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

Index: src/device/rcp/rsp/rsp_core.h
--- src/device/rcp/rsp/rsp_core.h.orig
+++ src/device/rcp/rsp/rsp_core.h
@@ -111,12 +111,12 @@ static osal_inline uint32_t rsp_mem_address(uint32_t a
 
 static osal_inline uint32_t rsp_reg(uint32_t address)
 {
-    return (address & 0xffff) >> 2;
+    return (address & 0x1f) >> 2;
 }
 
 static osal_inline uint32_t rsp_reg2(uint32_t address)
 {
-    return (address & 0xffff) >> 2;
+    return (address & 0x1f) >> 2;
 }
 
 void init_rsp(struct rsp_core* sp,
