Different Windows editions have had different upper maximum RAM limits. 64-bit Windows support different amounts of memory as a 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.
Read: Difference between 64-bit and 32-bit Windows.
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.