Mail Archives: djgpp/1999/04/21/07:50:53
> >Has anybody ever written any code to use the memory that is beyond 4GB on
> >the P6/PII/PIII chips? I am sure that this would have to be assembly code
> >mixed with C code, but all I need to do is move data in and out of the +4GB
> >region.
> >Brett Henning
> The first point is that: yes, the P6s do have 64GB addressing (36 address
> lines).
> Essentially, from what I can tell the way it is implemented is that you need
> to change the 4K page mode that is the basis of the 386 addressing scheme
> into a 2 Meg page mode via setting bit 5 on CR4 to 1. Really kind of nifty.
> The unfortunate point is that, that you cannot have programs running with
> both 4K mode and 2Meg mode operating at the same time, so it really needs to
> be enabled by an Operating System aware of how the new paging mechanism
> works.
> Number of OSes enabling new paging mechanism that I know of: 0 :(
> Basically, so unless you are programming OSes, and in this case you probably
> wouldn't be using DJ's *DOS* port of GCC/egcs, your probably SOL. It might
> be possible to recode the DPMI server to know about the 2 Meg page mode, but
> it seems like a lot of work for a feature isn't in a whole lot of demand at
> the moment.
> -- Kelley Cook
Well, since you brought it up ... :-)
CWSDPMI is currently limited to 255Mb physical memory due to being written with
a 16-bit compiler, and the 640K limit it has to live with...
I've considered hacking a fix in to suppport more than 256Mb of memory by
request - but then the page tables themselves (1 per 4Mb, so 1Kb/Mb) start to
limit you to about 512Mb or so before you start page faulting the tables
themselves (not a very useful design).
Then I decided anyone with that much memory probably doesn't want to page
anyway, and would not worry about wasting a few Mb here and there. So I
considered a physical memory only version - no virtual - using large page
support (4Mb pages or 2Mb pages).
Since I haven't used anything except Windows NT, Unixy flavors, and OpenVMS
for several years (no DOS) this is a purely entertainment exercise.
If anyone has exotic hardware to play with and is interested, drop me a note.
I already have some systems with 1Gb in them.
If anyone has a strong opinion or need, I may listen ...
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