Why is the virtual address space 4GB anyway?
The size of the address space is capped by the number of
unique pointer values. For a 32-bit processor, a 32-bit
value can represent 232 distinct values.
If you allow each such value to address a different byte
of memory, you get 232 bytes, which equals
four gigabytes.
If you were willing to forego the flat memory model
and deal with selectors, then you can combine a 16-bit
selectors value with a 32-bit offset for a combined 48-bit pointer
value.
This creates a theoretical maximum of
248 distinct pointer values,
which if you allowed each such to address a different byte
of memory, yields 256TB of memory.
This theoretical maximum cannot be achieved on the Pentium
class of processors, however.
On reason is that the lower bits of the segment value
encode information about the type of selector.
As a result, of the 65536 possible selector values, only
8191 of them are usable to access user-mode data.
This drops you to 32TB.
The real limitation on the address space using the
selector:offset model is that each selector merely describes
a subset of a flat 32-bit address space. So even if you
could get to use all 8191 selectors, they would all just
be views on the same underlying 32-bit address space.
(Besides, I seriously doubt people would be willing to return
the the exciting days of segmented programming.)
In 64-bit Windows, the 2GB limit is gone;
the user-mode virtual address space is now a stunning 8 terabytes.
Even if you allocated a megabyte of address space per second,
it would take you three months to run out.
(Notice however that you can set
/LARGEADDRESSAWARE:NO on your 64-bit program
to tell the operating system to force the program to live
below the 2GB boundary. It's unclear why you would ever want
to do this, though, since you're missing out on the 64-bit
address space while still paying for it in pointer size.
It's like paying extra for cable television and then not watching.)
Armed with what you have learned so far,
maybe you can respond to this request
that came in from a customer:
Oen of our boot.ini files has a /7GB switch.
Our consultant told us that we should set it to 1GB less than
the system memory. Since we have 8GB, 8GB - 1GB = 7GB.
The consultant said that setting this value allows an application to
allocate more than 2GB of memory.
We would like Microsoft to comment on this analysis.
