Holy cow, I wrote a book!
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.
Only programs marked as /LARGEADDRESSAWARE are affected.
For compatibility reasons, only programs that explicitly
indicate that they are prepared to handle a virtual address space
larger than 2GB will get the larger virtual address space.
Unmarked programs get the normal 2GB virtual address space,
and the address space between 2GB and 3GB goes unused.
Because far too many programs assume that the high bit of user-mode
virtual addresses is always clear, often unwittingly.
MSDN has a page listing some of the ways programs make this
One such assumption you may be making is taking
the midpoint between two pointers by using the formula
As I noted in a previous exercise,
this is subject to integer overflow and consequently can result
in an erroneous pointer computation.
Consequently, you can't just take an existing program that you didn't
write, mark it /LARGEADDRESSAWARE, and declare your job done.
You have to check with the authors of that program that they verified
that their code does not make any 2GB assumptions.
(And the fact that the authors
didn't mark their program as 3GB-compatible
strongly suggests that no such verification has occurred.
If it had, they would have marked the program /LARGEADDRESSAWARE!)
Marking your program /LARGEADDRESSAWARE indicates to the operating
system, "Go ahead and give this program access to that extra
gigabyte of user-mode address space,"
and as a result, addresses in the third gigabyte
become possible return values from memory allocation functions.
If you set
the "Top down" flag in the
memory manager allocation preferences mask
(search for "top down"),
you can instruct the memory manager to allocate high-address memory
first, thereby forcing your program to deal with those addresses
sooner than it normally would.
This is very handy when testing your program in a /3GB configuration
since it forces the troublesome memory addresses to be used sooner
Exercise: Find the bug in the following function.
Hint: What's today's topic?
#define BUFFER_SIZE 32768
BOOL IsPointerInsideBuffer(const BYTE *p, const BYTE *buffer)
return p >= buffer && p - buffer < BUFFER_SIZE;
Physical memory is not virtual address space.
In my opinion, this is another non sequitur. I'm not sure what logical process led to this myth. It can't be a misapprehension of a 1-1 mapping between physical memory and virtual memory, because that mapping is blatantly not one-to-one. You typically have far more virtual memory than physical memory. Free physical memory doesn't have any manifestation in any virtual address space. And shared memory has manifestations in multiple virtual address spaces yet correspond to the same physical page.
Though this brings up a historical note.
In Windows/386, the kernel just so happened to map all physical memory into the kernel-mode virtual address space. There was a function _MapPhysToLinear. You gave it a physical memory range and it returned the base of a range of linear addresses that could be used to access that physical memory. Some driver developers discovered that the kernel mapped all of physical memory and just handed out pointers into that single mapping. As a result, they called _MapPhysToLinear(0, 0x1000) and whenever they wanted to access physical memory in the future, they just added the address to the return value from that single call. In other words, they assumed that
_MapPhysToLinear(p, x) = _MapPhysToLinear(0, x) + p
In Windows 95, the memory manager was completely rewritten and the above coincidence was no longer true. To conserve kernel-mode virtual address space, physical memory was now mapped linearly only as necessary.
Of course, the drivers that relied on the old behavior were now broken because the undocumented behavior they relied upon was no longer present.
As a result, when it starts up, Windows 95 looks around to see if any drivers known to rely on this undocumented behavior are loaded. (Windows 3.1 didn't support dynamically-loaded kernel drivers so looking at boot time was sufficient.) If so, then it went ahead and mapped all of physical memory into the kernel-mode virtual address space to keep those driver happy. This wasted virtual address space but kept your machine running.
I can already hear people saying, "Microsoft shouldn't have made those buggy drivers work. They should have just let the computer crash in order to put pressure on the authors of those drivers to fix their bugs." This assumes, of course, that the cause of the crash could be traced back to the buggy driver in the first place. A very common manifestation of a stray pointer in kernel mode is memory corruption, which means that the component that crashes is rarely the one that caused the problem in the first place.
For example, nearly all Windows 95 bluescreen crashes in VMM(01) are caused by memory corruption. VMM(01) is the non-swappable part of the Windows 95 kernel which is where the memory manager lives. If a driver corrupts the kernel-mode heap, a bluescreen in the memory manager is typically how the corruption manifests itself.
Virtual memory is not virtual address space (part 2).
This myth is being perpetuated even as I write this series of articles.
