[amiga386]

> it's theoretically fastest to do `and d1,d0` e.g. in a loop

That's true, although I'd add that ASR/AND are destructive while BTST would be nondestructive, but we're pretty far down a chain of hypotheticals at this point (why would someone even need to test evenness in a loop, when they could unroll the loop to doing 2/4/6/8 items at a time with even/odd behaviour baked in)

> I thought BLTPRI enabled meant the blitter takes every even DMA cycle it needs, and when disabled it gives the CPU 1 in every 4 even DMA cycles

Yes, that is true: https://amigadev.elowar.com/read/ADCD_2.1/Hardware_Manual_gu... "If given the chance, the blitter would steal every available Chip memory cycle [...] If DMAF_BLITHOG is a 1, the blitter will keep the bus for every available Chip memory cycle [...] If DMAF_BLITHOG is a 0, the DMA manager will monitor the 68000 cycle requests. If the 68000 is unsatisfied for three consecutive memory cycles, the blitter will release the bus for one cycle."

> one of those cases is very likely to be true

It blew my mind when I realised this is probably why Workbench is 4 colours by default. If it were 8, an unexpanded Amiga would seem a lot slower to application/productivity users.