Holy cow, I wrote a book!
Ten thousand human beings in peak physical condition. All in one dormitory complex. With 130,000 free condoms. Let the games begin!
And you have to tip your hat (tam?) to The Scotsman for finding an athlete named "Randy Jones" for this article.
A table of contents now that the whole thing is over. I hope.
I'm not sure how successful this series has been, though,
for it appears that even people who have read the articles
continue to confuse virtual address space with physical address space.
(Or maybe this person is merely mocking a faulty argument?
I can't tell for sure.)
the interaction between PAE and NX on his own blog,
so I'll merely incorporate his remarks by reference.
(And notice again the concession to backwards compatibility.
Without the backwards compatibility work, XP SP2 would have shipped
with NX support and an asterisk,
"* and those of you who have device drivers that are
not PAE-ready will not be able to take advantage of these new security
enhancements. We could've done something to make your systems
secure, but we decided not to do it in order to teach you a lesson.")
Hereby incorporating by reference the "FakeMenu" sample
in the Platform SDK. It's in the winui\shell\fakemenu directory.
For those who don't have the Platform SDK, what are you doing
writing Win32 programs without the Platform SDK?
Download it if it didn't come with your development tools.
If for some reason you don't want the Platform SDK
yet you want to write Win32 programs (I bet you're the sort of
person who throws away the manual as soon as you buy something),
you can look at the version that
has stashed away
on this page.
Address Windowing Extensions (AWE)
does not require PAE.
I don't know why some people claim that it does,
since it is so easy to demonstrate otherwise.
Take a program that uses AWE.
If you don't have one handy, you can
use the one that comes in MSDN as a sample program that demonstrates
how to use AWE.
Grant yourself "Lock Pages in Memory" privileges and run the program.
Observe that it works.
Now remove the /PAE switch from your boot.ini, reboot, and run
the program again.
Observe that it still works.
Myth disproved by direct experimentation and observation.
This is another non sequitur.
PAE increases the amount of physical memory that can be
addressed by the processor, but that is unrelated to virtual
(Remember that PAE stands for
Physical Address Extensions.)
PAE increases the physical address space
(the address space that the CPU can use
to access the memory chips on your computer) from 32 bits
to 36 on a Pentium 2,
for a theoretical maximum physical memory capacity of 64GB.
However, the size of a pointer variable hasn't changed.
It's still 32 bits (for a 32-bit processor), which means
that the virtual address space is still 4GB.
With PAE enabled, the page table and page directory entries
double in size (to accomodate the additional bits in the page frame),
which significantly increases the amount of memory required for
page tables and page directories (since each page table describes
only half as much memory as it used to).
Notice that this has as a consequence that
PAE and /3GB conflict with each other
to some degree.
If you turn on both PAE and /3GB, then the kernel will limit itself
to 16GB of physical memory.
there isn't enough address space in the kernel
to fit all the necessary memory bookkeeping into the 1GB of memory
you told the kernel to squeeze itself into.
(On an AMD processor, the physical address space expands to 40 bits,
for a theoretical maximum of 1TB. However, the memory manager uses only
37 of those bits, for an actual maximum of 128GB. Why?
For the same reason that the kernel limits itself to 16GB in /3GB mode:
Not enough address space.
It's time to move to 64-bit processors...)
Apparently there is
some unrest in comment-land
with people who are
sick of this whole /3GB series.
Why have I been spending over two weeks exploring the consequences of the
/3GB switch and exploding various common myths about it?
Because too many people don't understand what /3GB means
but talk as if they do.
As you saw yesterday, there are still lots of people out
there that don't understand the differences between
physical memory, virtual memory, and virtual address space,
and end up misconfiguring
their computers or treating the switch as magic fairy dust.
I've gotten tired of explaining it to
misguided person after misguided person
over the years, so I figured if I wrote up the explanation
and debunkings once and for all, I won't have to visit the topic
You think you're sick of /3GB?
You've only had to deal with it
for two weeks. Imagine having to explain the /3GB switch for
At any rate, the 3GB series will draw to a close at the end of the week,
assuming everything goes according to schedule.
Then there will be other topics for you to be sick of.
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
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
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.
Just because the virtual address space is 3GB doesn't mean that you can
map one giant 3GB block of memory.
The standard holes in the virtual address space are still there:
64K at the bottom, and
64K near the 2GB boundary.
Moreover, the system DLLs continue to load at their preferred
virtual addresses which lie just below the 2GB boundary.
The process heap and other typical process bookkeeping also
take their bites out of your virtual address space.
As a result, even though the user-mode virtual address space is nearly 3GB,
it is not the case that all of the available space is contiguous.
The holes near the 2GB boundary prevent you from getting even 2GB of
contiguous address space.
Some people may try to relocate the system DLLs to alternate
addresses in order to create more room, but that won't work for
First, of course, is that it doesn't get rid of the 64K
gap near the 2GB boundary.
Second, the system allocates other items such as thread information
blocks and the process environment variables
before your program gets a chance to start running, so by
the time your program gets around to allocating memory, the space
it wanted may already have been claimed.
Third, the system really needs certain key DLLs to be loaded
at the same address in all processes.
For example, the syscall trap must reside at a fixed location
so that the kernel-mode trap handler will recognize it as
a valid syscall trap and not as an illegal instruction.
The debugger requires this as well, so that it can
use CreateRemoteThread to inject a breakpoint
into the process when you tell it to break into the
process you are debugging.
If you look through the
you'll see an article that say that
Exchange 2000 requires the /3GB switch with more than 1
gigabyte of physical RAM.
Yet I've been writing all this time that /3GB has nothing to do
with physical RAM. What's the deal?
The title of the article could be a bit clearer.
It really should be something more like
"Exchange 2000 requires the /3GB switch to take advantage of
more than 1 gigabyte of physical RAM".
It appears that Exchange 2000 doesn't use
the bank-switching technique I described in an earlier entry.
(I don't blame them. It's extraordinarily cumbersome.)
Consequently, for Exchange 2000, virtual address space equals
The capacity of a program is typically a combination of
multiple factors, the lowest of which sets the limit.
suppose you need two piece of bread, two pieces of
bologna, and a slice of cheese to make a bologna and cheese sandwich.
Whichever ingredient you run out of first determines how many sandwiches
you can make.
If you run out of cheese first, adding more bologna won't help any.
Okay, so what do bologna and cheese
sandwiches have to do with Exchange 2000?
From the description in the article, it appears that
program is RAM-constrained most of the time
(you run out of cheese first). But once you get the
memory on the machine up to one gigabyte, you have excess RAM
and address space becomes the new limiting factor.
(You added plenty of cheese and now you've run out of bologna.)
That's where the /3GB switch comes in.
It increases the user-mode address space, thereby relieving pressure
on the address space constraint.