diff --git a/src/mess/machine/thomson.c b/src/mess/machine/thomson.c
index 3c9eed60015..7e32d7c7233 100644
--- a/src/mess/machine/thomson.c
+++ b/src/mess/machine/thomson.c
@@ -4110,7 +4110,15 @@ MACHINE_START_MEMBER( thomson_state, to8 )
 	m_thom_cart_bank = 0;
 	m_thom_vram = ram;
 	membank( THOM_CART_BANK )->configure_entries( 0,  8, mem + 0x10000, 0x4000 );
-	membank( THOM_CART_BANK )->configure_entries( 8, 32, ram, 0x4000 );
+	if ( m_ram->size() == 256*1024 )
+	{
+		membank( THOM_CART_BANK )->configure_entries( 8,    16, ram, 0x4000 );
+		membank( THOM_CART_BANK )->configure_entries( 8+16, 16, ram, 0x4000 );
+	}
+	else
+	{
+		membank( THOM_CART_BANK )->configure_entries( 8, 32, ram, 0x4000 );
+	}
 	membank( THOM_VRAM_BANK )->configure_entries( 0,  2, ram, 0x2000 );
 	membank( TO8_SYS_LO )->configure_entry( 0,  ram + 0x6000);
 	membank( TO8_SYS_HI )->configure_entry( 0,  ram + 0x4000);
diff --git a/src/mess/video/thomson.c b/src/mess/video/thomson.c
index 727e19a34d2..235055788d6 100644
--- a/src/mess/video/thomson.c
+++ b/src/mess/video/thomson.c
@@ -1192,7 +1192,7 @@ WRITE8_MEMBER( thomson_state::to8_sys_hi_w )
 
 WRITE8_MEMBER( thomson_state::to8_data_lo_w )
 {
-	UINT8* dst = m_thom_vram + offset + 0x4000 * m_to8_data_vpage + 0x2000;
+	UINT8* dst = m_thom_vram + ( ( offset + 0x4000 * m_to8_data_vpage + 0x2000 ) & m_ram->mask() );
 	assert( offset < 0x2000 );
 	if ( *dst == data )
 		return;
@@ -1207,7 +1207,7 @@ WRITE8_MEMBER( thomson_state::to8_data_lo_w )
 
 WRITE8_MEMBER( thomson_state::to8_data_hi_w )
 {
-	UINT8* dst = m_thom_vram + offset + 0x4000 * m_to8_data_vpage;
+	UINT8* dst = m_thom_vram + ( ( offset + 0x4000 * m_to8_data_vpage ) & m_ram->mask() );
 	assert( offset < 0x2000 );
 	if ( *dst == data )
 		return;
@@ -1223,7 +1223,7 @@ WRITE8_MEMBER( thomson_state::to8_data_hi_w )
 /* write to video memory page through cartridge addresses space */
 WRITE8_MEMBER( thomson_state::to8_vcart_w )
 {
-	UINT8* dst = m_thom_vram + offset + 0x4000 * m_to8_cart_vpage;
+	UINT8* dst = m_thom_vram + ( ( offset + 0x4000 * m_to8_cart_vpage ) & m_ram->mask() );
 	assert( offset < 0x4000 );
 	if ( *dst == data )
 		return;