Fix zero-BIOS MSVC registry build

runtime/CMakeLists.txt

@@ -537,6 +537,9 @@ if(BUILD_TESTING)
         add_test(NAME cdrom_lid_integration_test
                  COMMAND ${Python3_EXECUTABLE}
                          ${CMAKE_CURRENT_SOURCE_DIR}/tests/test_cdrom_lid_integration.py)
+        add_test(NAME bios_registry_zero_entry_guard_test
+                 COMMAND ${Python3_EXECUTABLE}
+                         ${CMAKE_CURRENT_SOURCE_DIR}/tests/test_bios_registry_zero_entry.py)
     endif()
 
     # Full card protocol regression: read, write, address echo, per-slot state,

runtime/runtime.cmake

@@ -667,6 +667,15 @@ else()
     list(LENGTH _psxrt_bios_linked _psxrt_bios_count)
 endif()
 
+# ISO C requires at least one initializer. A setup host can intentionally have
+# zero linked BIOS backends, so give the generated array one unused null entry
+# while keeping its public count at zero. MSVC 19.44 otherwise crashes with
+# C1001 in CloseTypeServerPDB when it compiles an empty initializer list.
+if(NOT _psxrt_registry_entries)
+    set(_psxrt_registry_entries "    0, /* unused: registry count is zero */
+")
+endif()
+
 # Registry of the compiled-in backends, in preference order. Generated so the
 # stem list stays the single source of truth.
 set(_psxrt_registry_c "${CMAKE_BINARY_DIR}/psx_bios_registry.c")

runtime/tests/test_bios_registry_zero_entry.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""Keep setup-host BIOS registry generation valid ISO C."""
+
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[2]
+RUNTIME_CMAKE = (ROOT / "runtime" / "runtime.cmake").read_text(encoding="utf-8")
+
+guard = "if(NOT _psxrt_registry_entries)"
+sentinel = 'set(_psxrt_registry_entries "    0, /* unused: registry count is zero */'
+writer = 'set(_psxrt_registry_c "${CMAKE_BINARY_DIR}/psx_bios_registry.c")'
+
+assert guard in RUNTIME_CMAKE
+assert sentinel in RUNTIME_CMAKE
+assert RUNTIME_CMAKE.index(guard) < RUNTIME_CMAKE.index(writer)
+
+print("zero-entry BIOS registry guard passed")