wawa: yes, I was missing the arosbootstrap in the s-s, thanks! Works now, well, sort of ...
toni wilen: it looks like you're right, thanks! For starters, I added CS and CFG registers for Gayle, although they don't seem to be needed (I will need them later anyway, for PCMCIA ethernet emulation). I'm still looking into other things missing.
The 'fun' part: I looked through the AROS sources, and in the ata_ReadSignature() function, one of the tests if there is a valid IDE device attached is that it reads the ATA Error register and expects to be set to 1 (= AMNF bit set, Address Mark Not Found). I looked through the ATA spec and I didn't find anywhere mentioning that the ATA Error register should have this bit set. Anyone have any idea? I just permanently set the error register to 1, and the test in AROS ata_ReadSignature() now passes. I don't particularly like this solution, though.
AROS still doesn't boot, as later I get "ATA interrupt but BUSY flag set!?" error, still looking into that (it shouldn't happen).