MORE INFORMATION
Applications developed for the Windows NT Server platform continue to grow,
both in terms of size and performance demands. For applications that are
I/O intensive, such as database management systems (DBMS), the use of a
larger process space can provide considerable performance benefits as time-
intensive I/O access to media is reduced. With the current Windows NT
Server product, the per-process address limit is 2 GB. 4GT increases this
limit to 3 GB without introducing new APIs. 4GT does this by reducing the
potential RAM allocated to the Windows NT kernel from 2 GB to 1 GB.
This feature benefits applications that run on powerful computers with more
than 2 GB of physical RAM and that can take advantage of a larger address
space. The impact on developers and applications are summarized below.
Windows NT Server/E supports 4GT on Intel architecture servers only.
NOTE: The 4GT works only on 32-bit processors, so it does not increase the
addressable memory of Windows NT Server beyond its current limit of 4 GB.
Writing Applications for 4GT
User-mode Address Selection:
When 4GT is enabled, the highest bit of a virtual address cannot be used to
differentiate user-mode addresses from kernel-mode addresses.
Memory Allocation Issues:
Some dynamic link library (DLL) files load near the 2 GB boundary;
therefore, there is a region of the 2 GB space in which contiguous memory
cannot be allocated using VirtualAlloc.
Effects Visible in Kernel Mode:
Kernel-mode code can no longer assume the user/kernel boundary is at
0x80000000 or at any other number. Code that uses ProbeForRead or
ProbeForWrite macros must be rebuilt using new headers that no longer
contain assumptions about kernel space starting at 0x80000000.
Enabling 4GT Support in Your Applications
The changes to support 4GT are done at both the system and application
levels.
System Changes:
After you have installed Windows NT Server/E, you must modify the
Boot.ini file to enable 4GT. To enable 4GT, simply add the /3GB parameter
to the startup line.
NOTE: This change is only effective if you are using Windows NT Server/E.
On standard Windows NT, this flag will relocate the kernel, but
applications will be unable to access more than 2 GB.
For example:
[boot loader]
timeout=30
default=multi(0)disk(0)rdisk(0)partition(2)\WINNT
[operating systems]
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows NT Server Version 4.00"
/3GB
multi(0)disk(0)rdisk(0)partition(2)\WINNT="Windows NT Server Version 4.00
[VGA mode]" /basevideo /sos
: Some of the lines above have been wrapped for readability.
Application Changes:
No new APIs are required for 4GT support. Instead, memory allocations
remain the same, with the exceptions that are noted below in "Tips for
Effective 4GT Support." However, it would be ineffective for every
application to automatically be provided with a 3-GB address space. To
provide a selective use of 4GT, the following mechanism has been
implemented:
- Executables that must see the 3-GB address space are required to have
the bit IMAGE_FILE_LARGE_ADDRESS_AWARE set in their image header. This
can be done using the Imagecfg tool that is included in the Support
folder on the Windows NT Server/E compact disc. For example, to modify
the target file DBMSApp.exe, type the following at a command prompt:
NOTE: The linker also has a new switch (/ LARGEADDRESSAWARE) to link
executables with the IMAGE_FILE_LARGE_ADDRESS_AWARE bit. Setting this bit
and then running the application on a system that does not have 4GT support
should not affect the application.
Tips for Effective 4GT Support
The following guidelines are intended as a summary of changes that
developers will want to review when they consider enabling 4GT support
within their applications:
- Use GlobalMemoryStatus to get the amount of total user virtual space.
Avoid using hard-wired constant definitions such as "#define
HIGHEST_USER_ADDRESS 0xC0000000". Try to detect the real value at
runtime.
- Avoid signed comparisons with pointers. Some applications might crash on
a 4GT-enabled system for just this reason. A condition such as "if
(pointer > 40000000)" will be false for a pointer that is above 2 GB.
- Code using the highest bit to tag items (data value versus an address
value) will fail. For example, a 32-bit word might be considered a user-
mode address if below 0x80000000 and an error code if above. This does
not work anymore.
Setting a Paging File Size
If you are using the 4GT option and have a system with greater than 3 GB of
physical memory, you may want to consider changing the default size of your
page file. Note that this is changed by using the System tool in Control
Panel. In Windows NT 3.51, the default pagefile size is generally 11 MB
larger than physical memory. In Windows NT 4.0, the default pagefile size
is equal to the amount of physical memory on the system. On a 4-GB
computer, this would result in a 4.01-GB paging file. Under such
circumstances, the effectiveness of this paging file could be minimal,
based on the total size, which is 4 GB. Accordingly, a 256-MB page file
could be a more effective use of disk space. However, having a smaller
paging size will affect the total memory commit size for applications. You
should review these settings with the Independent Software Vendor (ISV) for
your applications that are being run with the 4GT option.