Windows® Internals, Sixth Edition, Part 2

Besides affecting performance, the amount of physical memory impacts other resource limits. For example, the amount of nonpaged pool, operating system buffers backed by physical memory, is obviously constrained by physical memory. Physical memory also contributes to the system virtual memory limit, which is the sum of roughly the size of physical memory plus the current configured size of any paging files. Physical memory also can indirectly limit the maximum number of processes.

Windows support for physical memory is dictated by hardware limitations, licensing, operating system data structures, and driver compatibility. Table 10-20 lists the currently supported amounts of physical memory across the various editions of Windows along with the limiting factors.

Table 10-20. Physical Memory Support

Version

32-Bit Limit

64-Bit Limit

Limiting Factors

Ultimate, Enterprise, and Professional

4 GB

192 GB

Licensing on 64-bit; licensing, hardware support, and driver compatibility on 32-bit

Home Premium

4 GB

16 GB

Licensing on 64-bit; licensing, hardware support, and driver compatibility on 32-bit

Home Basic

4 GB

8 GB

Licensing on 64-bit; licensing, hardware support, and driver compatibility on 32-bit

Starter

2 GB

2 GB

Licensing

Server Datacenter, Enterprise, and Server for Itanium

N/A

2 TB

Testing and available systems

Server Foundation

N/A

8 GB

Licensing

Server Standard and Web Server

N/A

32 GB

Licensing

Server HPC Edition

N/A

128 GB

Licensing

The maximum 2-TB physical memory limit doesn’t come from any implementation or hardware limitation, but because Microsoft will support only configurations it can test. As of this writing, the largest tested and supported memory configuration was 2 TB.

Windows Client Memory Limits

64-bit Windows client editions support different amounts of memory as a differentiating feature, with the low end being 2 GB for Starter Edition, increasing to 192 GB for the Ultimate, Enterprise, and Professional editions. All 32-bit Windows client editions, however, support a maximum of 4 GB of physical memory, which is the highest physical address accessible with the standard x86 memory management mode.

Although client SKUs support PAE addressing modes on x86 systems in order to provide hardware no-execute protection (which would also enable access to more than 4 GB of physical memory), testing revealed that systems would crash, hang, or become unbootable because some device drivers, commonly those for video and audio devices found typically on clients but not servers, were not programmed to expect physical addresses larger than 4 GB. As a result, the drivers truncated such addresses, resulting in memory corruptions and corruption side effects. Server systems commonly have more generic devices, with simpler and more stable drivers, and therefore had not generally revealed these problems. The problematic client driver ecosystem led to the decision for client editions to ignore physical memory that resides above 4 GB, even though they can theoretically address it. Driver developers are encouraged to test their systems with the nolowmem BCD option, which will force the kernel to use physical addresses above 4 GB only if sufficient memory exists on the system to allow it. This will immediately lead to the detection of such issues in faulty drivers.

32-Bit Client Effective Memory Limits

While 4 GB is the licensed limit for 32-bit client editions, the effective limit is actually lower and dependent on the system’s chipset and connected devices. The reason is that the physical address map includes not only RAM but device memory, and x86 and x64 systems typically map all device memory below the 4 GB address boundary to remain compatible with 32-bit operating systems that don’t know how to handle addresses larger than 4 GB. Newer chipsets do support PAE-based device remapping, but client editions of Windows do not support this feature for the driver compatibility problems explained earlier (otherwise, drivers would receive 64-bit pointers to their device memory).

If a system has 4 GB of RAM and devices such as video, audio, and network adapters that implement windows into their device memory that sum to 500 MB, 500 MB of the 4 GB of RAM will reside above the 4 GB address boundary, as seen in Figure 10-43.

The result is that if you have a system with 3 GB or more of memory and you are running a 32-bit Windows client, you may not be getting the benefit of all of the RAM. You can see how much RAM Windows has detected as being installed in the System Properties dialog box, but to see how much memory is actually available to Windows, you need to look at Task Manager’s Performance page or the Msinfo32 and Winver utilities. On one particular 4-GB laptop, when booted with 32-bit Windows, the amount of physical memory available is 3.5 GB, as seen in the Msinfo32 utility: