Oh those precious cycles 
According to this, and many other sources, https://www.msx.org/wiki/MSX-Music_programming#Direct_usage_... I get the info about 7C:12 and 7D:84 cycles.
But can someone explain whether the wait is "global" - so no writing to the chip at all for x cycles after a write.
OR
Is the wait pr port? So that, I can make sure I always wait 84 cycles between writes to 7D and 12 cycles between writes to 7C? My inner loop would like to know :)
