BPLCON1 AGA - odd behaviour

[buzzybee]

Hi guys

I am using FS-UAE for developing and testing Reshoot Proxima 3, a new game for AGA-Amigas. All the time, while developing the game, I had issues with bits PF1H0 and PF1H1 in BPLCON1. These two bits control subpixel horizontal scrolling on AGA-chipset.

All the time it seemed like either my code and/or these two bits do not work as intended on FS-UAE: If SPREN=1 (DMA control, Sprites on) and BRDRBLNK=1 (Border is black, not COLOR00), then FS-UAE basically seems to perform a logical NOT-operation on the the values of PF1H0 and PF1H1 before drawing the picture. Therefore, horizontal scrolling is jagging. So much so, that I decided not to use PF1H0 and PF1H1 at all. Which is a shame, as code and these two bits work on real hardware, and make the game look much smoother.

Now I noticed that PF1H0 and PF1H1 work as intended on FS-UAE, if SPREN=0 or BRDRBLNK=0. Any idea what might be cause for that - or even better, any workaround that I could implement into my code?

Btw: I am using FS-UAE version 3.1.47. Older versions show same results.