Re: [hatari-devel] TOS v4 / TT-RAM issue with floppy images

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 27/11/2017 à 17:12, Thorsten Otto a écrit :
> On Montag, 27. November 2017 11:24:37 CET Nicolas Pomarède wrote:
> > maybe just leave it as is and add a note that TT-RAM for
> > falcon is an emulation hack and might cause errors with floppy
>
> The problem is that it might not only cause read errors, but may accidently
> overwrite the ST-RAM instead of the intended TT-RAM address.

Yes :)

I checked the code ; the difference between ST and Falcon with FDC DMA 
transfer is that STF/STE are correctly masked to 0x3fffff (limited to 4 
MB) and Falcon will mask FDC DMA address with 0xffffff (limtied to 16 
MB, but a real falcon won't allow that config).

So, in all cases falcon's emulation will give bad result if trying to 
transfer FDC data in TT RAM, because TT RAM starts at 0x01000000 (just 
after 16 MB).

Also, DMA registers only exist for 3 bytes, you can't set the highest 
byte of a 32 bit address.

Any FDC access beyond 0x01000000 will indeed overwrite the corresponding 
normal RAM.