[stella] RFC: What happens when changing HM registers within 24 cycles of HMOVE?

[Stephen Anthony]

I've noticed that several of the TIA issues in Stella are due to 
improper emulation of the horizontal motions registers within 24 cycles 
of an HMOVE (which the Stella programmers guide says can't be done).

Right now, there are hacks present for 7 or 8 different games, which 
obviously isn't extensible to new ROMs.  I'd like to get this emulation 
fixed once and for all.  However, I need some feedback.

Can anyone describe exactly what happens when modifying HMP0, HMPl, 
HMM0, HMMl and HMBL within 24 cycles of an HMOVE?  Ideally I'd like a 
short description of the behaviour, and any other registers that might 
be affected.

Thanks in advance for any info,
Steve