mame Debugging port indirection?

[The Usotsuki]

I'm getting a bit frustrated trying to figure out why some code is going off the rails, except that I know what the cause is: some code is dinking with the PSG's "Port A".  So I'm trying to trace into the code to see what does this.  I've found several bits of code that do this, but I haven't found all of them.

 

I was able to get a window that shows me the current status of the port:

but I can't find a way to break when it gets altered (in case it's something nonobvious).  I've been Ring TFM and it hasn't given me any clues, and I tried several possible commands that don't work.

 

I managed to disassemble the game in its original form, but it's a completely uncommented disassembly.  This is the last thing hindering me.

[The Usotsuki]

wpi a0,2,w,1,{ tracelog "PSG: %04X: %02X: %02X\n",pc,wpaddr,wpdata;g }

Not perfect, but serviceable.  Found my missing hit.