This is complementary to port 22, this sets the lowest ROM page that can execute code. The value of this port is the lowest ROM page allowed minus one. This is adjusted as apps are loaded. With no apps, this port is 69h, thus allowing only the 6Ah-7Fh ROM area to execute. If a 16K app is loaded, it becomes 68h, allowing that app to run from 69h. Again it is only writeable from 7Ch-7Fh pages.