After reading from the test register area it's still possible to read from the wave RAM area. When writing the wave RAM area with pre-determined data, it's possible to detect changes and patterns during rotation using those wave RAM data values.

That's very interesting. Thank you for checking.

Data bus, not budget.

Does the internal data budget have pull up resistors? Or does it really depend on that outside factor?

I wonder what will happen if you read the frequency, volume and enable bits registers.

So when you read the test register, it'll actually contain all bits set? Is that what I understand from this? That's very interesting.