The user-mode virtual address space is normally 2GB, but that doesn't limit you to 2GB of virtual memory. You can allocate memory without it being mapped into your virtual address space. (Those who grew up with Expanded Memory or other forms of bank-switched memory are well-familiar with this technique.)
HANDLE h = CreateFileMapping(INVALID_HANDLE_VALUE, 0,
PAGE_READWRITE, 1, 0, NULL);
Provided you have enough physical memory and/or swap file space, that 4GB memory allocation will succeed.
Of course, you can't map it all into memory at once on a 32-bit machine, but you can do it in pieces. Let's read a byte from this memory.
BYTE ReadByte(HANDLE h, DWORD offset)
DWORD chunkOffset = offset % si.dwAllocationGranularity;
DWORD chunkStart = offset - chunkOffset;
LPBYTE pb = (LPBYTE*)MapViewOfFile(h, FILE_MAP_READ, 0,
chunkStart, chunkOffset + sizeof(BYTE));
BYTE b = pb[chunkOffset];
Of course, in a real program, you would have error checking and probably a caching layer in order to avoid spending all your time mapping and unmapping instead of actually doing work.
The point is that virtual address space is not virtual memory. As we have seen earlier, you can map the same memory to multiple addresses, so the one-to-one mapping between virtual memory and virtual address space has already been violated. Here we showed that just because you allocated memory doesn't mean that it has to occupy any space in your virtual address space at all.
[Updated: 10:37am, fix minor typos reported in comments.]
Virtual memory is not virtual address space (part 1).
I don't know where this myth comes from;
it's a non sequitur.
Virtual address space describes how addresses are resolved,
but since each process has its own virtual address space,
the amount consumed by one program has no effect on
that consumed by another program.
Say you have a program that allocates 1GB of memory.
Run three copies of it. Now you have a total of
3GB of allocated memory. And none of the programs came
even close to exhausting its 2GB virtual address space allotment.
Tomorrow, the debunking of a variation on this myth.
One of the adverse consequences of the /3GB switch is that
it forces the kernel to operate inside a much smaller space.
One of the biggest casualties of the limited address space is
the video driver. To manage the memory on the video card, the
driver needs to be able to address it, and the apertures required
are typically quite large.
When the video driver requests a 256MB aperture, the call
is likely to fail
since there simply isn't that much address space available to spare.
All of kernel's bookkeeping needs to fit inside that one gigabyte.
Page tables, page directories, bitmaps, video driver apertures.
It's a very tight squeeze, but if you're willing to cut back
(for example by not requiring such a large video aperture),
you can barely squeak it through.
(A later entry will discuss another casualty of the reduced address space.)
It's like trying to change your clothes inside a small closet.
You can do it, but it's a real struggle, you're going to have
to make sacrifices, and the results aren't always very pretty.
It's simple to explain what it does, but people often misunderstand.
The /3GB switch changes the way the 4GB virtual address space is split up.
Instead of splitting it as 2GB of user mode virtual address space
and 2GB of kernel mode virtual address space,
the split is 3GB of user mode virtual address space and 1GB of kernel mode
virtual address space.
And yet people think it does more than that.
I think the problem is that people think that "virtual address space"
means something other than just "virtual address space".
The term "address space" refers to how a numerical value (known
as an "address") is interpreted when it is used to access some
type of resource. There is a physical address space; each address
in the physical address space refers to a byte in a memory chip
somewhere. (Note for pedants: Yes, it's actually spread out over
several memory chips, but that's not important here.)
There is an I/O address space; each address in the I/O address space
allows the CPU to communicate with a hardware device.
And then there is the virtual address space. When people
say "address space", they usually mean "virtual address space".
The virtual address space is the set of possible pointer values
(addresses) that can be used at a single moment by the processor.
In other words, if you have
an address like 0x12345678, the virtual address space determines what
you get if you try to access that memory.
The contents of the virtual address space changes over time, for example,
as you allocate and free memory.
It also varies based on context: each process has its own virtual address
Saying that 2GB (or 3GB) of virtual address space is available to user mode
means that at any given moment in time, out of the 4 billion
virtual addresses available in a 32-bit value, 2 billion
(or 3 billion) of them are potentially usable by user-mode code.
Over the next few entries, I'll talk about the
various consequences and misinterpretations of the /3GB switch.