Re: [MiNT] Memory protection

[Henk Robbers <H.Robbers@cable.A2000.nl>]

Michael Schwingen wrote:
> 
> On Fri, Feb 19, 1999 at 02:16:58AM +0100, Henk Robbers wrote:
> >
> > I think you use it in such a way that the effect
> > is a mere amalgamating of ST and TT ram. The displacement from physical
> > space is a constant for all TT ram pages. Am I right?
> 
> No. We still have ST- and TT-RAM (ISA DMA is only possible in the lower
> 16MB, which is identical with ST RAM).
>
> The Offset in TT RAM is not constant - you can reduce the amount of ST RAM
> to 4 or 8MB, which means that the rest is moved up to TT-RAM, and each
> of the 8 memory banks start at a multiple of 64MB in address space, which
> means that you get holes in the physical address space unless you put 512MB
> into the machine.

I see. The physical address of each bank remains unchanged when you upgrade?
How will 16 or 32 Mb fit in this, in 1 or more bank, leaving empty banks?

Furthermore: to my regret I did not express myself correctly.
I wanted to say: Each TTram page's offset will remain constant
throuhgout a kernel session.
That is, if that reduction can only be done at bootup.
Is this the case?

> The PMMU is set up so that you have one contiguous block of TT RAM (in
> virtual address space).

Very well.

> This also means that drivers which do DMA in TT RAM (like my SCSI driver)
> must support scatter/gather DMA, since one transfer may span multiple
> physical blocks.

I am glad to hear that.
Because we need that too as we are going to scatter pages
all over, while franticly swapping them around. :-)

Thanks a lot.
Henk