some helpers to see if I can get to the bottom of the anpanmdx issue

src/mame/tvgames/xavix.cpp

@@ -465,7 +465,7 @@ void superxavix_state::superxavix_lowbus_map(address_map &map)
 	map(0x6f80, 0x6f99).rw(FUNC(superxavix_state::superxavix_crtc_1_r), FUNC(superxavix_state::superxavix_crtc_1_w)).share("sx_crtc_1"); // Super XaviX CRTC?
 	map(0x6fa0, 0x6fa7).rw(FUNC(superxavix_state::superxavix_crtc_2_r), FUNC(superxavix_state::superxavix_crtc_2_w)).share("sx_crtc_2"); // maybe more CRTC regs?
 
-	map(0x6fb0, 0x6fc7).ram().share("bmp_base");
+	map(0x6fb0, 0x6fc7).rw(FUNC(superxavix_state::bitmap_params_r), FUNC(superxavix_state::bitmap_params_w)).share("bmp_base");
 
 	// extended external bus stuff (possible banking control?)
 	map(0x7909, 0x7909).w(FUNC(superxavix_state::extended_extbus_reg0_w));

src/mame/tvgames/xavix.h

@@ -690,6 +690,9 @@ class superxavix_state : public xavix_state
 	void superxavix_pal_saturation_w(uint8_t data, bool bitmap);
 	void superxavix_pal_lightness_w(uint8_t data, bool bitmap);
 
+	uint8_t bitmap_params_r(offs_t offset);
+	void bitmap_params_w(offs_t offset, uint8_t data);
+
 	void superxavix_crtc_1_w(offs_t offset, uint8_t data);
 	uint8_t superxavix_crtc_1_r(offs_t offset);
 	void superxavix_crtc_2_w(offs_t offset, uint8_t data);

src/mame/tvgames/xavix_v.cpp

@@ -144,6 +144,18 @@ uint8_t superxavix_state::superxavix_crtc_1_r(offs_t offset)
 	return m_sx_crtc_1[offset];
 }
 
+void superxavix_state::bitmap_params_w(offs_t offset, uint8_t data)
+{
+	logerror("%s bitmap_params_w %02x %02x\n", machine().describe_context(), offset, data);
+	m_bmp_base[offset] = data;
+	// if anything changes these mid-screen we may need to trigger a partial update here
+}
+
+uint8_t superxavix_state::bitmap_params_r(offs_t offset)
+{
+	return m_bmp_base[offset];
+}
+
 void superxavix_state::superxavix_crtc_2_w(offs_t offset, uint8_t data)
 {
 	logerror("%s superxavix_crtc_2_w %02x %02x\n", machine().describe_context(), offset, data);