The OS/2 subsystem implements the protection between OS/2 applications. It constructs their address spaces (both the flat address space and LDTs) and implements the same protection as exists in OS/2.
Some of the memory management limitations of OS/2 1.x are removed. The most important of these is the limit of 16 MB of physical RAM; the OS/2 subsystem uses the large memory capability of Windows NT. This translates into increased performance for applications that can use the additional memory, such as Microsoft's SQL Server. SQL Server asks for the physical memory available in the system at setup time. It then uses this number to determine the level of caching it will use. In OS/2, you can't use more than 16 MB; however, in the OS/2 subsystem in Windows NT, you can use 32 MB (for example) and double your caching capability.
The OS/2 subsystem uses the Windows NT paging mechanism; no segment swapping is performed. Segment swapping is inferior to paging and exists in OS/2 only to support the 80286 processor, which is not supported for Windows NT.