diff --git a/emulators/a5200.yml b/emulators/a5200.yml
index 015f839a..a4de1b28 100644
--- a/emulators/a5200.yml
+++ b/emulators/a5200.yml
@@ -35,11 +35,11 @@ systems:
 files:
   - name: "5200.rom"
     path: "5200.rom"
-    size: 2048
+    min_size: 2048
     required: false
     hle_fallback: true
     validation: [size]
     note: >
       Atari 5200 OS ROM. Core option "a5200_bios" controls official vs internal.
       Altirra replacement has lower compatibility with some titles.
-    source_ref: "libretro/libretro.c:58-59, libretro/libretro.c:416-507, emu/memory.c:46"
+    source_ref: "libretro/libretro.c:58-59, libretro/libretro.c:416-507, libretro/libretro.c:456-468, emu/memory.c:46"
diff --git a/tests/test_profile_checks.py b/tests/test_profile_checks.py
index b38c0ce4..46b0c18d 100644
--- a/tests/test_profile_checks.py
+++ b/tests/test_profile_checks.py
@@ -148,5 +148,16 @@ class AresAtari5200Bios(unittest.TestCase):
         )
 
 
+class A5200Bios(_Probe):
+    """The core reads 0x800 bytes and falls back to the Altirra OS on a short
+    read; bytes past them are never read (libretro/libretro.c:456-468 at
+    40c6f2f)."""
+
+    def test_short_falls_back_and_longer_is_read(self):
+        self.assertFalse(self.accepts("a5200", "5200.rom", b"\0" * 1024))
+        self.assertTrue(self.accepts("a5200", "5200.rom", b"\0" * 2048))
+        self.assertTrue(self.accepts("a5200", "5200.rom", b"\0" * 4096))
+
+
 if __name__ == "__main__":
     unittest.main()