Windows System
Windows and 4GB RAM — The Frustration, Confusion and Solution(Part 4)
The 4GB limit is cause by IA32 microstructure, which debuted in 1986’s INTEL i386 processor(and it servers us a good 20 years!). For modem x86 processors with 64bit capacity, while they support more than 4GB memory, are still limited to the 4GB virtual address barrier when running a 32bit OS.
These articles are mostly focus on how to get Windows “see” 4GB, but this doesn’t imply that a specified application can use all the 4GB memory. In 32bit Windows, by default, the lower 2 GB are reserved for user-mode programs and the upper 2 GB are reserved for kernel-mode programs. We can use the /3GB switch under Windows Server 2003, Windows XP, Windows 2000, and Windows NT. or IncreaseUserVA element in BCDEdit in Windows Vista to increase user-mode space, which is called 4 GT RAM Tuning. Anandtech features a nice article about this issue, the benefit is obvious but there are also some potential problems when enabling this tweak. The best choice is go for 64bit Windows.
The solution, finally
To get full 4GB support(and make full use of them), a prerequisite is to get around the MMIO issue, so:
- Motherboards with Memory Remapping Support, Intel P945 or older wouldn’t work.
- An 32bit OS with PAE support and can address more than 4GB or a 64bit OS. So 32bit XP or Vista is out of luck here.
- Fine tune: Enable 4 GT RAM Tuning under 32bit Windows or switch to 64bit alternatives to get more space for application.
Before using Memory Remapping to sort out the MMIO issue, make sure your MB/OS supports it properly, so don’t turn it on when using 32bit Windows XP or Vista.
In Windows, PAE or X64 is required to get all 4GB RAM, but that’s not enough.