Tiles on/off commands can cause lockups

This happens consistently on the actual F256K2 hardware in my office and in the IDE emulator. I haven't tested mame at this point.

To replicate type:

`
tiles on
`
then immediately type:
`
tiles off
`

The system will lock and not allow further input. This occurs in the IDE emulator as well. 

Found following the example in the documentation.