In this paper, the authors analyze the trade-offs in architecting stacked DRAM either as part of main memory or as a hardware-managed cache. Using stacked DRAM as part of main memory increases the effective capacity, but obtaining high performance from such a system requires Operating System (OS) support to migrate data at a page-granularity. Using stacked DRAM as a hardware cache has the advantages of being transparent to the OS and perform data management at a line-granularity but suffers from reduced main memory capacity.