It was recently brought to my attention that Darren Atkinson has a really good summary of the invalid register exchange and transfer opcodes in the 6809.
So good, that I should use the information to make MAME’s 6809 better match real hardware.
That will be a future post. In the mean time, I wrote a little program to exercise all of the invalid combinations and print results to the CoCo’s screen memory. This way I can, eventually, verify the implementation.
The software package is here: Test-Exg-TFR.zip
By “invalid”, I mean the combinations of registers that Motorola didn’t document. Combinations like specifying different width registers when exchanging or transferring. Or the use of the register bit patterns not defined by Motorola.