A method to request memory from a far memory cache and implement, at an operating system (OS) level, a fully associative cache on the requested memory. The method includes pinning the working set of a program into the requested memory (pin buffer) so that it is not evicted due to cache conflicts and is served from the fast cache and not the slower next level memory. The requested memory extends the physical address space and is visible to and managed by the OS. The OS has the ability to make the requested memory visible to the user programs. The OS has the ability to manage the requested memory from the far memory cache as both a fully associative cache and a set associative cache.