diff --git a/src/mess/drivers/m20.c b/src/mess/drivers/m20.c
index b5de310407e..652a1b77bbd 100644
--- a/src/mess/drivers/m20.c
+++ b/src/mess/drivers/m20.c
@@ -755,6 +755,9 @@ void m20_state::machine_reset()
 	else
 		m_port21 = 0xff;
 
+	if(system_bios() > 0)  // bits have different meanings?
+		m_port21 &= ~8;
+
 	m_fd1797->reset();
 
 	memcpy(RAM, ROM, 8);  // we need only the reset vector