rsp/rsp_core.c.orig
+++ src/device/rcp/rsp/rsp_core.c
@@ -304,11 +304,11 @@ void read_rsp_regs2(void* opaque, uint32_t address, ui
     struct rsp_core* sp = (struct rsp_core*)opaque;
     uint32_t reg = rsp_reg2(address);
 
-    *value = sp->regs2[reg];
+    if (reg < SP_REGS2_COUNT)
+        *value = sp->regs2[reg];
 
     if (reg == SP_PC_REG)
         *value &= 0xffc;
-
 }
 
 void write_rsp_regs2(void* opaque, uint32_t address, uint32_t value, uint32_t mask)
@@ -319,7 +319,8 @@ void write_rsp_regs2(void* opaque, uint32_t address, u
     if (reg == SP_PC_REG)
         mask &= 0xffc;
 
-    masked_write(&sp->regs2[reg], value, mask);
+    if (reg < SP_REGS2_COUNT)
+        masked_write(&sp->regs2[reg], value, mask);
 }
 
 void do_SP_Task(struct rsp_core* sp)
