TROGBlog

When it says HMOVE at 73 cycles, does that mean the HMOVE write starts at the 73rd cycle, or ends at the 73rd cycle?

I'm guessing that all the move registers have to be set to 8 to prevent any motion. This makes the HMCLR register useless in this mode.

 

The HMOVE instruction has to end at cycle 73 (or 74) for this trick to work, which means that you have to do the sta HMOVE on cycle 70 or 71. This will move the sprites left by 8 pixels, so you will need to set the HMPx registers to 8 unless you want this movement. However, you can also use other combinations of HMOVEs and offsets if these cycles are inconvenient. There is a handy table at the end of the advanced programming guide which shows the movement generated by the different HMOVE/cycle combinations.

 

Chris

Thanks Chris. I figured out what I was doing wrong. After trying various values, I finally got wise and opened up the Stella TIA debugger. I've never had to use the TIA panel before, but this time it proved essential for finding the bug. The sprites were strobed in the right locations and the HMOVE was executing at the correct cycle, but I noticed the HM values for P0 and P1 were both 0, even though I had just written an 8 to both registers. Well, that's the problem. I wrote an 8. What I needed to write was an $80, because the HM registers only use the upper 4 bits. Once I changed that value, everything fell into place, and I now have a title screen with no artifacts. Thanks again.