Different Windows editions have had different upper maximum RAM limits. 64-bit Windows support different amounts of memory as an SKU differentiating feature, with the low end being 512MB for Windows XP Starter to 128GB for Vista Ultimate and 192GB for Windows 7 Ultimate.
All 32-bit Windows client SKUs, however, including Windows Vista, Windows XP and Windows 2000 Professional, support a maximum of 4GB of physical memory. 4GB is the highest physical address accessible with the standard x86 memory management mode. Hardware Specifications for Windows 7 lays down the minimum memory (RAM) requirements.
Maximum memory (RAM) limits
So, while the maximum RAM limit for 32-bit Windows 7 / 8 editions is 4GB, when it comes to the 64-bit editions, the amount of memory that the OS can address depends on which edition you are running.
Here are the upper RAM limits for the different editions of Windows 7 64-bit:
- Starter: 2GB
- Home Basic: 8GB
- Home Premium: 16GB
- Professional: 192GB
- Enterprise: 192GB
- Ultimate: 192GB
These limits are similar to those for Windows Vista editions, except that Vista Enterprise and Vista Ultimate have had their upper limits raised from 128GB to 192GB.
Windows 8 and Windows 8.1, 64-bit has different memory limits depending on the edition.
- Windows 8: 128GB
- Windows 8 Professional: 512GB
- Windows 8 Enterprise: 512GB.
For Windows Server 2008 R2 Enterprise edition, the limit could extend to even 2 TB.
Hope this helps.