Re: [hatari-devel] defaulting to SMALL_MEM

[Eero Tamminen <oak@xxxxxxxxxxxxxx>]

Hi,

On 13.2.2022 14.56, Nicolas Pomarède wrote:
> Le 13/02/2022 à 11:46, Eero Tamminen a écrit :
> > Especially considering that non-existing ST-RAM areas can be *read* 
> > without exceptions, at least in some cases...
>
> regarding non existing RAM, it all depends if you read RAM areas using 
> the bank memory functions in stMemory.c that check for boundary (which 
> is a little slower), or if you read memory by directly accessing the 
> allocated memory, which can indeed go out of bound if all the proper 
> check are not there (but is faster, which matters a lot when doing 
> screen convert code)

There was recent discussion on EmuTOS devel list that non-existent RAM 
can be *read* without exception on *Atari* machines in certain cases. [1]

Thomas' change skips updating the internal video.c variables if (any 
part of) the current video line is past end of RAM.

While it's unlikely any program would intentionally rely on screen being 
mapped outside of existing RAM area temporarily, accidents can happen. 
And they might be relying on internal video variables being updated.


	- Eero

[1] David Henderson:

On my STE (stock processor with 4MB STRAM, no expansion) reads of the 
D00000 range don’t generate bus errors
....
NB. writing to that address *does* generate bus error.


Christian Zietz:

Yes, the STE has some additional unused ROM decodes, that allow reads 
from 0xD0xxxx.